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

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Basic Controls

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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

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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

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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

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THANK YOU

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