oscilloscope fundamentals, hands-on course at eelive 2014
TRANSCRIPT
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Oscilloscope Fundamentals
Workshop AKA: A few things you may not have known, or
may have forgotten about scopes!
Dave Rishavy
Product Manager – Rohde-Schwarz North America
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Agenda
ı Choosing an Oscilloscope ı RTE Tour ı Probing Basics
Workshop: Passive probe compensation Workshop: Ground lead effects
ı Vertical System Overview Workshop: Channel input coupling Workshop: Effective use of vertical scale
ı Sampling & Acquisition Workshop: Aliased Signal Capture Workshop: Acquisition Rate
ı Horizontal Systems Horizontal measurements
ı Trigger System Workshop: Runt Trigger
ı Other Things a Scope can do: EMI Debug Workshop: A quick look at EMI
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Choosing an Oscilloscope
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Bandwidth Definition ı Bandwidth is THE single-most
crucial parameter used for the
oscilloscope selection:
Ensure the scope has enough
bandwidth for the application!
ı Oscilloscope bandwidth is
specified at -3dB (-29.3%)
Frequency
Att
en
uati
on
0dB
-3dB
fBW
0 dB 6 div at 50 kHz
- 3 dB 4.2 div at bandwidth
The maximum bandwidth of an oscilloscope: The frequency at which a sinusoidal
input signal amplitude is attenuated by -3dB.
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Bandwidth – Requirements of the Test Signal
ı Required scope bandwidth depends on test signals frequency components Digital “square” waveform is composed of
odd sine wave harmonics
Frequency
Am
pli
tud
e
fFundamental f3rd harm. f5th harm.
Rule of thumb:
BWScope = 3-5x fclk of Test Signal
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Bandwidth – Application Mapping
l Data rates of typical I/O interfaces
Interface Data Rate Clock
Frequency
Oscilloscope Bandwidth
Requirement Oscilloscope
Classes 3rd harmonic 5th harmonic
I2C 3.4 Mbps 1.7 MHz 5.1 MHz 8.5 MHz Value
LAN 1G 125 Mbps 62.5 MHz 187.5 MHz 312.5 MHz Lower mid-range
USB 2.0 480 Mbps 240 MHz 720 MHz 1200 MHz Mid-range
DDR II 800 Mbps 400 MHz 1.2 GHz 2.0 GHz
SATA I 1.5 Gbps 750 MHz 2.25 GHz 3.75 GHz Upper Mid-range
PCIe 1.0 2.5 Gbps 1.25 GHz 3.75 GHz 6.25 GHz High-end entry
PCIe 2.0 5.0 Gbps 2.5 GHz 7.5 GHz 12.5 GHz High-end
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Bandwidth – Technology Mapping
Logic
Family
Typical Signal
Rise Time
Calculated Signal
Bandwidth
Oscilloscope Band-
width Requirement
TTL 2 ns 175 MHz 525 - 875 MHz
CMOS 1.5 ns 230 MHz 690 - 1150 MHz
LVDS 400 ps 875 MHz 2625 - 4375 MHz
ECL 100 ps 3.5 GHz 10.5 - 17.5 GHz
ı Digital technologies have characteristic rise times, e.g:
BW * trise_10-90 = 0.35
trise_10-90 = 0.35 / BW
Bandwidth x Risetime = 0.35
e.g. 100 MHz Bandwidth = 3.5 nsec Risetime
Measured rise time depends on intrinsic rise time of the scope
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RTE Tour
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The RTE
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Some Favorite Buttons
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Interface Overview
Signal Bar (Location to where
active waveforms and
results reside in icon
form. Can contain
both Signal icons and
result icon.)
Tool Bar (Quick access to commonly used functions)
Smart Grid (Flexible drag and drop
diagram / measurement
display)
Menu Bar ( Complete Access to all
functionality)
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Reference: Tool Bar
UNDO / REDO
Tool Tip
Save Set
Signal Bar On/Off
Select
Cursor
Histogram
Zoom
Note the Arrow
indicated there
are multiple
selections
Measure / Quick Meas
Mask Test
FFT Analysis
Search Entry into configuration
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Reference:
Signal Bar Waveforms,
Measurements,
decode tables, (and
nearly anything) can be
dragged onto the
signal bar
Signal bar will highlight
when something is
ready to be dropped
onto it
Active signals will show
information about the
signal and be
displayed in the
SmartGrid.
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Reference:
SmartGrid
SmartGrid positions
1 = Placement will be in existing diagram (overlay of signals),
creates floating icon for results.
2 = New diagram (Grid) on the left or right
3 = New diagram (Grid) above or below
4 = New tab (similar to a sheet in an Excel notebook)
5 = XY-diagram 6 = YX-diagram (only available in certain configurations)
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Our Target
ı Small Digital Stimulus Board
Toggle
Mode Back
Flooded
Areas are
Ground
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Probing Basics
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Probe Basics:
ı These three factors – Encompass most of what goes into
proper selection of a probe
physical attachment
minimum impact on circuit operation
adequate signal fidelity
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Probe Basics: Passive Probes
ı Passive Probes
Least Expensive
No active components, essentially wires with an RC
network
Input impedance decreases as the frequency of the
applied signal increases
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Probe Basics: Active Probes
ı Active Probes
ı Low loading, Adjustable DC offset, Auto recognition by
instrument
ı Incorporate field effect transistors that provide very high
input impedance over a wide frequency range.
ı In short, Active probes are recommended for signals with
frequency components above 100MHz.
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Probing Best Practices
ı Use appropriate probe tip adaptors whenever possible:
Even an inch or two of wire can cause significant
impedance changes resulting in distorted wave forms at
high frequencies
ı Keep ground leads as short as possible:
Added inductance of an extended ground lead can cause
ringing to appear on a fast transition wave-form
ı Compensate the probe:
An uncompensated probe can lead to various
measurement errors, especially in measuring pulse rise or
fall times
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Probe Options
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Workshop: Probe Compensation
ı Matches the probe cable capacitance to the scope input capacitance.
ı Assures good amplitude accuracy from DC to upper bandwidth limit
frequencies
ı A poorly compensated probe can introduce measurement errors
resulting in inaccurate readings and distorted waveforms
ı Connect Probe to compensation output on RTO, Use Favorite Buttons
ı Use small screw driver to adjust POT in probe body to adjust wave-form
Affects amplitude, rise time, etc
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Workshop: Probes Ground Loop Effects
ı Study the effects of extended ground wires on wave-forms
Use passive probe on 10_MHz_clock output
Measure overshoot with long ground lead
Replace long ground lead with short spring lead
Do a single shot to stop acquisition and compare the two waveforms
Take a measurement of the positive and compare
Affects overshoot, rise time, etc
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Vertical System Overview
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The Function Blocks of a Digital Oscilloscope The Vertical System
ADC Acquisition
Processing
Memory
Post-
Processing
Display
Trigger
System Horizontal
System
Att. Amp
Amp
Vertical System
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Vertical System Overview
ı The controls and parameters of the Vertical System are used to
scale and position the waveform vertically
ı The vertical system detects the analog voltage and conditions the
signal by the attenuator and signal amplifier for the analog-to-digital
converter (ADC)
Scale Position
Offset Bandwidth
Input Coupling
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Workshop: Channel Input Coupling
ı Broadest BW is achieved with 50 Ohm DC input coupling
ı Passive probe is typically 1 M Ohm coupled limiting the bandwidth to
500 Mhz under all conditions
ı Benefit to 1 M Ohm coupling is protection from high voltages
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Workshop: Channel Input Coupling
ı Study the effects of scope termination on signaling
Connect to the ANA signal on the demo board.
Toggle until only this light is illuminated
PRESET and AUTOSET the RTE.
Select a vertical scale of 400mV/div on CH1
Note the default to DC coupling. DC coupled includes the DC level of
the signal.
Select AC couple from the channel menu. Note the signal floats to the
zero level. This will reject the DC offset of the signal.
ı 50 Ohm setting will be covered in Near Field Probe Sections
Affects impedance considerations
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The Function Blocks of a Digital Oscilloscope The Vertical System – Analog-to-Digital Converter
ADC Acquisition
Processing
Display
Trigger
System Horizontal
System
Att. Amp
Amp
Vertical System Memory
Post-
Processing
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Analog-to-Digital Converter (ADC)
ı The ADC in the acquisition system samples the signal at discrete points
in time converts the signal's voltage at these points to digital values
called sample points
ı Most Oscilloscopes use 8-bit ADCs
ı ADC for a scope is not typically “off the shelf”
Technology is highly sensitive
ı Parameters:
Sample rate: Clock rate of ADC – typically 5 times higher than
oscilloscope bandwidth
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Analog-to-Digital Converter (ADC) Sampling
ı Samples are equally spaced in time ı Sample Rate measured in Samples/Second (Sa/s, kSa/s, MSa/s, GSa/s) ı Clock rate of ADC – typically 5 times higher than oscilloscope bandwidth
Taking samples of an input signal at specific points in
time.
Samples
Hold Time Needed for Digitizing
Sample Interval TI
Interpolated Waveform
{
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Maximizing the ADC input range
ı Input range and position directly affects the resolution of the waveform amplitude
ı The 10 vertical scales correspond to the full ADC input range
Signal amplitude:
0.5 V
Scale/div = 50 mV/div Scale/div = 100 mV/div
Best ADC resolution
8 bit => 2 mV / bit
reduced ADC resolution
8 bit => 4 mV / bit
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Demo: Vertical Scale
ı Examine quantization errors introduced by using only half
the ADC
Affects Most Measurements
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Sampling and Acquisition
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Vertical System
ADC Acquisition
Processing
Display
Trigger
System Horizontal
System
Att. Amp
Amp
Sampling Methods & Acquisition Modes
Memory
Post-
Processing
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Aliasing (Sampling too slow)
ı Nyquist Rule is violated:
Sampling rate is smaller than 2x highest signal frequency
Signal is not sampled fast enough -> aliasing
False reconstructed (alias) waveform is displayed !!!
Example
-input: 1 GHz sine wave
-sample rate: 750 MSa/s
-alias: 250 MHz
input signal
alias
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Workshop: Affects of Aliasing
ı Connect to the 10_MHz_Clk signal
Preset / Autoset
Zoom Around Trigger
Force the acquisition length to 5KSa Press Res/Rec Length
Select Record Length Limit and set to 5KSa. Change Acquisition time to
500us. This will force the sample rate to 10MSa/s.
The signal is heavily aliased here. It will also lose trigger.
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Workshop: Affects of Aliasing ı Adjust Acquisition time back with Nav knob to see the effects as the sample rate
is brought to 500MSa/s and beyond.
Ensure proper sample rate for your signal in question.
(This is where Autoset may not be your friend)
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Sampling Methods: Interpolation between points
>10 samples
Real-time Sampling •over-sampling following Nyquist rule
Interpolation
linear sine (sin(x)/x)
>2 samples; improves interpretation of the samples
Dots
I Interpolate between the samples
I Linear interpolation computes record
points between actual acquired samples
by using a straight line fit.
I Sin(x)/x interpolation computes record
points using a curve fit between the actual
values acquired.
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l Digital Oscilloscope have significant blind-times
typical ratio: max. 0.5% active –> 99.5% blind (=50,000 wfm/s)
Wfm Update Rate: Issue of Digital Oscilloscopes
acquisition
of 1st wfm blind time acquisition
of 2nd wfm
acquisition cycle
for 1 waveform
e.g. 100 ns e.g. 19.9 us
Scope display
is missing the
critical signal
faults!
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Benefit of High Capture Rate Glitch Capture Probability vs. Test Time
ı Glitch Capture Probability
ı Test time decreases
tremendously with
higher acquisition rates
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Workshop: Display Update Rate
ı Glitch Example
Connect passive probe to SIGNAL
Setup demo board to have the NARROW, FREQ and
RARE all illuminated. This will generate a glitch at 100/s
Preset / Autoset
Observe glitch @ 40ns/div
Set demo board to mode with only NARROW and RARE
illuminated.
Observe rare glitch (occurs once per second)
Set a Mask on the signal to capture the glitch.
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Horizontal System
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Horizontal System
Vertical System
ADC Acquisition
Processing
Display
Trigger
System Horizontal
System
Att. Amp
Amp
Memory
Post-
Processing
l The horizontal system's sample clock determines how often the
ADC takes a sample; the rate at which the clock "ticks" is called
the sample rate and is measured in samples per second
l The sample points from the ADC are stored in memory as
waveform points; these waveform points make up one waveform
record
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Sampling Rate
Record Length Resolution
Time Scale
Acquisition time
Sample Rate Time
Scale
# of
Div’s
Record
Length
• # of samples
• time / div’s
• 10 * time / div’s
• time between
2 samples
x x =
e.g. 10 GS/s x 100 ns/div x 10 Div’s = 10K samples
10 GS/s x 100 s/div x 10 Div’s = 10M samples
Acquisition time
1 / Resolution
Horizontal System Buzz Words
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ı What are the advantages of higher sample rates? Increased signal fidelity (more accurate signal reproduction)
Better resolution between sample point
Higher chance of capturing glitches or anomalies
Can observe high frequency noise in low frequency signal
ı What are the advantages of deep memory? Capturing of longer time periods while maintaining high resolution (fast
sample rate)
Better zoom in capability
Horizontal System Summary
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Trigger System
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Trigger System
Channel
Input
Vertical System
ADC Acquisition
Processing
Display
Trigger
System Horizontal
System
Att. Amp
Amp
Memory
Post-
Processing
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Trigger System
ı Motivation
Get stable display of repetitive waveforms
In 1946 the triggered oscilloscope was
invented, allowing engineers to display a
repeating waveform in a coherent, stationary
manner on the phosphor screen
Isolate events & capture signal before and after
event
Define dedicated condition for acquisition start
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Types of Triggers: Runt Trigger
3/30/2014 FAST: Advanced Triggering
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Workshop: Runt Trigger
ı Demo Board – RUNT, FREQ, RARE
illuminated.
Probe SIGNAL
ı PRESET, then AUTOSET
ı Observe/ Identify the amplitude of the runt
pulse
Infinite Persistence (DISPLAY key) is
another method to see a rare event
ı Note the amplitude of the runt pulse. Jot this
down.
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Workshop: Runt Trigger
ı Toggle change button down demo board to only have RUNT and RARE illuminated.
ı Press Preset
ı Press Autoset.
ı Change horizontal scale to 20ns/div
ı The runt should be hard or impossible to see.
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Trigger Menu ı Keep the same Demo board configuration.
ı Enter TRIGGER system
ı Select trigger type “RUNT”.
ı Set the upper and lower limits to the “Open space” around the runt we saw
ı Why is does it not appear triggered?
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Other things a scope can do:
EMI Debug
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Workshop: A Quick Look at EMI ı With a sensitive front end and a fast FFT, some oscilloscopes can also assist in
looking at EMI issues.
ı Near Field Probes allow us to pick up radiated emissions
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Workshop: A Quick Look at EMI
ı Attach Loop Probe to CH1
ı Near Field Probes are 50Ohm coupled.
ı PRESET/AUTOSET
ı Set vertical scaling to 1mV
ı Observe the emissions in the time domain at this sensitivity
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Workshop: A Quick Look at EMI
ı Perform an FFT
ı Settings: CF: 250MHz, Span: 500MHz, RBW: 100KHz
ı From DISPLAY button, select a color table of choice
ı Adjust FFT window size
ı Move the near field probe around to see the FFT effects.
ı Bonus, use a mask to stop on a random event.
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