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

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

Choosing an Oscilloscope

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.

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

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

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

RTE Tour

The RTE

Some Favorite Buttons

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)

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

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.

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)

Our Target

ı Small Digital Stimulus Board

Toggle

Mode Back

Flooded

Areas are

Ground

Probing Basics

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

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

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.

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

Probe Options

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

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

Vertical System Overview

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

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 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

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

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

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

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

{

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

Demo: Vertical Scale

ı Examine quantization errors introduced by using only half

the ADC

Affects Most Measurements

Sampling and Acquisition

Vertical System

ADC Acquisition

Processing

Display

Trigger

System Horizontal

System

Att. Amp

Amp

Sampling Methods & Acquisition Modes

Memory

Post-

Processing

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

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.

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)

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.

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!

Benefit of High Capture Rate Glitch Capture Probability vs. Test Time

ı Glitch Capture Probability

ı Test time decreases

tremendously with

higher acquisition rates

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.

Horizontal System

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

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

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

Trigger System

Trigger System

Channel

Input

Vertical System

ADC Acquisition

Processing

Display

Trigger

System Horizontal

System

Att. Amp

Amp

Memory

Post-

Processing

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

Types of Triggers: Runt Trigger

3/30/2014 FAST: Advanced Triggering

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.

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.

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?

Other things a scope can do:

EMI Debug

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

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

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.

THANK YOU