Mike Schnecker
Getting the most out of your Measurements
Workshop
Agenda
Oscilloscope Basics
Probing Basics Passive probe compensation Ground lead effects
Vertical System Overview Channel input coupling Effective use of vertical scale Ways to get more vertical resolution
Horizontal Systems Sampling Methods Acquisition Rate Relationship of memory depth and sample rate
Trigger System Trigger specifications Advanced triggers
Using a RTE1000 Series
Oscilloscope.
But the majority of what we’ll
discuss is scope agnostic – it
could be done with any other
R&S scope or any digital scope
for that matter.
10/1/2015 Oscilloscope Fundamentals 2
Basic things I assume you know…
ı Oscilloscopes measure
Voltage (y-axis) vs. Time (x-axis)
ı …so that means oscilloscopes work in the
“time domain”
ı Oscilloscopes have been around a long
time (1930s!)
Started out with analog implementations
Now we have digital storage oscilloscopes
(DSOs)
The General Radio Oscilloscope (1931),with sweep circuit (right).
3
Bandwidth Definitionı 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 dB6 div at 50 kHz
- 3 dB4.2 div at bandwidth
The maximum bandwidth of an oscilloscope: The frequency at which a sinusoidal
input signal amplitude is attenuated by -3dB.
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
plitu
de
fFundamentalf3rd harm.f5th harm.
Rule of thumb:
BWScope = 3-5x fclk of Test Signal
(1.5 – 2.5x of the bit rate)
5
Bandwidth – Application Mapping
l Data rates of typical I/O interfaces
Interface Data Rate‘101010…’
Frequency
Oscilloscope Bandwidth
Requirement Oscilloscope
Classes3rd 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 MHzMid-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
8
Oscilloscope Basics – viewing the signal
ı Viewing the signal on the oscilloscope
Best done using the basic 3 knobs on the instrument:
1. Vertical scale and position – get the signal on screen
2. Horizontal scale – see the lowest frequency signal variation
3. Trigger level – get a stable signal
ı Possibly change trigger type to get a better picture
ı Once the signal is on the screen:
Measure sung observation
Measure using markers
Use automated measurements
10
Basic Controls
11
Workshop: Basic Controls
ı Connect the passive probe to the 10MHz_CLK signal on the demo board
ı Preset the oscilloscope
ı Use vertical – horizontal – trigger controls to get a stable signal on the screen
ı Connect the passive probe to the I2C_SCLK signal
ı Preset the oscilloscope
ı Use vertical – horizontal – trigger controls to get a stable signal on the screen
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Probe Basics:
ı These three factors – Encompass most of what goes into
proper selection of a probe
Physical attachment
Minimum circuit loading – what the circuit sees
Adequate signal fidelity – what the scope sees
13
Probe Basics:
ı This situation is frequently
encountered:
Signal is not easy to reach
Source impedance can vary widely
Setup is sensitive to noise and will be
frequency dependent
For more than one signal to be
measured, there will be slight
propagation differences between probe
tip and instrument input (skew)
14
Probe Basics: The ideal probe
15
ı The ideal probe:
Does not influence the source
Displays the signal without
distortion
Probe Basics: The real probe
16
ı The real probe:
Does influence the source
Displays a distorted signal
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
17
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.
18
Passive Probe Input Impedance
19
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
ı Add test points when possible to original design
20
Probe Options
21
Probe Summary – Best Practices
ı Circuit changes when the probe is attached
ı Probe effect minimized by having:
High resistance
1MOhm vs 10MOhm probes, specified on data sheet
Low capacitance
Probe tip geometry (passive versus active, specified on data sheet)
Generally active probes have lower capacitance
Low inductance
Lead length shorter has lower inductance
Not specified in a data sheet, multiple accessories
22
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
ı Use small screw driver to adjust POT in probe body to adjust wave-form
ı Zoom in on wave-form for better resolution
ı Press Measure/Acquisition/select averaging in wfm arithmetic
ı Compensate probe
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Affects amplitude, rise time, etc
Workshop: Probes Ground Loop Effects
ı Study the effects of extended ground wires on wave-forms
Use passive probe on probe compensation output
Measure overshoot with long ground lead
Zoom into edge and study positive overshoot
Take a reference acquisition to save the wave-form to the screen
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 negative overshoot
01.10.2015 Footer: >Insert >Header & Footer 24
Affects overshoot, rise time, etc
Workshop: Passive Probe vs Active Probe Rise Time
ı Connect passive probe to Ch1 and Active probe to Ch2
ı Probe 10_MHZ_CLK on Demo Board
ı Zoom in and measure rise time
ı Rise time of passive probe wave-form is slower
ı Discussion
ı This capacitive loading affects the bandwidth and rise time characteristics of the
measurement system by reducing bandwidth and increasing rise time.
ı Passive probe = 9.5 pF
ı Active has .8 pF
01.10.2015 Footer: >Insert >Header & Footer 25
Affects rise time, overshoot, etc
The Function Blocks of a Digital OscilloscopeThe Vertical System
ADC Acquisition
Processing
Memory
Post-
Processing
Display
Trigger
SystemHorizontal
System
Att. Amp
Amp
Vertical System
27
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
Workshop: Channel Input Coupling
ı Defines how the signal spans the path between its capture by the probe through
the cable and into the instrument.
ı Broadest BW is achieved with 50 Ohm input coupling
ı Passive probe is typically 1 M Ohm coupled limiting the bandwidth to 500 Mhz
under all conditions
ı Benefits to 1 M Ohm coupling is protection from high voltages
ı Study the effects of scope termination on signaling
Connect coaxial cable to SMA connector labeled RF OUT
Select 50 Ohm coupling and measure signal amplitude
Select 1 M Ohm coupling and measure signal amplitude
Do you know what’s happening?
Affects impedance considerations
29
The Function Blocks of a Digital OscilloscopeThe Vertical System – Analog-to-Digital Converter
ADC Acquisition
Processing
Display
Trigger
SystemHorizontal
System
Att. Amp
Amp
Vertical SystemMemory
Post-
Processing
30
Analog-to-Digital Converter (ADC) Sampling
ı Samples are equally spaced in timeı Sample Rate measured in Samples/Second (Sa/s, kSa/s, MSa/s, GSa/s)ı Sample rate: Clock rate of ADC – typically 5 times higher than oscilloscope
bandwidth
Taking samples of an input signal at specific points in
time.
Samples
Hold TimeNeeded forDigitizing
Sample Interval TI
Interpolated Waveform
{
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
Workshop: Vertical Scale
ı Examine quantization errors introduced by using only half
the ADC
Connect passive probe to 10_MHZ_CLK signal on demo
board
Use half grid display and compare to full grid
Save as reference
Compare saved ½ grid and see levels
Measure positive overshoot and show effects
Affects Most Measurements
35
Vertical Summary – Best Practices
ı Signal should occupy 80% of the screen (not off screen
causing amplitude overload)
ı Avoid having any part of signal off the screen
ı Avoid making the signal too small (<30%), reduces
resolution
36
Improving Vertical ResolutionWhen might it be helpful
ı Typically low bandwidth signals where you want to see a small signal in the
presence of a larger one
ı Common Applications
Power Analysis
Measurement of small voltage variation in presence of large voltage, e.g.
conduction Loss
Small current analysis on component sleep state
Accuracy in Ripple Voltage measurements
Medical
Weak cardio or neural signal
Wireless communication
High resolution suitable for NFC, Wireless Power Charging & design using
small Amplitude Shift Keying in data transmission. Typically in lower BW.
Embedded circuit designs
Low power circuit with weak signals
Sub threshold leakage measurements
Improving Vertical Resolution
ı There are three ways to increasing
vertical resolution in an oscilloscope
Averaging
High resolution (Enhanced res)
BW Filtering
ı Each of these increases signal to
noise ratio thereby giving more
signal resolution
“HighRes” Decimation
BW Filtering
Improving Vertical Resolution
ı Benefits of improving vertical resolution
Increases resolution up to 16-bits
8-bit = 256 levels
10V full screen = 39mV
16-bit = 65,536 levels
10V full screen = 152uV
ı There are potential drawbacks
Averaging:
Requires a repeatable waveform
High resolution:
Sample rate reduction
Unknown BW
Higher resolution signal is not seen by the trigger
Improving Vertical Resolution
ı BW Filtering
Doesn’t have drawbacks of averaging or high-res
No sample rate reduction
Known BW
Higher resolution signal is seen by the trigger
Quantization steps clearly visible. “Hidden” low level signal becomes visible.
Signal characteristics can be measured.
Improving Vertical ResolutionLab
ı Small Demo Board
ı Channel 1 to I2C_SCL
ı Preset
ı Autoset
ı Turn sample rate to 10GS/s
ı Note small glitch to left of trigger
ı Go to HD Mode
Set to 3MHz and 16bits
ı Draw a zoom box that covers just
the glitch and the current edge we
are triggering on
ı Go to trigger mode and change
hysteresis to zero
ı Adjust to trigger on rising edge of
glitch
Vertical System
ADC Acquisition
Processing
Display
Trigger
SystemHorizontal
System
Att. Amp
Amp
Sampling Methods & Acquisition Modes
Memory
Post-
Processing
42
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
43
Sampling Methods - Effects of Different Sample
RatesA 10 kHz
Sine Wave SignalNyquist/ Shannon
The sampling rate
must be:
iS ff 2Sf
Sf : sampling frequency
if : frequency of the
input signal
Input Signal: 10kHz Sine Wave
Sampling Rate: 200kHz
Sampling Rate: 50kHz
Sampling Rate: 25kHz
Sampling Rate: 12,5kHz
Workshop: Affects of Aliasing
ı Connect to the RF OUT signal (50 ohm coupling)
Add a frequency measurement to verify 825MHz sine
wave
Set the horizontal scale to 1us/div
Lower sample rate to 5GSa/sec, then 2GSa/sec and
observe the frequency measurement
45
Acquisition Summary – Best Practices
ı Beware of Autoset – sample rate may be too low
ı Adjust record length first, sample rate second
Sample rate maintains signal fidelity. As you adjust the sample rate
lower, you lose signal detail (especially bad with transitions/edges)
ı Faster update rate increases chance of catching signal details or
glitches
ı Screen refresh rate is much slower than the update rate
50
Agendaı System Bandwidth Definitionı Probing Basicsı RTO Tour Workshop Passive probe compensation Ground lead effects De-skewing probes
ı Vertical System Overview Workshop Channel input coupling Effective use of vertical scale
ı Sampling & Acquisition Workshop Acquisition Rate
ı Horizontal Systems Workshop Horizontal measurements
ı Trigger System Workshop Edge Trigger Runt Trigger
51
Horizontal System
Vertical System
ADC Acquisition
Processing
Display
Trigger
SystemHorizontal
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
52
Sampling Rate
Record LengthResolution
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 SystemBuzz Words
53
Horizontal System Memory
ı Purpose of Memory
Every sample has to be stored in acquisition memory
Deeper memory of course stores more samples
Longer periods of time captured means more samples to store if
sample rate wants to be maintained (better signal reproduction &
zoom)
1 0 1 1 1 0 0 1
Sampling Digitizing
(Sample & Hold)(Convert to
Number)
1 0 1 1 1 0 0 1
10111001
11110101
(SequenceStore)
Scope Screen
Memory Storage
…
54
Workshop: Horizontal Measurements
ı Understand the sample rate of the scope to ensure the
measurement is accurate
Use passive probe on probe compensation and take long
acquisition
Measure rise time
Zoom in and see how many points are on the edge.
Increase sample rate and check the rise time again
(should be around 1.5ns)
Ch1/Acquisition/Resolution Tab. Adjust Record Length
Limit to 20Msa and Sample Rate to 2Gsa/s
Affects Most Measurements
56
Horizontal Summary – Best Practices
ı More record length and faster sample rate together expand the length of time
scales you can view
Higher memory results in longer time capture
Higher sample rate results in shorter time scales
ı Autoset does not give you the optimum sample rate
Minimizes record length for a given time scale
ı Higher sample rate and deep memory advantages: Increased signal fidelity (more accurate signal reproduction)
Better resolution between sample points
Higher chance of capturing glitches or anomalies
Observe high frequency noise in low frequency signal
Capturing of longer time periods while maintaining resolution (fast sample rate)
Agendaı System Bandwidth Definitionı Probing Basicsı RTO Tour Workshop Passive probe compensation Ground lead effects De-skewing probes
ı Vertical System Overview Workshop Channel input coupling Effective use of vertical scale
ı Sampling & Acquisition Workshop Acquisition Rate
ı Horizontal Systems Workshop Horizontal measurements
ı Trigger System Workshop Edge Trigger Runt Trigger
58
Trigger System
Channel
Input
Vertical System
ADC Acquisition
Processing
Display
Trigger
SystemHorizontal
System
Att. Amp
Amp
Memory
Post-
Processing
59
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
60
Trigger Accuracy
ı Key accuracy parameter:
Minimum detectable glitch (a small signal spike):
what is the smallest pulse that can be triggered on
typically [ps]
Sensitivity:
minimum voltage amplitude required for valid trigger
typically [mV or div]
Jitter:
timing uncertainty of trigger,
determines smallest measurable signal jitter
typically [ps rms]
61
Trigger Types (I)
ı Edge Trigger is the original, most basic and most common trigger type
triggering is executed once a signal crosses a certain threshold
rising edge falling edge rising and falling edge
62
Workshop: Trigger Basics
ı Runt Pulse Example
Set the demo board to mode 9. Runt 100/s
Use passive probe on RARE_SIG
Press Trigger/Type=Window/Vertical condition=stay within//time
condition=longer/Width=50ns
Adjust upper and lower trigger limit until runt is visible in wave-form
Change board mode to 0. Runt 1/s What is happening?
Change trigger mode between Auto and Normal – observe what
happens
Advanced
While Auto triggering, use digital filter and observe change in signal.
Set to 1MHz and apply to trigger. Observe what happens. Increase
filter BW and observe changes
65
Trigger Summary – Best Practices
ı Advanced triggers can more accurately represent the signal on the scope
Protocol triggers can capture specific addresses, values
ı Trigger sensitivity is difference from the acquisition sensitivity
May be able to see something but not trigger on the event
Digital triggering eliminate this problem
ı Auto trigger mode acquires waveforms even if a trigger event is not present.
Normal mode will wait until a trigger event happens
Normal mode is very useful for advanced triggers
67
High Definition Mode Summary – Best Practices
ı Uses oversampling and low pass filtering to enhance ADC resolution
Superior to other methods since it does not require a repetitive signal and
does not reduce sample rate
ı Minimizes waveform distortion
ı Combined with digital triggering, can trigger on very small signal changes
ı Great measurement technique for:
Small current levels in low power devices
Measuring dynamic on resistance of transistors
Low speed serial buses – prevents
68
THANK YOU
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