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Prctica 3

Empleo de un Analizador Vectorial de Seales

Agilent 89600

Tutorial

Laboratorio de Procesado de Seal en Comunicaciones


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Vector Signal Analyzer ApplicationLoading and Running the Vector Signal Analyzer Application

To load the 89600 Vector Signal Analyzer application:

Click Start > (All) Programs > Agilent 89000 VSA > Vector Signal Analyzer

Or

Double-click the icon on the desktop.

The application checks for installed hardware to establish system configuration and presets. If nohardware is detected a warning message is shown and the analyzer will initialize using thesimulated hardware configuration and associated presets.

The following figure shows the initial display with simulated signals and default configuration. Ifyour analyzer includes measurement hardware, the trace displayed will be different.

The application is now ready for use!


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Vector Signal Analyzer ApplicationUsing the Vector Signal Analyzer

In this section, we will learn to navigate the Vector Signal Analyzer's user interface.

Main Measurement ProcessAnalyzing signals with the 89600 generally starts with this sequence:

1. Preset the analyzer.

2. Choose an Input to the analyzer: RF, baseband, or file.

3. Range the analyzer for the expected signal power.

4. Set Frequency and Span to include the signal(s) of interest.

5. Configure the Display for the number and orientation of measurement traces.

6. Choose the desired TraceData.

7. Choose a Trace Format.

8. Scale or Autoscale the trace.

9. Apply Marker features to yield measurement results.

Load and runyour vector signal analyzer application and perform the steps along with thetutorial.

Let's first look at the user interface:

Click the different areas of the display below for explanations.


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Let's now execute the main measurement process on the user interface.

This process is implemented in "left to right" fashion on the main menu bar.

Step 1. Preset the analyzer

Click File > Preset to recall a "safe" initial instrument setup.

The selection Preset Setup will initialize the vector signal analyzer and many of its parameters, butwill not reset the demodulator setup, data registers, math functions or other instrument states thathave been saved. Preset Menu/Toolbars will set any menu/toolbar customization back to thefactory-shipped state. Custom setups can be saved under File, Save.

Preset Display Appearance will set any display customization, colors, fonts, etc., back to thefactory shipped state. Custom display setups can be saved under File, Save.

Preset All is a complete system preset to the factory-shipped state.

The file menu also allows you to: Save/recall instrument setups. Save/recall custom menus and toolbars.

Save/recall display setups (appearance). Save/recall I/Q demodulation state definitions

Copy trace data to a data register

Save/recall recorded signals.

Print the active trace.

Close (exit) the VSA window.

Step 2. Choose an Input to the analyzer: RF, baseband, or file

The Input menu configures the analyzer to accept data from the VXI hardware or from a data filereferred to as a "recording".


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The Input menu is also accessed to setup:

The input channel (when the optional second channel is available).

Set up and control signal recording and playback.

Step 3. Range the analyzer for the expected signal power

Also found in the Input menu is the Range function. Set the range for the maximum total signalpower at the input of the analyzer and the system will conveniently configure all attenuators andgain stages to maximize dynamic range and minimize analyzer distortion. Range can also be setdirectly on the measurement grid. Just click the Range field and step the value or type a value indirectly.

.

Note

Failure to set the range above the power of the signal will result in an "overload"condition and significant measurement distortion.

Step 4. Set Frequency and Span to include the signal(s) of interest.

Moving now to the MeasSetup menu, select the appropriate Frequency Band (DC - 36 MHz or36 - 2700 MHz) for your signal. This will configure the hardware to use or bypass the RFtuner section.

Next, click Frequency and set the Centerand Span, or Start and Stop frequencies(select the ShowCenter/Span check box to enter center frequency and span; clear it to enter the start and stopfrequency).


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These parameters can also be set directly on the measurement grid by clicking on the field andstepping the value using the up/down keys on the keyboard or entering the value directly.

The MeasSetup menu is also used for:

Setting resolution bandwidth and time parameters

Averaging

Turning on and setting up the Analog and Digital Demodulators.

Step 5. Configure the Display for the number and orientation of measurement traces.

There are six measurement grids that can be configured and displayed. From the Display menu,select Layout to specify which of the 6 grids to be displayed.

The display tool bar can also be used to select the layoutas well as the "active" trace. The View/Overlay Trace item can be selected to lay traces on top ofeach other for direct comparison.


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Other aspects of the display like color and fonts can be set under Display Appearance. Thesesetups can then be saved under the File menu.

More about the display menu

Step 6. Choose the Desired Trace Data.

The real flexibility of the vector signal analyzer is realized here. There are many choices of data tobe analyzed, the most common of which would be frequency or time.

To select data, click Trace > Data and choose from the list, or click the data field on themeasurement grid and step through the choices.

More on trace data

Note

Other trace data types will be made available when demodulation is turned on.

Step 7. Choose a Trace Format

For any trace data type, a number of formats may be appropriate. For example, a log magnitudeformat is useful on frequency domain data. The analyzer allows virtually any format to be applied


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to any data.

Click Trace > Format or double-click the format field on the measurement grid and select from thepop-up menu.

More about trace formats

Step 8. Scale or Autoscale the trace

A good first step to scaling a trace is to Autoscale. Do this by clicking on the Trace menu and

choosing Autoscale. Another easy way is to right-click the measurement grid and selectAutoscale from the pop-up menu.

Specific scaling and scale units can be specified under Trace, and X-scale or Y-scale. Shownhere is the Y-scale dialog box.


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Note

At this point in the process, repeat steps 6, 7, and 8 until all traces to be usedare defined and scaled.

More about trace scaling

Step 9. Apply Marker features to yield measurement results

Measurements can be made directly from the scaled grids, but using markers will be more precise.Full access to marker capability is gained under the Marker menu.

A quicker way to start using a marker is to click the (marker tool) and then click the trace. Thiswill drop a marker on the trace and the mouse can be used to position the marker.


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More about markers


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Frequency Domain MeasurementsAcquiring a signal

Signal analysis on the 89600 is really data processing. Therefore, signal data can come from ahardware subsystem that has digitized the signal, or from a data file generated mathematically.

89600Software

Front endHardware

Design Modelingor

ComputationSoftware

To start the signal acquisition process:

1. First, load and run the Vector Signal Analyzer application and then Preset the analyzer byclicking File > Preset > Preset Setup.

2. Configure the display layout by clicking Display > Layout. Here you have a choice of viewinga single measurement grid or up to 6 grids at a time. Select Single. You should nowbe viewing a spectrum on trace A.

If you have a hardware front end, acquire signals following these steps:

1. Configure the Input menu to use Data From the Hardware.

2. From the MeasSetup menu, select the appropriate Frequency Band. On the Input menu orthe measurement grid, adjust the Range for the most sensitive setting without indication ofoverload.

3. From the MeasSetup menu or the measurement grid, adjust the Frequency parameters forCenter Frequency and Span, or click the parameter fields on the measurement grid and typethe values directly. For most measurements, choose a span that includes all significantmodulation sidebands. To scale the signal, click Trace and then Autoscale or choose Y Scaleto set specific scales and units from the pop-up menus. Right-clicking on the measurementgrid will also pop-up an Autoscale choice.

Scaling can also be performed directly on the measurement grid by clicking on the scaleparameters

To analyze a recorded file:

1. Click Input > Data From > Recording.

2. Click File > Recall > Recall Recording. The program will then prompt you for the location andname of the recorded file.

3. Once the recording has been loaded, playback can be started by pressing the key.

4. The can be pressed at any time to stop playback. Pressing it again will resume playback.

More about recording

Try recalling the recording named 50PCAM.DAT in the "C:\Programfiles\Agilent\89600 VSA\Help\Signals" directory.


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Frequency Domain MeasurementsMeasuring Frequency and Amplitude

Once a signal of interest has been established on the display, (the example here is the recordedsignal 50PCAM.DAT found in the "C:\Program files\Agilent\89600 VSA\Help\Signals" directory),

the marker can be used to measure frequency and amplitude. Try this procedure:1. To drop a marker onto the trace, click the marker tool.. In this mode the pointer becomes a

crosshair. Placing this crosshair on the trace and left clicking the mouse will drop a marker atthat position. To drive the marker to the peak of the signal, click Markers > Search > Peak.This tool can also be found by right-clicking on the measurement grid, or by pressing Page Upon the keyboard.

2. The marker frequency and amplitude results are displayed at the bottom of the measurementwindow.

Saving Instrument States

Once the analyzer has been set up, saving the instrument state can save time when returning tothis setup.

To do so, click File > Save > Save Setup.

The program will then prompt for a file name.


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To recall the setup click File > Recall > Recall Setup, and then choose the desired setup filename.

What else can be saved?


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Frequency Domain MeasurementsChannel Power/ Band power / adjacent channel power

Measuring the power of a modulated signal or "channel power," cannot be accomplished at onefrequency point. This measurement requires the integration of the frequency domain over a

specified bandwidth. The bandpower marker feature is used in this procedure.1. Acquire a modulated signal or load the recorded signal, 50PCAM.DAT found in the

"C:\Program files\Agilent\89600 VSA\Help\Signals" directory. View this signal in a frequencyspectrum.

2. Reduce the span around the signal until the modulation sidebands are clearly visible (2/3 ofthe frequency span).

3. To drop a bandpower marker onto the trace, click the tool. On the trace, move the mouseto the center frequency of the band to be measured and click to drop the marker. To expandthe bandpower marker to the desired bandwidth, place the mouse pointer on the verticalbandpower marker and click and drag/expand the bandpower marker to include the bandwidthof interest.

Note

The band power is now being displayed at the bottom of the window. This is the

total power inside the bandwidth of the bandpower marker.

4. For more precise control of the bandpower marker, open the Markers menu, and selectCalculation.

The Marker Calculation dialog box will allow you to precisely control the center and span or


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upper and lower limits of the marker. Here is the resulting measurement.

Note

Adjacent channel power can be measured by changing the bandpower markercenter frequency to that of an adjacent channel.

Another Plus!:Band Power markers can be applied to any rectangular display format and used tocalculate total area or the RMS of that format.

To turn the band power calculation off, click Markers > Calculate, and clear the Band Power boxin the pop-up menu.

More about band power markers


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Frequency Domain MeasurementsRelative Frequency and Amplitude

Relative frequencies and amplitudes or the differences between two points on a trace can bemade with the Offset Marker. To make a relative measurement:

1. Display a signal in the frequency domain.

2. Position a marker at the peak of the signal. Right-click the measurement grid, and from thepop-up menu, click Peak.

3. To establish a reference point at the location of the marker, right-click the measurement gridand select Move Offset to Mkr. The reference point is shown as a box at the location of themarker. Their difference (zero) is shown at the bottom of the window.

To make relative measurements, click and drag the marker to another point on the trace. The


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frequency and amplitude difference between the marker and the reference point are displayed atthe bottom of the display.

Note

The offset marker can be used in any domain or format.

More on the Offset Marker


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Frequency Domain MeasurementsImproving Resolution

VSA Facts:

The frequency domain is computed from the time domain via an FFT More time information yields better frequency resolution (effective resolution bandwidth or

RBW)

Reasons to improve resolution:

Resolve low amplitude signals that are close to larger signals

Reduce the noise floor (noise power is proportional RBW)

To force the VSA to acquire more time data:

Decrease the frequency span

Change the RBW Mode from 1-3-10 to Arbitrary

Increase the number of Frequency Points After any changes, click RBW on the display and step down. If more resolution is

available, RBW will decrease

Try improving the frequency resolution of the recorded signal 50PCAM.DAT found in the"C:\Program files\Agilent\89600 VSA\Help\Signals" directory.

First click MeasSetup from the tool bar and then ResBW. From here, you can increase theFrequency Points from the default of 801 points.

Now change the ResBW Mode to Arbitrary. Then select either RBW on the measurement gridand step down or fill in a low value into the ResBW field in the menu and let the system computethe lowest RBW value available.


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Shown here is the difference between a trace stored with a 3 kHz RBW (blue) versusa 372.984 Hz (green) resolution. Signal resolution and the noise floor have both been improved.

More about resolution bandwidth


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Frequency Domain MeasurementsSpectrogram Displays

Set up the spectrogram.

1. Preset the analyzer. (Click File > Preset > Preset All.)

2. Set up the display.

a. Click in Trace A, then click Trace > Spectrogram.

b. Select the Show Spectrogram check box and click Close.

c. Click in Trace B, then click Trace > Data > Spectrum.

3. Recall a recording.

a. Click File > Recall > Recall Recording.

b. Browse to the directory "C:\Program files\Agilent\89600 VSA\Help\Signals".

c. Select Bstiming.dat and click Open.

4. Click the Restart button, .

5. When the spectrogram display is full, auto scale the Y axis for both traces.

a. Pause the measurement when the signal is at its maximum. (Click the pause button, )

b. Right-click in each trace and click Y Auto Scale.


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Notice that every fifth pulse is shorter than the others. This is because the recording does notinclude an integer number of pulses. The recording begins in the middle of a pulse.

Spectrogram markers

1. Click in Trace A, then click Markers > Spectrogram.

2. Select the Trace Select check box.

3. Notice the number displayed in the Trace box. This is the last (bottom) trace in thespectrogram display. Click in the Trace box, and use the down arrow key or the mouse wheelto change the number. Notice that the trace select marker moves up in the spectrogram, andtrace B updates.

Raising the threshold to eliminate noise

The pulsed signal currently displayed (Bstiming.dat) is good for demonstrating the spectrogramthreshold feature. By raising the threshold, you can eliminate noise from the display.

1. Click in trace A, then click Trace > Spectrogram.

2. Click in the Threshold box and type 50. Click OK.

Notice that the noise is significantly reduced. Any part of the signal below 50% of full scale hasbeen removed from the display.


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Time Domain MeasurementsSignal Acquisition and Triggering

To start the signal acquisition process we will view both the frequency & time domains:

1. First, preset the analyzer and configure the display layout by clicking Display > Layout. Hereyou have a choice of viewing a single measurement grid or up to 6 grids at a time. SelectStack 2. Next, view the frequency domain on trace A by clicking on the measurement grid or

clicking , the Trace A button. Then click the Trace > Data > Spectrum. Set-up trace A'sformat by choosing Trace > Format > Log Mag. These parameters can also be adjusteddirectly on the display grid by double-clicking on these fields and then choosing from thepull-down menu.

2. To set up trace B for the time domain, click or click the B measurement grid. Then clickTrace > Data > Main Time. Set-up trace B's format by choosing Trace, Format, and Real.

If you have a hardware front end, acquire signals following these steps:

1. Configure the Input menu to use Data From the Hardware.

2. From the MeasSetup menu, select the appropriate Frequency Band (0 - 36 MHz,or 36 MHz - 2.7 GHz).

3. From the MeasSetup menu or the measurement grid, adjust the Frequency parameters forCenter Frequency and Span. For most measurements, choose a span that includes allsignificant modulation sidebands.:

Note

For baseband signals do not "span in" on the signal (Zoom Mode). Insteadleave the start frequency at zero Hertz and reduce the stop frequency asnecessary (Baseband Mode).

4. To scale the signal, click Trace and then Autoscale or choose Y Scale or X Scale to setspecific scales and units from the pop-up menus.Scaling can also be performed directly on the measurement grid by clicking on the scaleparameters.

5. For best sensitivity, go to the Input menu or the Range field on the measurement grid, andadjust the Range for the lowest setting without indication of overload.

To analyze a recorded file:

1. Click Input > Data From > Recording.

2. Click File > Recall > Recall Recording. The program prompts you for the location and name

of the recorded file. Once the recording has been loaded, playback can be started by clickingthe key.

The can be clicked at any time to stop playback. Clicking it again will resume playback.

More about recording signals

Example:

1. Load the signal bstqpsk.dat from the "C:\Program Files\Agilent\89600 VSA\Help\Signals"directory.

2. Set up the display using the process shown above.

3. Start the recording playback by clicking .

4. Autoscale the traces.


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The display should look something like this, with the frequency domain on trace A and the timedomain on trace B.

As the signal plays back, the modulated RF burst will scroll by making analysis difficult. You can

stabilize the playback by clicking the pause key or by applying a trigger.

Triggering

Time-variant signals like RF bursts will generally require stabilization via triggering for reliableanalysis results.

Triggering controls for both live signals and recorded signals are found by clicking the Input menuand then Trigger This will bring up the Input Properties window.

The Playback Trigger

On the Input Properties window, select the Playback Trigger tab. Change the trigger Type fromFreeRun to Magnitude and set the Mag Level to 500 mV. To view the beginning of the burst, seta Delay of -1 mSec (pretrigger).

Trigger hold-off will prevent retriggering on complex signals like this one. Set Hold-off to 2 mSec.


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Shown here is the triggered burst.


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Triggering On Live Signals

The trigger modes found under the Trigger tab of the input properties menu are much like those ofan oscilloscope. Triggering can be from the measurement channels as well as an external triggerinput. The IF Mag type trigger is unique because it is both a level and frequency qualified trigger.All triggers have adjustable level, hold-off, and pre and post delay.

More on Triggering


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Time Domain MeasurementsPower Envelope

Building upon the example loaded under triggering, let's measure the peak and average power ofthe RF burst.

To measure peak power:

1. View the signal in the time domain by clicking Trace, Data, and Main Time.

2. A useful format for viewing the power of a RF burst is log magnitude. Click Trace > Format >Log Mag or double-click the format field of the Y-axis in the measurement grid and select LogMag from the pull-down menu.

3. To change the units of the Y-scale to dBm, click Trace > Y-scale. From the pop-up menu clickY-Unit > Power.

4. Stabilize the signal using triggering, click Input > Trigger.. > Playback Trigger, and set thetrigger parameters found in the pop-up window.

Note

To clearly see the beginning of the burst, use pretriggering by entering anegative value for trigger Delay. Find the peak power with a marker by

right-clicking on the measurement grid, and selecting Peak from the pop-upmenu.


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6. Read the peak carrier value from the marker at the bottom of the display.

Shown here is the recorded signal bstqpsk.dat in the time domain displayed in a log magnitudeformat. This is a useful way to see the power envelope of a signal. The marker can be used tomeasure power or voltage at an instant in time. The time axis is relative to the beginning of theacquisition record or the trigger point.

Average Power in the Time Domain

Average power of the burst can be calculated in the time domain by the band power marker


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feature. There are several ways to turn the band power marker on. Here is one simple way:

1. Click the area tool

2. Left-click and drag over the portion of the time domain trace that you would like averaged.

3. Select Band Power Marker from the pop-up window. Band power markers are then turned onand the resulting average over time is shown at the bottom of the display.


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Time Domain MeasurementsTiming

The 89600 Vector Signal Analyzer actually acquires its data from the time domain so it is naturallysuited to make timing measurements like burst pulse width and period.

To try these measurements, let's build upon the example loaded under triggering, or use your ownlive RF burst signal.

To measure pulse timing parameters:

1. Acquire and trigger the signal. The Log magnitude format is helpful when viewing an RF burstenvelope.

2. Adjust trigger Delay in order to view the beginning of the burst to be measured.

3. If a longer time record is required to see the whole burst try any or a combination of:

a. Increasing the amount of data acquired by increasing the number of Frequency Points.

b. Setting the ResBWMode to arbitrary

c. Reducing the Span.

4. Click the marker tool and click the trace to drop a marker.

5. Position this marker at a reference point on the trace.

Note

The time value displayed at the bottom of the display for this marker is relative

to the trigger event. This is useful for measuring RF performance relative toother events in the device under test used to externally trigger the analyzer.

6. To make relative timing measurements between two points on the waveform, right-click thegrid again and select Show Offset. A second box-shaped marker (the offset marker) willappear on the trace . The measurements shown on the bottom of the display will be thedifference between the reference marker and the offset marker.

This is a burst width measurement.


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Period, rise, and fall measurements can all be made as easily.


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Time Domain MeasurementsComplex or I/Q

We know from the Vector Signal Analyzer's theory of operation, the system digitizes the signal andconverts it to two arrays of information; I the in phase or "real" array, and Q the quadrature or

"imaginary" array. It is there for the analyzer to easily display the signal as real, imaginary orcomplex.

Shown here is the recorded signal qpsk.dat with a DisplayLayout of 2 grids stacked. Trace Datafor both grids is Time. Grid A is a Trace Format of Real and grid B has a Trace Format of Re-Im(I-Q). This complex view allows you to see the whole signal (magnitude and phase) versus time.

Notice that the four distinct magnitude and phase states of the QPSK (quadrature phase shiftkeying) signal can be easily seen. The phase shift can be seen in this display because thedemodulation process of carrier and symbol clock recovery has not yet occurred. These processeswill be discussed in the digital demodulator section of this tutorial.


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Time Domain MeasurementsGating

Gating is an 89600 feature that allows you to select a time-qualified portion of the signal to beprocessed further by the analyzer's signal processor. Shown here is the recorded signal

Gate2bursts.dat. This signal has two TDMA (time division multiple access) bursts. Both bursts areQPSK modulated at 50 kilo-symbols per second. The first burst is modulated with a random bitstream with an equalization sequence in the middle. The second burst is 10 dB lower that the firstand modulated with a bit pattern of 8 one's and 8 zero's. Trace B shows these two successivebursts.

Trace A is the FFT of trace B showing the composite of both bursts in the frequency domain. Toanalyze each burst separately, a gate can be applied.

Setup details: Span 78.125 kHz, RBW; coupling fixed, mode arbitrary, 801 frequency points,recording trigger; magnitude 400mv, delay -1ms. Trace B is log format.

Time Gating

To set up the time gate, Click MeasSetup > Time. From the pop-up menu, select the time gatebox to turn the gate on. Vertical bar markers will appear on the time trace. These markers definethe right and left edges of the time gate. The markers can be positioned by filling in the left and

right reference fields of the menu or click the gate marker tool and drag and drop the markersdirectly on the trace display.


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Once the gate has been positioned over the second burst, the spectrum of that burst will be seenin trace A as shown below. This spectrum clearly shows the non-random nature of the modulationof the second burst.


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NoteWhen the gate is on, only the time information inside the gate is allowed to passon to the other signal processing functions of the analyzer such asdemodulation. The 89600 gives you 2 levels of signal time qualification,triggering and gating.

More about gating

Be aware that when gating the time domain and viewing the results in the frequency domain, theresolution of the frequency spectrum will be a function of the length of the time gate. The shorterthe gate length, the poorer the frequency resolution.

Here is an example of setting a 320 microsecond gate over the equalization sequence of the firstburst. This short gate yields limited frequency resolution in the spectrum of trace A.


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More about resolution


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Time Domain MeasurementsRecording Signals

Why Record Signals?

No gaps! Continuous time record at full bandwidth. Long records. Up to 192 megasamples (with option 001 for E1439 or E1438 module).

Powerful post processing. More control over the analysis.

Analyze in the frequency, time, analog or digital demodulation domains.

Slow playback with overlap processing.

Porting of simulations back to design software.

Archiving. Save signal records for future analysis.

To record a signal:

1. Acquire the signal in the normal fashion. See Acquiring a signal.Be sure that the Span chosen will contain the signal characteristics of interest.

Choose the most sensitive Range without the possibility of overload.2. Use triggering to ensure when the recording will start.

Consider pretriggering (negative trigger delay) to start recording before the event of interestoccurs.

3. Set up recording length. Click Input > Recording > Length.This can be specified in points, records, or seconds.

4. Click Control > Record or click the record button.

5. When a qualified trigger occurs the recording will begin.Recording is immediate when no triggering is specified.

How Much can be Recorded?

Recording time is a function of the memory size of the ADC module (E1438 or E1439) and theeffective sample rate set by your choice of span.

Here are some examples:

89600System

Memorysize

Number ofsamples

Recording timeat Full BW

Recording timeat 40 kHz BW


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Standard 89600 18 M-byte 12,582,912 0.125 sec 120 seconds

89600 opt 288 288 M-byte 201,326,592 1.92 sec 32 minutes

89600 opt 001 1.2 G-byte 805,306,368 7.68 sec 128 minutes

PlaybackRecording playback can be started by clicking the key or Control > Restart.

Or

Click Control > Player.

The player lets you:

Start and pause the playback

Drag the bar to any position in the record

Back up and rewind

Loop the recording

Set start and stop times

Rerecord

Overlap Processing

Recording playback requires that the recording buffer be broken into time records for processingand display. Shown in blue below, is the normal way to do that. This method results in the fastestplayback speed.

Overlap processing advances through the recording buffer by "sliding" the time record windowthrough the buffer. You controls the rate at which data is replayed (0% to 99.99%). The result is aslower playback. Data lost by the FFT windowing process is now analyzed as well.

Overlap can be set by clicking MeasSetup > Time.

There are two fields; one for when averaging is off and another for when averaging is on.


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The higher the overlap percentage, the slower the playback.

Shown here is the default state.

More about recording signals


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Analog Demodulation MeasurementsAnalog Demodulator Theory of Operation

What is the Analog Demodulator?

The analog demodulator performs processing on the digitized time record of the signal andrecovers the signal's amplitude, frequency, and phase characteristics. The analyzer then makesthe demodulated results available for viewing in a number of useful ways.

Why Demodulate?

To analyze intentional modulations like FSK (frequency shift keying).

To analyze unintentional modulations like phase noise or AM to PM conversion.

Single-shot signal parameters like frequency or phase settling or pulse shaping.

Demodulating a signal might seem complicated, but because of the basic nature of a vector signalanalyzer, it is very straightforward.

The heart of the VSA is the A to D converter (ADC). The digitized signal is then split and multiplied

by sine and cosine. The results are a real and an imaginary array of numbers that represent thesignal's complex characteristics versus time. That signal and its amplitude, frequency and phasemodulations can be expressed by the equation for V below.

Solving for A (amplitude modulation) and (phase modulation) is easy and the solution for

(Frequency modulation) is just the derivative of the phase.More about the analog demodulator


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Analog Demodulation MeasurementsSetting up the Analog demodulator

1. Acquire the signal. See Acquiring a signal.

2. Choose a span that includes the modulation sidebands of interest.

3. For burst signals, apply triggering.

4. Turn the demodulator on by clicking MeasSetup > Demodulator > Analog Demod.

Note

The display will not change until the demodulator has been set up.

5. Set up the demodulator by clicking MeasSetup > Analog Demod and select the AM, FM, orPM check boxes. This will make the demodulated results available.

6. View demodulated results by clicking Trace > Data > Ch 1 Demod, and select from thepop-up menu, or double-click the "Trace Data" field on the measurement grid.

7. Typical choices would be Main Time, or Spectrum to view the recovered modulation waveformor its spectrum.

8. Format and scale the trace as before.

Analog Demodulation Measurement Tutorials

AM Modulation Measurement Tutorial

FM Modulation Measurement Tutorial

Phase Noise Modulation Measurement Tutorial

AM to PM Characteristics Measurement Tutorial

See Also

Analog Demodulation Theory of Operation


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Analog Demodulation MeasurementsAM Modulated Signal Measurement Example:

Acquiring the signal

Recall the recording named 50PCAM.DAT in the "C:\Program

files\Agilent\89600 VSA\Help\Signals" directory as was done in Acquiring a signal.

Using 89600 default setup, this signal, 50% amplitude modulated by a 25 kHz sinewave, shouldlook like this:

Shown above is the amplitude modulated spectrum in Trace A including carrier, upper and lowersidebands and distortion sidebands. Trace B shows the modulated RF envelope versus time

(unscaled) varying at a 25 kHz rate.

Turning the Demodulator on

To select and activate the demodulator, click MeasSetup > Demodulator > Analog Demod.

Nothing new will happen until we set up the demodulator.

Setting Up the Demodulator

Click MeasSetup again. Click Demod Properties to open the Analog Demo Properties dialogbox, from which you can select AM, FM, or PM demodulation. Select AM.


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View Demodulated Results

Now that the AM demodulator is running, let's view the results.

For Trace A, double-click the Trace Data field of measurement Grid A, and in the pop-up windowclick Ch1 Demod > Spectrum.

For Trace B, Select trace data of Ch1 Demod, and Main Time.

Format and Scale the Demodulated Results

The result of the AM demodulation is shown below.Trace A is the spectrum of the recovered 25kHz modulation tone (note the marker). The other toneis a distortion harmonic created by the modulation process.

Trace B is the recovered 25 kHz modulation waveform, but it is hard to recognize as a sinewave inthis time span.


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Changing the Time Span

The time span can be changed by using the inverse relationship between time and frequency. Inthis relationship, frequency resolution can be traded off for a shorter time span.

Click MeasSetup > ResBW to change the resolution bandwidth from 1 kHz to 3 kHz.

The frequency resolution of Trace A will be reduced and therefore requiring a shorter time recordto produce that spectrum. Here is the result.


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The spectrum now has a lower resolution but the corresponding shorter time record shows therecovered modulating sine wave. The offset marker shows the period to be about 40microseconds or a frequency of 25 kHz.


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Analog Demodulation MeasurementsFM Modulated Signal Example

Frequency control of oscillators is a key parameter in all RF systems. Applying analog FMdemodulation is an easy way to verify dynamic oscillator behavior.

FM demodulation applications:

Peak deviation of FM or MSK transmitters

Frequency settling of synthesizers

Frequency pulling in burst transmitters

Frequency agility of hopping oscillators

Acquiring the signal

Recall the recorded signal named "xmitter.dat" in the "C:\Programfiles\Agilent\89600 VSA\Help\Signals" directory.

Shown here is the "xmitter.dat" file that is a recording of a FM transmitter turning on. The recordingwas allowed to play and then paused when the carrier appeared.

Trace A shows the carrier settling from frequencies above the intended center frequency.

Trace B shows the carrier building in amplitude and changing frequency during the turn-on process.

Using the process found under Setting up the analog demodulator ,

1. Set up for FM demodulation

2. Set up Trace C for Trace, Data, CH1 Demod, Spectrum and Autoscale

3. Set up Trace D for Trace, Data, CH1 Demod, Main time and Autoscale

4. View traces C and D

Here is the result:

Trace D displays frequency versus time. Seen from left to right is the demodulation of noise(before the carrier is on), carrier on and starting to lock, settling, and finally settled. The markermeasures 4.83 kHz above the center frequency at 10.4 mSec from the trigger point of therecording.

Trace C is the FFT of trace D with resonances showing as deterministic spectra. The marker

at 4.8 kHz indicates the bandwidth of the PLL (phase locked loop) of the transmitter.


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Peak deviation and settling time can easily be measured with a marker.

Note

Phase deviation and settling is just as easily determined by selecting PM for thedemodulation mode.

Now that all four traces are set up, the vector domain and the analog demodulation domain can beshown simultaneously by clicking Display > Layout > Grid 2x2.

More about FM Demodulation


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Analog Demodulation MeasurementsPhase Noise Measurement Example

Phase noise measurements are popular for the evaluation of oscillator stability.

The 89600 offers several ways to measure phase noise. Described here are two methods:

Direct measurement

Phase demodulation

Direct Phase Noise Measurement

The examples here use the recorded signal; "C:\Programfiles\Agilent\89600 VSA\Help\Signals\sinewpn.dat".

1. Load and play the recording or use a live sine wave (see Acquiring a signal.)

2. Set up Trace A for a spectrum measurement with a span greater than 20 kHz.

3. Turn on averaging by clicking Measure > Average > Average Type > RMS Video

Exponential.4. Set Count to 100.

5. Place a marker on the peak of the carrier.

6. Set up the band power marker 1 kHz wide, 10 kHz a Way from the carrier and set the bandpower marker to Calculate: C/No as shown in the picture below.

7. Read the noise side band level at the bottom of the display as C/No. This is the single sideband noise power normalized to 1 Hz at a 10 kHz offset from the carrier.


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The problem with this measurement is that the power in the side band might not be just phasenoise. AM noise side bands and spurious signals can obscure the true level of phase noise.

The Phase Demodulation Method

Continuing with the same signal and set up from the direct measurement method:

1. Turn the analog demodulator on. Click MeasSetup > Demodulator > Analog Demod.

2. Set up for phase demodulation. Click MeasSetup > Analog Demod > PM.

3. View the power spectral density of the recovered modulation. Click Trace > Data > Ch1Demod > PSD.

4. Set up the Y-scale. Click Trace > Y-scale > Y-Unit > RMS as shown in the picture below.


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5. Move the marker to 10 kHz. The marker is reading a double-side band level for phase noise.To compensate, subtract 3 dB from the reading. This is the single sideband phase noise levelat a 10 kHz offset from the carrier.

The 89600's phase noise performance is shown here.

Phase Noise,BB Opt.

withdown

converter

withoutdown

converter


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dc < Fin < 39 MHz100 Hz Offset:1 kHz Offset:> 10 kHz Offset:

-97 dBc/Hz

-122 dBc/Hz

-137 dBc/Hz

Phase Noise, RF Opt.

36 MHz < Fin < 1 GHz> 20 kHz Offset:> 100 kHz Offset:

< -97 dBc/Hz

< -115 dBc/Hz

1 GHz < Fin < 2.7 GHz> 20 kHz Offset:> 100 kHz Offset:

< -97 dBc/Hz

< -115 dBc/Hz

More about PM Demodulation


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Analog Demodulation MeasurementsAM to PM Characteristic Measurement Example

In modern I/Q modulated systems, the phase of the carrier must be stable for information to beencoded upon it. Many of these same systems use RF burst techniques in order to increase

system capacity. When these rapid amplitude changes occur, power supplies are stressed,oscillators are pulled and reactive components are stimulated inducing unwanted phase shifts inthe carrier. This AM to PM characteristic must be understood for the system to perform well.Shown below is an example of a carrier signal, Amplitude Modulated by a square wave. (Recordedsignal "C:\Program files\Agilent\89600 VSA\Help\Signals\ampmsqr.dat".).

Trace A shows the RF AM envelop versus time.

Trace B shows the results of an analog PM demodulation. This is unintended phase modulationdue to the amplitude modulation process.

During the rising and falling edges of the burst, meaningful phase modulation must be delayeduntil the carrier has settled.

Shown below is the rising edge of one of the bursts. This was scaled by:

1. Clicking the area tool

2. Dragging a box around a rising edge

3. Scaling in the X-axis.


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Digital Demodulation AnalysisDigital Demodulator Theory of Operation

The digital demodulator is a powerful tool for viewing and measuring the quality of I/Q or "digitally"modulated signals. This is a general-purpose demodulator that needs a minimum of prior

information about the signal and therefore can be used on a wide variety of modulations. Thedigital demodulator works by:

1. Starting with the digitized signal in I/Q form:

2. Recovering or "locking" to the carrier.

3. Recovering the symbol clock

4. Applying a reconstruction filter.

5. Detecting the symbols.

The demodulator also models or "predicts" what an ideal signal would have looked like andcompares this ideal result with the measured result. The differences between the measured andmodeled results are displayed as modulation errors.

To learn more about the theory of operation, see the next topic: Digital Demodulator Block Diagram


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Digital Demodulation AnalysisDigital Demodulator Block Diagram

Shown here is the processing diagram of the digital demodulator.

Click the blocks to find out what they do.

Narration


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Digital Demodulation AnalysisQPSK Digital Demodulation Measurement Example

Preset the analyzer, load the signal qpsk.dat from the "C:\Programfiles\Agilent\89600 VSA\Help\Signals" directory, and start playback. This signal is QPSKmodulated at 50 ksymbols/sec, root raised cosine filtered with an alpha of .35. All measurementson the VSA should begin with the same first 3 steps:

1. Set center frequency and span for signal under test

2. Set the center frequency and span properly. Center frequency does not need to be exact(better than a few percent) because the VSA will make the final adjustments automatically.The span must include all of the significant modulation side bands. You should always viewthe signal in the vector mode to be sure that the signal is present. Set input range as low aspossible without overload

3. Range the signal for best sensitivity. This will ensure the best signal-to-noise ratio fordemodulation.

4. Set up triggering (if required)

a. Set up a trigger if you are looking for a specific burst or transmission relative to a systemtrigger. Triggering is not required to randomly demodulate blocks of data.

b. Once the signal is clearly viewed in the vector mode, then proceed to the demodulationmode.

5. Select digital demodulation mode

Turn the demodulator on under the MeasSetup menu.

6. Specify correct modulation format

The demodulator set up menu is found under MeasSetup and Demod Properties. Here themodulation format (such as FSK, PSK or QAM) is selected.

For our example, select QPSK.


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7. Specify correct symbol rate

a. The intended symbol rate must also be entered. Be careful not to confuse bit rate forsymbol rate. The symbol rate is the bit rate divided by the number of bits per symbol. Inthe QPSK examples shown here the four modulation phase states represent 2 bits ofinformation each so the QPSK symbol rate will be the bit rate /2.

b. The symbol rate must be set precisely. Errors of a few percent will be displayed assignificant modulation error.

c. The result length field specifies how many symbols will be demodulated and displayed intrace. If the signal is a burst, the result length should be the number of symbols in a burst

or less, otherwise, the demodulator will try to demodulate noise in the absence of thesignal.

8. Select result length and points/symbol

a. The result length determines how many symbols will be demodulated and displayed.

b. The points-per-symbol parameter sets the effective sample rate.

9. Select filter shapes and alpha (measured and reference)

a. The step is to specify the filtering to be used by the demodulator to receive the signal. Themeasured filter is the same type of filter used by a receiver. The reference generator in thedemodulator uses the reference filter to predict ideal signal behavior. This filter representsthe total filtering of both the transmitter and receiver combined.

b. The filter alpha or BT (time bandwidth product) must also be specified. These parametersexpress the bandwidth of the filter relative to the symbol rate of the modulation


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10. Turn on Pulse Search and set search length (burst signals only)

a. Turn this feature off for continuously modulated signals.

b. Pulse search and sync search are features that work like triggering for IQ modulated andburst signals. Burst signals can be stabilized using the triggering features of the 89600 andthis will be the best way to synchronize data acquisition to external trigger signals. Pulsesearch is an easy way to automatically detect and demodulate burst signals without settingup a trigger.

c. Sync search will search the I/Q modulation for a user specified bit pattern and display thedemodulated results relative to this sync pattern.

d. Pulse search and Sync search can be used together for more powerful qualification of thesignal.


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11. Checks: shape of eye/constellation diagrams, EVM

To confirm proper operation of the demodulator, view the signal as a vector diagram orerror vector magnitude. A stable vector diagram will build confidence that the demodulator isset up and functioning correctly and matches the incoming signal.

More on the digital demodulator


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Digital Demodulation AnalysisEDGE Digital Demodulation Measurement Example

This example shows you how to measure and analyze an EDGE (Enhanced Data rates for GSMEvolution) signal. We will be using a recorded EDGE signal with these characteristics; 5 MHz carrier

frequency, 0 dBm amplitude, and framed (pulsed) data format.

1. Initialize the analyzer: Click File > Preset > Preset Setup

2. Load and play back the recorded EDGE signal (file name Edge_5MHz.dat):

a. Click File > Recall > Recall Recording, type the file name [C:\Programfiles\Agilent\89600 VSA\Help\Signals\ Edge_5MHz.dat], click Open.

b. Start playback: clickControl > Restart

3. Set the center frequency and span:

a. Click MeasSetup > Frequency > Frequency tab, type 5 MHz in the Center text box.

b. Click RecordingSpan to set the span to the 625 kHz.

c. Click Close.

Tip

If you're measuring an unknown signal the following guidelines will help set thecenter frequency and span properly.

You should always view the signal in the vector mode to be sure that the signal ispresent.

Center frequency does not need to be exact (better than a few percent) becausethe VSA will make the final adjustments automatically.

The span must include all of the significant modulation side bands. Set the analyzer todigital demodulation mode:

d. Click MeasSetup > Demodulator > Digital Demod

More on the digital demodulator


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4. Set input range as low as possible without overload:

If you were measuring a real or simulated signal you would need to set the range, however,this step is not required with a recorded signal. The analyzer will set its range to the recordedrange value (see Selecting the Optimum Range).

5. Configure the analyzer to demodulate the EDGE signal:

We will use the digital demod Standard Preset feature to auto-configure the analyzer. TheEDGE standard setup automatically sets the demodulation format, frequency span, symbolrate, filtering, and several other demodulation parameters (see the EDGE Standard Setuptable for a complete list).

a. Click MeasSetup > Demod Properties > Preset to Standard.

b. Select Cellular > EDGE > Close.


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6. Set the display layout to show these four trace views; the IQ Vector Diagram (trace A), theSpectrum data (trace B), the Error-Vector Magnitude data (trace C) and the Symbol Table(trace D):

Click the down arrow ( ) on the right side of the Active Trace toolbar (short cut for Display,Layout) and click Grid 2x2.


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7. Let's compare the measured signal shown in trace A to an ideal signal in trace B.

a. Make trace B the active trace: Click trace B.

b. Display an ideal signal in trace B:

Click Trace > Data > IQ Ref Time.

8. Now we will investigate the effects ISI (Inter-Symbol Interference).


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Symbol locations on the IQ vector and constellation diagrams will appear random due to the

combined effects of the 3/8 symbol rotation interval and significant ISI (Inter-SymbolInterference) introduced by the EDGE transmit and measurement filters. However there's goodnews, at the 1-point/symbol setting (the value set by the EDGE standard setup), the analyzerremoves the effects of ISI so you can view a clean vector diagram. Above one point/symbol,the analyzer does not remove the effects of ISI

Change the points/symbol from 1 to 2 points/symbol and compare the difference.

a. Click MeasSetup > DigitalDemod > Format tab.

b. Type 2 in the Points/Symbol text box, click close.

c. After viewing the difference, reset the points/symbol to 1 points/symbol.

9. Let's use Sync Search and Search Pattern to correlate measured data to a known EDGE TSC(Training Sequence Code):

The TSC (Training Sequence Code) synchronizes the measurement to the timing of thedemodulated training sequence in the EDGE burst. This does require the EDGE burst to havea valid TSC, which is true for our recorded EDGE signal. Eight standard EDGE TrainingSequence Codes (0 through 7) have been included as Search Pattern optional selections. Wewill use TSC 0 "(EDGE)TSC0" in the following example.

a. Configure the display layout to show the IQ Vector diagram (trace A) in the upper grid andthe Symbol/Error Table (trace B) in the lower grid:

1. Click the Active Trace Toolbar Display Layout menu and click Stacked 2.

2. Click the Active Trace Toolbar Trace A button and then click Trace > Data > IQ Meas Time.

3. Click the Active Trace Toolbar Trace B button and then click Trace > Data > Sym/Errs.

b. Click MeasSetup,DigitalDemod > Search tab.

c. Select the Pulse Search and the Sync Search check boxes.

Note

d. Pulse search and sync search are features that work like triggering for IQ modulated andburst signals. Click Search Pattern > (EDGE)TSC0.


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e. We will set up the markers to easily measure individual symbol data:

1. Make trace B the active trace: Click the trace B grid.

2. Click Markers > Position Tab

3. Select the Marker and Couple Marker check boxes

4. Click Close.Notice that the search pattern is highlighted in the symbol table. You can watch the trace A andtrace B markers move from symbol to symbol by pressing the left and right keyboard arrowkeys.


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Analysis Tools

Once the digital demodulator is set up and running, these analysis tools are available for digitaldemodulation analysis (These data results use the QPSK data file, see QPSK DemodulationMeasurement example):

Vector Diagram

Constellation Diagram

Eye Diagram

I and Q Versus Time

Demodulated Spectrum

Error Vector Magnitude Versus Time

Magnitude and Phase Error

Error Vector Spectrum

Symbol Table/Error Summary

Adaptive Equalizer Results: Impulse Response and Channel Response


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Digital Demodulation AnalysisAnalysis Tools: Vector Diagram

A good place to start viewing demodulation results is the vector diagram. This tool shows thecomplex recovered signal at all moments in time with the symbol clock marked with red dots. The

practiced viewing of the vector diagram can see proper modulation format, filtering and otherqualitative aspects of the signal.

Note

For this topic, use the signal and configure the analyzer as shown earlier inSetting Up the Digital Demodulator.

To view the vector diagram, click Trace > Data > IQ Meas Time and then Trace > Format > I-Q.or double-click directly on the "trace data" and "trace format" fields of the measurement grid anduse the pop-up menus to select IQ Meas Time and I-Q format. Shown here is an ideal QPSKmodulated carrier plotted onto the vector diagram.


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Digital Demodulation AnalysisAnalysis Tools: Constellation Diagram

The constellation diagram shows the carrier's magnitude and phase synchronous with the symbolclock. This view gives some insight as to what a typical receiver will "see" and need to deal with in

order to demodulate the signal. Modulation errors will appear as deviations of the red dots from theideal targets marked by circles (default) or crosses (the illustration below uses crosses). Typicalmodulation errors will be shown in upcoming topics in this tutorial.

Note


To view the constellation diagram, click Trace > Data > IQ Meas Time and then Trace > Format> Constellation.


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Digital Demodulation AnalysisAnalysis Tools: Eye Diagrams

Eye diagrams are a traditional tool observed by connecting an oscilloscope to the I/Q test points ofa digital radio. Shown are the I and Q voltages versus time. These voltages are retraced every few

symbols to show in a statistical way, the repeatability of the signal. The opening of the "eye" shapein the trace is a qualitative indication of noise on the signal.

Note


To view the eye diagram, click Trace > Data > IQ Meas Time, and then Trace > Format > I-eyeor Q-eye.


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Digital Demodulation AnalysisAnalysis Tools: I and Q Versus Time

The recovered I and Q voltages can also be viewed versus time by clicking Trace > Format and Ior Q. As in all of the time formats, the symbol clock is marked by bars (default) or dots. The

following illustration uses dots (see note after illustration).

Note


NoteThe red dots or vertical bars shown on demodulated time traces mark therecovered symbol-clock timing. This feature can be controlled by clicking Trace> Format and the Digital Demod tab of the pop-up menu.

More on IQ versus time


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Digital Demodulation AnalysisAnalysis Tools: Demodulated Spectrum

As seen in vector analysis of signals before, the FFT of the time record will produce a spectrum.When the demodulated time record is transformed, the result is a baseband spectrum of the

recovered signal.

Note


This demodulated spectrum was produced by clicking Trace > Data > IQ Meas Spectrum andthen Trace > Format > Log Mag.

More on IQ spectrum


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Digital Demodulation AnalysisAnalysis Tools: Error Vector Magnitude Versus Time

The formats seen to this point have great qualitative merit, but a major feature of the demodulatoris its ability to measure a real signal and then to model an ideal reference signal from it. The

difference between the measured signal and the modeled reference produces a number ofquantitative formats. Among the most useful is EVM (error vector magnitude). Here modulationdeviation from ideal is expressed as percent error that makes EVM an easy to evaluate modulationaccuracy.

Note


To view EVM click Trace > Data > Error Vector Time. As with all data in the 89600, a number of

formats can be used.

This plot shows EVM in a linear format with percent error on the vertical axis versus 26 symbols oftime in the horizontal axis (marked with red dots). It is easy to observe the generally low EVM of anaccurately modulated signal.

More on error vector versus time


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Digital Demodulation AnalysisAnalysis Tools: Magnitude and Phase Error

When modulation errors are observed from the vector, constellation, or EVM diagrams, the nextstep might be to determine what the nature of these errors are. EVM can be broken down into its

magnitude and phase components.

Note


To view these errors click Trace > Data > IQ Mag Error or IQ Phase error.

More on magnitude and phase error


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Digital Demodulation AnalysisAnalysis Tools: Error Vector Spectrum

As seen before, the FFT of a time trace will produce a spectrum. The same can be done with theEVM time trace. The result is the error vector spectrum. This format can reveal the spectral

content of the unwanted signals that drive the modulated carrier a Way from its ideal path. If thoseerror components are deterministic then they will show up in the EV spectrum as spectra.Measuring these spectra can give added insight into the nature and origin of these error signals.

Note


To view Error Vector Spectrum, click Trace > Data > Error Vector Spec.

More on error vector spectrum


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Digital Demodulation AnalysisAnalysis Tools: Symbol Table/Error Summary

This result can be the most powerful of the digital demodulation tools. Here, demodulated bits canbe seen along with error statistics for all of the demodulated symbols. Modulation accuracy can be

quickly assessed by reviewing the rms EVM value. Other valuable errors are also reported as seenin the image below.

Note


To view the symbol table click Trace > Data > Syms/Errs.

More on Symbol Table


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Digital Demodulation AnalysisTroubleshooting: Quadrature Error

Quadrature error is due to the I and Q channels of a transmitter not operating precisely at 90degrees to each other. Shown here is the recorded signal, "C:\Program

files\Agilent\89600 VSA\Help\Signals\ Qpskquad.dat". The Error table QuadErr parameter showsthat the signal has a 5-degree quadrature error. Also notice that the constellation diagram shows aparallelogram shape instead of an ideal square shape.

Note

For this signal, use the same demodulator setup as used in the previous topic:format; QPSK, symbol rate; 50 ksymbols/sec, meas. filter; root-raised cosine,ref filter; raised cosine, alpha; 0.35.


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Digital Demodulation AnalysisTroubleshooting: IQ Offset

This example uses the signal "C:\Program files\Agilent\89600 VSA\Help\Signals\Qpskioff.dat". Inthis case everything looks good on the Vector diagram.

Note


However, during the demodulation process, certain errors like carrier frequency error and IQ offset(also known as IQ origin offset) were measured and removed. These errors are reported in theerror summary/symbol table.

Click Trace > Data > Syms/Errs to view this result.

Observe the IQ offset error at -22 dB compared to the un-impaired signal seen before (-60 dB orgreater).

The measurement of IQ offset indicates the magnitude of carrier feedthrough. This is a keyindicator of a properly balanced modulator.


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Digital Demodulation AnalysisTroubleshooting: Symbol Rate Errors

Small deviations in the symbol clock can show up as significant errors in the modulation. Thisexample, "C:\Program files\Agilent\89600 VSA\Help\Signals\Qpsksmrt.dat", has a 0.1% symbol

rate error.

Note


This small error causes a significant spread of the constellation clouds. Trace B shows a "V"shape in the EVM versus time trace. This characteristic is caused by the demodulator aligning theexpected symbol clock rate with the clock rate of the signal for best fit at the mid point of the trace.The differences in the two clocks then show increasing "slip" or deviation further from the center ofthe trace.

To find the actual symbol rate of the signal, try adjusting the symbol rate of the demodulator untilthe error is minimized.


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Digital Demodulation AnalysisTroubleshooting: Filtering Errors

Filtering errors are among the most common in digital communication design. Typical errors canbe due to; errors in filter alpha, wrong filter shape or incorrect filter coefficients. The 89600

supplies a great set of tools to diagnose and even fix filtering problems.

Alpha Error

The first example, "C:\Program files\Agilent\89600 VSA\Help\Signals\Qpskalfa.dat", is a signal withan alpha of 0.2 instead of 0.35.

Note

For the signals in this topic, use the same demodulator setup as used in theprevious topic: format; QPSK, symbol rate; 50 ksymbols/sec, meas. filter;root-raised cosine, ref filter; raised cosine, alpha; 0.35.

The modulation errors can be seen in a number of analyzer displays. Notice in the error summary

below that the rms EVM is greater than 1.5 percent instead of less than 1 percent as seen before.The symbol table offers the fastest way to detect modulation problems.

The filter alpha problem is evident from the EVM versus time display as well.

To look a the EVM more closely, click the select area tool, drag a box around a few symbols,and rescale the x-axis to zoom in. Shown here is a rise in EVM between the symbol dots. This isinter-symbol interference or ISI. This is typical of filtering problems and incorrect alpha.


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Wrong filter shape

There are many reasons why a filter will end up with the wrong shape:

Too few coefficients Incorrect coefficients or wrong sign

Improper truncation or interpolation

Confusion (which is the right filter?)

This is an example of the confusion case. The transmitter has implemented a raised cosine filterinstead of a root-raised cosine filter. The result is the same in most of these cases; the modulationis distorted by improper filtering and the result is a signal that is hard for the receiver to reconstruct.

Shown here is the recorded signal; "C:\Programfiles\Agilent\89600 VSA\Help\Signals\Qpsknqst.dat". This signal has the same attributes as theexamples before except that the filter used was a raised cosine or Nyquist filter with alpha of 0.35.The resulting modulation errors are obvious from this vector diagram.

The problem can be detected by looking at the high EVM reported by the symbol table/errorsummary.


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EVM versus time also does not look good.

The problem is, when a distorted signal like this appears, it is difficult to determine the source ofthe problem. Is it noise? Software? What?

The adaptive equalizer

The adaptive equalizer was built-in to the 89600 to help identify problems like this.

It is implemented as a blind, feed-forward equalizer

Can reduce linear errors like filtering or multipath reflections

Cannot reduce non-linear errors like intermodulation distortion or noise

Works differently from dedicated equalizers in communication products

I/QMeasured EQ filter

Coefficients

LMSAlgorithm

EQFilter

MeasFilter

DetectorDetected Bits


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Why use the adaptive equalizer?

It will identify problems that can be solved by filtering

It will show what the signal performance could be with a filter applied

You can retrieve the coefficients to help check or improve the design

The frequency response and group delay of the entire transmission path can be calculated fromthe application of the equalizer. Let's apply the equalizer to this signal. To set up the equalizer,access the menu below by clicking MeasSetup, and Digital Demod... , and select theCompensate tab.

To use the adaptive equalizer:

1. Set up a Filter length of 21 symbols

2. Set a Convergence factor of 1E-7.

3. Select the Equalization Filter check box in order to initiate the filtering process.

4. Watch the vector or constellation diagram, the error summary, or EVM versus time to see ifthe filter improves modulation accuracy.

5. When errors have been minimized, stop the equalizer by changing the Adaptive field fromRun to Hold.If errors get worse, reduce the Convergence factor by clicking on this field and "stepping"the value down then click Reset Equalizer. It might also be necessary to increase the lengthof the filter to reduce errors that are delayed in time. Note Reset the equalizer wheneverchanges to its configuration are made.

Shown below is the signal after the equalizer had run for 15 seconds. The signal quality hasimproved confirming that the major error contributor was linear. This is an important finding whensearching for causes of modulation error.


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Checking EVM will show errors back down near 1%.

Adaptive Equalizer Results

To view the impulse response of the equalizing filter click Trace, Data, and Eq ImpulseResponse. Here the impulse response is viewed in a Log Mag format to make the smallercoefficients easier to see.

The filter coefficients that make up this trace can be transferred to a spreadsheet or CAD programfor analysis. These coefficients could even be implemented in the transmitter's software to "cleanup" the signal output.


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One other significant application of the adaptive equalizer is the "measurement" or derivation ofthe frequency response of the signal source that produced the signal. If the adaptive equalizer cancompute a filter that will "clean up" the modulation errors, it makes sense that we can compute thesource's frequency response.

To view the frequency response of the signal source click, Trace, Data, and Ch FrequencyResponse. This result can be displayed as log magnitude as seen in Trace A below, or in a groupdelay format as seen in Trace B.

Note

This display is the composite frequency response of the entire signal pathincluding baseband filtering, IF and RF filtering, mismatch, transmissionchannel effects such as fading and multi-path.

Here is shown a suggested Display > Layout > Grid 2x2 traces to try when using the adaptiveequalizer.


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Digital Demodulation AnalysisTroubleshooting: Spurious and Interfering Signals

When integrating a communications system, many signals-digital, baseband, IF and RF-arepresent. The close proximity of the components is an invitation to crosstalk and can lead to

unwanted signals in the signal output.This example signal; "C:\Program files\Agilent\89600 VSA\Help\Signals\Qpskspur.dat" is a QPSKmodulated signal with a spurious signal added 36 dB below the carrier.

Note

For this signal, use the same demodulator setup as used in previous topics:format; QPSK, symbol rate; 50 ksymbols/sec, meas. filter; root-raised cosine,ref filter; raised cosine, alpha; 0.35.

Observing the constellation on Trace A, a small spreading of the clouds is noted. The EVM onTrace B is elevated, but there are not many clues here as to the source of the problem.

Zooming in on the Constellation diagram reveals a doughnut shape to the clouds. This is indicativeof interference, but still no clues as to the origin of the spurious signal. Try viewing the spectrum ofthis signal. See anything? Probably not, because the spur is buried in the modulation sidebands.

By calculating the FFT of the EVM versus time trace, any deterministic components in the error


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trace will show up as spectra in the error vector spectrum.

To view the error vector spectrum click Trace, Data, and Error Vector Spec.

The result for this example shows a spur about 11 kHz below the center frequency. This confirmsthat the problem is a spur and noting the absolute frequency and the frequency offset from thecarrier will usually identify the source of the interference.


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Digital Demodulation AnalysisTroubleshooting: Compression

This recorded signal; "C:\Program files\Agilent\89600 VSA\Help\Signals\Qpskcomp.dat", has beencompressed by an amplifier.

Note


Notice the difference between this signal on trace A below and an uncompressed signal in trace B.The radial power excursions of trace A are reduced and the constellation clouds are expanded.

The image below shows the peak to average ratio of the compressed signal.

This is measured by:

1. Viewing the signal in the time domain in a log-mag format

2. Measuring the average power with the band power markers

3. Comparing to the marker placed at a peak.

The result here is about 1.7 dB. Comparing this to the uncompressed peak to average ratio ofabout 3.6 dB.


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Shown below is yet another way to detect compression problems. This data type is called CCDF or

the complementary cumulative density function. This curve shows the probability that the power isequal or above a certain peak to average ratio.

From this plot, compression (or expansion) is easy to see.

More on CCDF curves


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W-CDMA(3GPP)/HSDPA MeasurementsW-CDMA(3GPP)/HSDPA Overview

Option B7N adds W-CDMA/HSDPA (Wideband Code Domain Multiple Access) with HSDPA andcdma2000/1xEV-DV (Code Domain Multiple Access 2000) with 1xEV-DV demodulation capability

to the 89600-Series Vector Signal Analyzer software application. W-CDMA demodulation lets youmeasure W-CDMA(3GPP) and HSDPA modulated signals that are compliant to the following3GPP Technical Specifications:

3GPP TS.34.121 v.3.13.0 (2003-06) R1999 (for user equipment or mobile stations)

3GPP TS.25.141 v.3.13.0 (2003-03) R1999 and 3GPP TS.25.141 v.4.8.0 (2003-03) Rel 4 (forbase stations or base transmission stations).

Option B7N: W-CDMA (3GPP)/HSDPA also allows you to measure HSDPA signals that arecompliant with the following 3GPP Technical Specifications:

3GPP TS.25.211 V5.4.0 (2002-06) Rel 5 (for physical channels and mapping of transportchannels onto physical channels - FDD)

3GPP TS.25.213 V5.3.0 (2003-03) Rel 5 (for spreading and modulation - FDD)

W-CDMA (3GPP)/HSDPA demodulation descrambles, despreads, and demodulatesW-CDMA(3GPP)/HSDPA uplink and downlink signals. The application software automaticallyidentifies all active channels regardless of the Symbol Rate or Spread Code Length.

If you have an 8961x or 8964x series Vector Signal Analyzer, an optional second basebandchannel allows IQ baseband measurement capability. You can also perform IQ basebandmeasurements on data that is from a file or the stream interface.

See Also

About Opt B7N:W-CDMA(3GPP)/HSDPA Demodulation

W-CDMA(3GPP)/HSDPA Available features


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W-CDMA(3GPP)/HSDPA MeasurementsMaking a W-CDMA Measurement

The 89600-series application software provides an interface that makes it easy to demodulateW-CDMA(3GPP) and HSDPA modulated signals, yet provides wide flexibility in adjusting many

measurement parameters, such as chip rate, scramble code, sync type, and result length. Thefollowing steps are the core steps you should take when demodulating anyW-CDMA(3GPP)/HSDPA signal. The next two topics, Demodulating a Downlink Signal andDemodulating an Uplink Signal show you how to use these steps to demodulate aW-CDMA(3GPP) downlink and uplink signal.

Complete setup information for making W-CDMA(3GPP)/HSDPA signals is located in Setting up aW-CDMA(3GPP) Measurement and Setting up a HSDPA Measurement.

Eight Steps to Successful W-CDMA(3GPP) Demodulation

1. Set center frequency and span for signal under test.

2. Set input range as low as possible without overload.

3. Set up triggering (if required).4. View your signal in the frequency domain to verify the settings of the parameters that you set

in steps 1-3.

5. Select the W-CDMA(3GPP)/HSDPA demodulator.

6. Select Uplink or Downlink. Verify that the following W-CDMA(3GPP)/HSDPA parameters areset correctly:

Chip Rate

Scramble Code and Scramble Type

Sync Type

Mirror Frequency Spectrum7. Run the measurement.

WCDMA(3GPP) Measurement Tutorials

Demodulating a Downlink Signal

Demodulating a Uplink Signal

Analyzing a WCDMA(3GPP) Signal

Selecting a Slot for Analysis

Troubleshooting WCDMA(3GPP) Measurements Tutorial

See Also

W-CDMA(3GPP)/HSDPA Overview


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7. Click Preset to Default. Because this is a downlink signal, select Downlink.

8. If necessary, change the chip rate, scramble code and scramble type, and sync type to match

your signal. For the 3GPPDown.sdf recorded signal, use the default settings.

Select Quad 4 display to see four different displays for your signal.


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W-CDMA(3GPP)/HSDPA MeasurementsDemodulating an Uplink Signal

This topic uses the recorded signal, 3GPPUp.sdf, located in the "C:\Programfiles\Agilent\89600 VSA\Help\Signals" directory (see Step 1) to show you how to demodulate a

W-CDMA(3GPP)/HSDPA uplink signal. More detailed setup information for W-CDMA signals islocated in Setting up a W-CDMA(3GPP)/HSDPA measurement.

Note

Because this is a recorded signal, you do not have to perform step 2. However,if you are measuring your own signal, the parameters in step 2 must be setcorrectly to demodulate your signal.

1. Preset the analyzer, load the recorded signal 3GPPUp.sdf, and start playback.

2. Click File > Recall, Recall Recording, and select "C:\Programfiles\Agilent\89600 VSA\Help\Signals\3GPPUp.sdf". Set center frequency, span, range, andtrigger for your signal.

For details, see steps 2-5 in Demodulating a Downlink Signal.

3. View the spectrum of your signal to verify that the parameters in step 2 above are set correctly.The span that you set should be the smallest span that allows you to see all the energy in yoursignal. Excessive spans add unwanted noise to the measurement.

4. Select the W-CDMA(3GPP)/HSDPA demodulator.


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Select Quad 4 display to see four different displays for your signal.

By default, Traces A, C, and D show you composite displays of your signal. Compositedisplays show you the results of all channels and layers in your signal, and include data forboth I and Q. Notice that the trace A, "Ch1 Composite CDP" (Code Domain Power) display,looks different than it did for the 3GPP downlink signal. Uplink signals use separate channelsfor I and Q data--the Ch1 Composite CDP display shows channel data for I above the x-axisand channel data for Q below the x-axis. For details about 3GPP displays, see About TraceData (W-CDMA).

Tip

If some of the error information in the Ch1 Composite Error Summary table (in


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W-CDMA(3GPP)/HSDPA MeasurementsAnalyzing a W-CDMA Signal

The following steps show you how to use markers and Copy Marker to Despread Chanto quicklyview trace data for a single channel in your W-CDMA signal. This procedure uses the recordeddownlink signal, 3GPPDown.sdf, located in the "C:\Program files\Agilent\89600 VSA\Help\Signals"directory (see Step 1).

Note

The following steps apply to both W-CDMA uplink and downlink signals. If you'dlike to try the steps using an uplink signal, load 3GPPUp.sdf instead of3GPPDown.sdf (both signals are located in "C:\Programfiles\Agilent\89600 VSA\Help\Signals").. Demodulate the 3GPPDown.sdfrecorded signal, as shown in Demodulating a Downlink Signal. On Trace A (Ch1 Composite CDP), set a marker to the channel with the peak power.

1. Right-click trace A again, then click Peak.

2. Use Copy Marker to Despread Chan to select the marked channel (right-click trace A, then clickCopy Marker to Despread Chan).

Copy Marker to Despread Chansets the Channel Spread Code Lengthand Code Channelparameters. These parameters are located in the dialog box that is displayed when you clickMeasSetup > Demod Properties > Channel/Layer.


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Channel Spread Code Lengthand Code Channelspecify the channel used for channeltrace-data displays, such as Chan Syms/Errs (which displays the symbol table for the selectedchannel), Chan Mag Error, Chan Phase Error, and Chan Error Vector, Chan IQ Meas Time.

3. Change Trace C to display the vector diagram for the selected channel.

Double-click the trace title for Trace C to display the Trace Data dialog box. Then clickChannel 1 Chan > IQ Meas Time.

Tip

There are several trace data listed under Channel 1 Chan. All of these showinformation for the selected channel. For details about each selection, seeCh1

Data (W-CDMA).


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4. Change Trace D to display the symbol table for the selected channel.

Double-click the trace title for Trace D to display the Trace Data dialog box. Then clickChannel 1 Chan > Syms/Errs.

5. Change Trace B to display the Error Vector Time trace for the selected channel.

Double-click the trace title for Trace B to display the Trace Data dialog box. Then click Channel1 Chan > Error Vector Time.

The following illustration shows the results of the previous steps. The symbol table in Trace Dprovides both error summary information and demodulated bits for the selected channel. For

W-CDMA downlink signals, the symbol table also shows information about the demodulatedchannel, such as the number of Pilot Bits detected in the DPCH channel, the tDPCH timing valuefor the DPCH channel, and the first slot used in the measurement (Slot). For details about thesymbol table, see About the Channel Symbol Table (W-CDMA). For details about error informationin the symbol table, see About Channel Error Summary Data (W-CDMA)

Notice the errors shown in Trace B, the Error Vector Time trace. These errors arise due to theSync signal that comprises 10% of each slot in a 3GPP downlink signal. Because the Sync signalis not orthogonal, it increases the EVM, as indicated by the Error Vector Time trace.

Tip

If some of the error information in the symbol table (in trace D) is missing,resize the Agilent 89600 Vector Signal Analyzer window.


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W-CDMA(3GPP)/HSDPA MeasurementsSelecting a Slot for Analysis

The following steps show you how to select a slot other than the first slot when viewing Channel 1Chan trace data for your W-CDMA signal. This procedure uses the recorded signal,

3GPPDown.sdf, located in the "C:\Program files\Agilent\89600 VSA\Help\Signals" directory (seeStep 1).

1. Perform the steps in Analyzing a W-CDMA Signal to display demodulated data for a singlechannel.

At this point, Trace D shows the symbol table for the selected channel. Channel information isshown in the trace title. In this example, the selected channel is Channel 119 on layer

30ksym/s (S128 is another way of referring to the layerS128 refers to spread code length128).

Note

By default, all Channel 1 Chan trace-data show demodulated data only for thefirst slot in the measurement. The following steps show you how to viewdemodulated data for a slot other than the first slot and how to viewdemodulated data for more than one slot.

2. Display the Timetab to access the Result Length, Measurement Offset, and MeasurementIntervalparameters.

Click MeasSetup > Demod Properties > Time.

The Result Lengthdetermines the number of slots that are demodulated with eachmeasurement. The Measurement Intervaland Measurement Offsetlet you isolate a specificportion of the Result Lengthfor analysis. The Measurement Intervaldetermines how much ofthe Result Lengthis displayed, the Measurement Offsetdetermines the start position of themeasurement within the Result Length. For complete details, see About Measurement Interval

and Offset.

Tip

The parameter that most impacts measurement speed for W-CDMAmeasurements is the Result Length. To increase measurement speed, reducethe Result Length. However, you'll usually want to keep the result length at orabove 6 slots, because a result length under 6 slots may cause inaccuratemeasurement results.


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W-CDMA(3GPP)/HSDPA MeasurementsTroubleshooting Measurements

When demodulating W-CDMA signals, you may encounter problems demodulating your signal oryou may be able to demodulate your signal, but your signal contains errors.

If you're unable to demodulate your W-CDMA signal or you encounter errors such as those listedbelow, see Troubleshooting (W-CDMA) for assistance:

The analyzer is unable to lock to your signal.

Excessive noise floor.

The wrong channel is marked as active.

Tslot and Tframe or tDPCH is incorrect (DATA? error message is displayed in the symboltable).

If you've successfully demodulated your W-CDMA signal, but your signal has errors, such asquadratur

tutorial vs a

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