amplified rf measurements

8/3/2019 Amplified RF Measurements

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Contents Preface ......................................................................................................................... 2

1. Introduction ................................................................................................................. 4

1.1 Linear Measurements ............................................................................................. 4

1.1.1 Signal Measurements ...................................... ............................. .......... 4

1.1.2 Network Measurements ............................. ............................. ............... 5

1.2 Nonlinear Measurements ......................... ............................. ............................. ... 5

2. Measurement Devices ................................................................................................. 6

2.1 Spectrum Analyzer ......................... ............................. ............................. ..................... 6

2.1.1 Mixer .............................................................................................................. 7

2.1.2 IF- Filter ......................................................................................................... 72.1.3 Detector.......................................................................................................... 8

2.1.4 Video Filter .......................... ............................. ............................. ................ 8

2.2 Network Analyzer ............................ ............................. ............................. ................... 8

2.2.1 Source............................................................................................................. 9

2.2.2 Signal Separation Device ............................ ............................. .................... 9

2.2.3 Detector ....................................................................................................... 10

2.2.4 Processor/Display ......................... ............................. ............................. ... 10

2.3 Noise Figure Meter ........................... ............................. ............................. ................. 10

3. Amplifier Measurements.......................................................................................... 123.1 Linear Measurements ............................ ............................. ............................. ............ 12

3.1.1 Gain Measurement .................................................... ........................... 12

3.1.2 Return Loss Measurement .......................... ............................. ............ 13

3.2.3 Reverse Isolation Measurement........................................................... 14

3.2 Nonlinear Measurements............................................................................................ 14

3.2.1 Gain Compression Measurement ........................... ........................... 15

3.2.2 Harmonic and Intermodulation Distortion Measurement ............... 16

3.3 Linear Measurements ............................. ............................ ............................. ............ 18

3.4 Noise Figure Measurement ............................. ............................. ............................. .. 18

4 Safety Precautions ........................................................................................................... 20

5 Summary ....................................................................................................................... 21

6 Glossary ....................................................................................................................... 22

7 References ....................................................................................................................... 24



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1. IntroductionThe RF amplifiers are found in a wide variety of applications like wireless

communications systems, TV and Radar transmission to name a few. They are classified

according to their output characteristics. The most important parameters of an amplifier

are stability, noise figure, gain, output power, linearity, efficiency, ruggedness and DC

supply voltage. The use of an amplifier is application dependent.

The amplifiers are designed to meet the strict performance specifications and it is

important to measure their fundamental characteristics as accurately as possible. The RF

measurements fall into two broad categories: Linear and Nonlinear measurements.

Linear measurements are used to measure the stability, gain, output power and noise

figure of the system. Under large scale signal condition, the amplifier enters a nonlinear

state and nonlinear measurements are then used to measure harmonic distortion, gain

compression and intermodulation distortion.

1.1 Linear Measurement

Linear measurements measure the linear behavior of a device. A device exhibit linear

behavior when the frequencies at the input and output are same and the output

frequencies only undergo magnitude and phase change. There are two types of linear

measurement: signal measurements and network measurements. Signal measurements

use the time and frequency domain data to determine the characteristic of the waveform.

Network measurements determine the signal transfer characteristics of the device with

any number of ports [1].

1.1.1. Signal Measurements:

Signal measurements are taken either in time domain or in frequency domain. For low

frequency signals the most common measurement is in time domain. The instrument

used to capture the time domain data of the device is oscilloscope. Oscilloscopes plot the

amplitude of the signal over a period of time.

Frequency domain measurements are made for complex signal and signals with

varying amplitude. A spectrum analyzer is used to make frequency domain

measurements. More details about the spectrum analyzer will be presented in chapter 2.



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1.1.2. Network Measurements:

Network measurements are used at relatively high frequency to measure the quantities

like power, gain and port to port transfer functions. The power of RF signals cannot be

detected by the equipment used at low frequencies such as voltmeter and oscilloscope.

To facilitate the measurements, reference impedance is used e.g. 50-ohm as the typical

input and output impedance of an RF measurement instruments. A Network Analyzer

is usually used for network measurements, which measures the scattering parameters of

the device under test (DUT).

1.2 Nonlinear Measurement

Nonlinear behavior of a device is detrimental to the signal that passes through it as the

device may add new frequencies to the input signal and/or the output frequency may

undergo frequency shift. Nonlinearity effects include harmonic distortion, gain

compression, intermodulation distortion and adjacent channel interference. There are

some standard measurements which are used to evaluate the performance of a circuit.

This include 1dB compression point, Third order intercept point, Spurious Free dynamic

range and Noise power ratio [1].



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2. Measurement Instruments

Amplifiers linear and nonlinear parameters can be measured using Network and

Spectrum analyzers. For noise figure measurements, either a spectrum analyzer or Noise

Figure meter is used.

To make the measurements and interpreting the results correctly, it is important to

understand the theory of operation of these devices. This chapter gives a brief

introduction to the major components inside the measurement instruments and why

these components are important will be examined.

2.1 Spectrum Analyzer

The spectrum analyzer is used for frequency domain measurements. The spectrum

displayed on the screen of the analyzer contains the power corresponding to each

harmonic component of the signal. This information can be used for modulation,

harmonic distortion, output power, occupied bandwidth, carrier-to-noise ratio, and a

host of other measurements.

Frequency domain measurements can be made using Fourier Analyzer and swept-tuned

analyzer. The Fourier Analyzer digitizes the input signal and then performs themathematics to display the spectrum. However they have limitations in the areas of

frequency range, sensitivity and dynamic range. Swept-tuned Super-heterodyne is the

most common type of spectrum analyzer, which plots the spectrum of the signal by

translating the signal above the audio range. It has the advantage of making the

measurements over a large dynamic range and a wide frequency range. Figure-1 depicts

the block diagram of the swept-tuned spectrum analyzer.

In the analyzer, the input signal travels through an attenuator to limit the amplitude at

the mixer, and then is filtered by a low pass filter to remove the undesirable frequency

components. Then it gets mixed with the signal generated by the local oscillator (LO).The frequency of LO is controlled by the sweep generator, DC voltage ramp generator in

this case. As the frequency of LO changes, the signal at the output of the mixer gets

filtered by the resolution bandwidth filter (IF filter) and amplified by the IF amplifier.

An envelope detector, following the IF amplifier, then rectifies the signal producing a

DC voltage which is used to drive the vertical portion of the display. As the sweep

generator sweeps the frequency, a trace is drawn across the screen which shows the

spectral contents in the signal.



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Figure-1: Block Diagram of Super-heterodyne Spectrum Analyzer [2]

In the following sections, major components in a spectrum analyzer are discussed in

detail.

2.1.1 Mixer

A Mixer, as shown in the block diagram, is a three port device which is used to translate

the signal from one frequency to another. It is a nonlinear device and its output consists

of the frequencies of local oscillator (LO) signal (fLO), input signal (fIN) and the sum and

difference frequencies of LO and input signal (fLO + fIN & fLO - fIN). Spectrum analyzer

uses only the difference frequencies and this signal is called IF signal.

2.1.2

IF FilterIF-filter is a band-pass filter whose bandwidth is known as resolution bandwidth (RBW).

The resolution bandwidth setting determines the ability of the spectrum analyzer to

resolve the signals in frequency domain. With the narrow RBW the displayed average

noise level (DANL) of the spectrum analyzer is lowered, thus increasing the dynamic

range and improving the sensitivity of the spectrum analyzer. The drawback of narrow

RBW is in sweep speed. The narrower the RBW the more time spectrum analyzer would

take to sweep the frequencies across a given span.



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2.1.3 Detector

Envelope Detector is used to convert the IF-signal to baseband or video signal so that it

can be viewed on the screen. The output of envelope detector is digitized with an ADC

which is then represented as the signal amplitude on the vertical axis of the display.

2.1.4 Video Filter

Video Filter is a low pass filter located after the detector and before ADC. The filter

determines the bandwidth of the video amplifier and is used to average or smooth the

trace seen on the screen. Spectrum analyzer displays signal plus noise. By changing the

VBW settings, we can decrease the peak-to-peak variation of noise [2]. With small valueof VBW, we can identify a signal embedded in noise floor. A general Rule of thumb is

VBW = RBW/100

2.2 Network Analyzer

Network analyzer is used to measure both linear and nonlinear behavior of the devices.

There are two types of network analyzers: vector network analyzer (VNA) and scalar

network analyzer (SNA). VNA can measure frequencies from 5Hz to 110GHz and can

measure the S-parameters, magnitude and phase, standing wave ratios, gain,

attenuation, group delay, return loss, reflection coefficient and gain compression [3]. A

scalar network analyzer provides fast and economical measurements of many

amplifiers. SNA can measure only the amplitude portion of the S-parameters, resulting

in measurements like transmission gain and loss, return loss and standing wave ratio.

Figure-2 shows a generalized block diagram of a network analyzer. The analyzer has an

RF signal source that produces an incident signal which is used as stimulus to the DUT.

The device responds by reflecting a portion of the incident signal and transmitting the

remaining signal. The transmitted and reflected signals are measured by comparison to

the incident signal. The network analyzer couples off a portion of the incident signal

which is used as reference signal. It then sweeps the source frequencies, resulting in a

measured and displayed response of the device



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Figure-2: Network Analyzer Block Diagram [3]

Following four sections are required to measure the incident, reflected and transmitted

signals [3]:

1- Source for stimulus

2- Signal separation device

3- Receiver that provides detection

4- Processor/display for displaying results

2.2.1 Source

The signal source supplies the stimulus for the device under test (DUT). The frequency

or power of the source can be swept. Traditionally the network analyzers used separate

sources but the modern network analyzers have integrated sources which provide

excellent frequency resolution and stability.

2.2.2 Signal Separation Device

Network analyzer has a signal separation device known as test set. It performs two

functions.

1- Test set measures a portion of the incident signal to provide it as reference signal.

This can be done with splitters or directional couplers. For the microwave network

analyzers, directional couplers are usually used as they have very low insertion loss

and good isolation and directivity.



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2- Test set also separates the incident and reflected signals at the input of the DUT.

Again, couplers are used for this purpose as they have low loss and high reverse

isolation.

2.2.3 Detector

The next portion is signal detection device. The signal detection can be done using a

diode detector or tuned receiver. Diode detector converts the RF signal level to a

proportional dc level but the phase information of the RF carrier is lost in this case.

Modern network analyzer use the tuned receiver approach. The tuned receiver uses a

LO to down convert the signal from RF to intermediate (IF) frequency. The IF signal is

later bandpass filtered that narrows the bandwidth of the receiver and greatly improves

the sensitivity.

2.2.4 Processor/Display

This block process the reflection and transmission data and display the results. Most

network analyzers have similar features such as linear and logarithmic sweeps, linear

and log formats, polar plot and smith chart, etc.

2.3 Noise Figure Meter

Noise figure of an amplifier characterizes its ability to process low level signals. It is a

key parameter that differentiates an amplifier from another. The noise figure of an

amplifier can be measured using a spectrum analyzer or with a Noise Figure (NF) meter.

Noise figure measurement with the spectrum analyzer uses the Gain method and is

useful for measuring very high noise figure. But it will not be discussed in this report as

the spectrum analyzers needed for making noise figure measurement should be able to

provide very high resolution bandwidth and noise floor in the order -130dBm [4]. Most

of the noise figure measurements are done using a noise figure meter, which gives

accurate results for small NF measurement. Many types of noise figure measurement

equipment uses Y factor method, which will be illustrated later in the report.

A block diagram of the HP 8970A noise figure meter is shown in Figure-3. Themicroprocessor controls the input and IF attenuators and the first LO, reads the analog-to-digital

converter, provides output data for digital storage circuits, and turns the noise source on and off

[5].



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Figure-3: Noise Figure Meter [5]



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3. Amplifier Measurements

This chapter contains the methods for making the measurements of an amplifier. Bothlinear and nonlinear measurements are discussed in detail. Network analyzers are

traditionally used for making both linear and nonlinear measurements, while some of the

nonlinear measurements are done using a spectrum analyzer

3.1 Linear Measurements

Linear measurements include the determination of gain, reverse isolation, return loss and

noise figure.

3.1.1

Gain MeasurementAmplifier small signal gain is the gain in linear region and is defined as the ratio of the

amplifier’s output power delivered to the load Z0 to the input power delivered from a Z0

source, where Z0 is the characteristic impedance in which the amplifier is used [3]. The

gain can be expressed as follows:

( ) ( ) ( )out in

Gain dB P dBm P dBm

Before the measurements started, it is important to know the input and output power

levels of the amplifier under test (AUT) and the type of calibration required. Given the

approximate small signal gain and output power at 1dB compression, the input power

level can be estimated with the following formula.

1( ) ( ) ( ) 10in dBcompressionP dBm P dBm Gain dB dB

It is also important to estimate the output power from the AUT to avoid overdriving or

damaging the test port of the network analyzer.

Once the appropriate measurement parameters are selected, the next step is to perform

two port calibration of the network analyzer. If the attenuators are used at the output of

AUT they should also be included in calibration. If any change is made in themeasurement setup, it is necessary to calibrate the network analyzer again.

The block diagram of the measurement setup for small signal gain is shown in Figure-4.

The gain is measured at constant input power over a swept frequency range and it is

enough to measure the S21. The result can be displayed on a logarithmic scale or on a

smith chart. Figure-5 depicts the gain measured with a network analyzer. Markers can

be used to determine the gain at a particular frequency.



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Figure-4: Basic Setup for amplifier measurement [3]

Figure-5: Gain Measurement [3]

3.1.2 Return Loss Measurement

Return loss is a measure of the quality of the match of the input and output of the

amplifier, relative to system impedance [3]. Reflection coefficient contains the

magnitude and phase of the reflected signal. Return loss considers only the magnitudeof the reflected signal. The formula for the reflection and return loss are as follows:

20log( )

reflected

incident

V

V

RL

Measurement setup shown in Figure-4 can be used for return loss. The network analyzer

feeds RF power to port 1 of the AUT and measures the reflection at the same port. Plot

the S11 versus frequency and using the formula 20log(S11) gives the input return loss.



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The output return loss can be measured by repeating the same procedure for S22.

Figure-6 illustrates the linear plots of S11 and S22 against frequency. Again, markers can

be used to find the return loss at the desired frequency.

Figure-6: S11 and S22 measurement results [5]

3.1.3 Reverse Isolation Measurement

Reverse isolation is a measure of transmission from output to input [3]. The

measurement of the reverse isolation is the same as the small signal gain

measurement except the stimulus signal is applied at the output of the AUT and S12

is measured. The measurement setup is same as for small signal gain. A measurementresult of S12 using network analyzer is shown in Figure-7.

Figure-7: Reverse Isolation Measurement [3]

3.2 Nonlinear Measurements

This section describes how to measure the amplifier’s output power when 1-dBcompression occurs and the intermodulation distortion.



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3.2.1 Gain Compression Measurement

Gain compression is determined by measuring the amplifier’s 1dB gain compression

point (P1bB) which is the output power at which the gain drops 1dB relative to the

small signal gain [3]. Amplifier’s output power versus input power plot is shown in

Figure-8.

Figure-8: Amplifier Output vs. Input Power Plot [3]

Notice that the amplifier has constant gain in the linear region of operation and is

independent of the power level, but as the input power increases, the amplifier gain

appears to decrease.

The gain compression can be determined using a network analyzer. The procedure is

to apply a fixed frequency power sweep to the input of the amplifier. The fixed

frequency could be the centre frequency of the amplifier. The setup and calibration

data remains the same as for the gain measurement. 1dB compression point can be

easily measured by displaying the normalized gain (Ratio of transmitted and incident

signal). The result would appear as shown in Figure-9. Flat part of the trace

represents the small signal gain in the linear region and curved part corresponds to

the compression caused by higher input power level. 1dB compression point can be

found using marker.



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Figure-9: Amplifier 1dB compression point measurement [3]

3.2.2 Harmonic and Intermodulation Product Measurement

Harmonic and intermodulation products of an amplifier can be measured using a

spectrum analyzer. When the input signal is small, the amplifier is operating in the

linear region and harmonic components are almost non-existent. But increasing the

input signal level drives the amplifier to the nonlinear region and products of second

and third order of fundamental frequency are generated. The generation of harmonic

components reduces the gain of the amplifier as some of the output power at the

fundamental frequency is shifted to the harmonics. Harmonic products can be

visualized on a spectrum analyzer by selecting appropriate span.

Intermodulation distortion occurs when several signals are applied to an AUT when

it is operating in nonlinear region. Third order intermodulation products are most

problematic as they have frequencies close to the useful signal. Figure-10 shows how

the levels of useful signal and intermodulation products are related.

Figure-11 shows the measurement setup that consists of two signal generators, a

coupler to combine two signals and the spectrum analyzer. The frequency of the twosignals should be close to the centre frequency of the amplifier and the amplitude

should be equal. Select the appropriate reference level and attenuation, if needed. The

resolution bandwidth of the spectrum analyzer, in this case, must be carefully chosen

so that the signals can be separated in frequency domain. Figure-12 depicts the

intermodulation products of an amplifier whose centre frequency is 1.5GHz and the

frequency of two tones is 1.49995GHz and 1.50005GHz. 1MHz span in this case

would be sufficient to analyze the frequency products.



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Figure-10: Level of 3rd

order intermodulation product as a function of the level of usefulsignals [6]

Figure-11: Measurement setup for intermodulation distortion

Figure-12: Intermodulation Products of an amplifier[8]



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From the given plot, input 3rd order intermodulation product (IIP3) and output IP3

(OIP3) can be determined easily using following formulas.

@ .

13 ( ) 3( );

2

3 3

out fundamentalfreqOPI P dBm IMD dB

IIP OPIP Gain

Where IMD3=diff. between power of fundamental and intermodulation frequencies

3.3 Stability Measurement

As shown earlier, the transmission and reflection coefficient of an amplifier can be

measured using a network analyzer. With the given information of S-parameters,

designers can easily calculate the various parameters e.g. stability of an amplifier.

K-factor is a significant parameter which determines the stability of an amplifier. This

factor can be calculated using the S-parameters. When K-factor is greater than 1 while

delta is smaller than 1, the amplifier is unconditionally stable.

2 2 2

11 22

12 21

11 22 12 21

1 | | | | | |

2 | |

S SK

S S

where

S S S S

3.4 Noise Figure Measurement

IEEE standard definition of Noise Figure is “Noise figure is the ratio of total noise power

output to that portion of the noise power output due to noise at the input when the input source

temperature is at 290K”[5]

The setup is shown below. First of all, the noise figure instrument must be calibrated.

Calibration involves inputting the Excess Noise Ratio (ENR) as given on the noise source

in to the noise figure meter at the desired frequency range. This will establish the baselevel against which the DUT noise figure will be measured.



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Figure-13: Noise Figure Measurement using Noise Figure Meter [3]

Once the instrument is calibrated, noise figure meter calculates the gain and noise figure

of the AUT according to the following equation:

/10

/10

1010log

10 1

ENR

Y NF

Where ENR provides the information of the noise source at two temperatures: a hot T =

Th and a cold T = Tc = 290K. Y is the ratio of hot and cold noise powers.

0

( )

( , 2 9 0 )

hc

h

c

T T E N R

T

w h e r e

T H o t T e m p e r a tu r e N o i s eS o u r c e O N

T C o ld T e m p e r a tu r e N o is e S o ur c eO F F u s u all y K



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4. Safety precautionsFollowing are the few basic safety instructions which apply to all devices

1. Read carefully the data sheet, specification and safety instruction of the devices

before operating them. Operate the device within their performance limits.

2. Never use a device with a broken power cable.

3. Never pour any liquid into or onto the housing of the device, doing so can cause

short circuits, fire or injuries.

4. Ground yourselves to discharge any accumulated static charge by touching an

electrically neutral surface, for example a wooden table or the ground itself. Another

way is to wear one of the static electricity discharging bracelets while working. These

bracelets automatically ground the user and hence do not allow static charge to

accumulate in the first place.



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5. SummaryThis report addressed all issues related to the amplifier measurement. First, a distinction

was made between the measurement techniques and the amplifier measurements were

categorized into linear and nonlinear measurements. Then, the architecture of the

commonly used lab equipment was explained using block diagram. Finally, several

measurement topologies were presented in order to facilitate the reader in amplifier’s

measurements. Some safety measures are presented at the end.



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6. Glossary

Spectrum Analyzer:

Span: It is the range of frequencies that are displayed on the screen of a SpectrumAnalyzer or Network Analyzer.

Resolution Bandwidth: It is the bandwidth of the internal intermediate frequency filterof the Spectrum Analyzer. It is the minimum resolution that a Spectrum Analyzer canhave in terms of spacing between adjacent peaks. Therefore, two peaks will appeardistinct in a Spectrum Analyzer only if their distance on frequency scale is greater thanthe resolution bandwidth.

Video Bandwidth: It is the bandwidth of the low pass filter located after the detectorand before ADC. It is used to average or smooth the trace seen on the screen.

Sweep Time: When we narrow the resolution bandwidth, we must consider the time ittakes to sweep through them. Narrower bandwidth requires a long time. When sweeptime is very short, the RBW filter cannot fully respond, and displayed response becomesun-calibrated both in amplitude and frequency [2].

Reference Level: It is used to describe how much level is present in dB above or belowthis reference.

Attenuation: refers to the attenuation, and used to attenuate the signal. The attenuatedsignal is required at the input of mixer as if the amplitude is too high it could createdistortion and may damage the mixer.

Measurement Un-calibration: is related to sweep time. When the sweep time is veryshort, the RBW filter cannot fully respond both in amplitude and frequency due to thedelay in filter and measurement becomes un-calibrated. If MEASU UNCAL warningappears, try to change the sweep time or use wider filter.

Network Analyzer:

Calibration: Before we can actually use the VNA, we need to calibrate it. We can

calibrate the VNA using 4 knows standard terminations: Open, Short, 50ohms Matchedand through. Open the calibration menu on the VNA and it will progressively ask toconnect one of the terminations to the RF OUT port and then calibrate the losses of theRF cables. For the Through calibration, we connect the two RF cables to each other usingthe through calibration standard. We may calibrate both terminals of the VNA. Thepurpose of calibration can be explained from the following block diagram.



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Figure-14: Calibration Setup for network analyzer

For the user, the plane for RF measurements is at the terminals of AUT, however, for the

VNA, the plane for measurements is at its own terminals. In other words, in the default

case, neither the user nor the VNA is taking into account the RF cables (and the losses or

attenuation associated with them) connecting the AUT to the VNA. These cable losses

can considerably alter the RF measurement results and hence, to move the plane of

measurement for the VNA beyond the cables, we perform calibration. Now that the

cable losses are stored in the memory of the VNA, this set-up should not be disturbed,

meaning, the same RF cables, attenuators, connectors and adapters have to be used for

the entire measurement.

Directional Coupler: It is a transmission coupling device for separately sampling(through a coupling loss) either the incident or the reflected wave in a transmission line.

Splitter: Splitters are used to divide the input signal into two or more output signals.The performance of a splitter is evaluated according to loss and isolation. Loss is theamount of attenuation that a signal receives as it passes through input to output.Isolation means that the two input signals do not mix up.



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7. References

[1]. Mike Golio, “RF and Microwave Handbook,” CRC Press, 2001

[2]. Rhode & Schwarz, “Hints for Better Spectrum Analysis”, Application Note 1286-1

[3]: Agilent Technologies, “Testing Amplifiers and Active Devices with Agilent8510C Network

Analyzer ”, Product note

[4]:http://www.tutorialsweb.com/rf-measurements/noise-figure/noise-figure-measurement-

techniques.htm; November 30, 2008

[5]: HP, “Noise Figure Meter, Model 8970A”,Product Note

[6]: Agilent Technologies, “Fundamentals of RF and Microwave Noise Figure Measurements,”

Application Note 57-1

[7]: Agilent Technologies, “Microwave Component Measurements Amplifier Measurements

Using the Scalar Network Analyzer,” Application Note 345-1

[8]: Rhode & Schwarz, “Operating Manual of FSG Spectrum Analyzer,” Product Document

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