Waveform 2.1

Basic Digital Waveforms 2 Measurement

Paul GodinUpdated October 2009

Waveform 2.2

Measurement Tools

◊ Several instruments are used to analyze Digital Electronic circuits, including:◊ Logic Probes◊ Oscilloscopes◊ Logic Analyzers◊ Tracers◊ Function generators

Waveform 2.3

Logic Probes

◊ Logic Probes display static logic states at specific points on a digital logic circuit. These states include:◊ High◊ Low◊ Float

◊ Logic probes are easy to use.

Waveform 2.4

Logic Probes

◊ Logic Probes are usually connected to the power supply of the circuit under test.

◊ Some logic probes can also detect the presence of an oscillation.

◊ Related to probes are:◊ Current tracers, used to detect the location of a short

circuit in digital circuits. Works with induction.◊ Pulsers, used to inject an oscillation into the circuit to

trace its route.

Waveform 2.5

Logic Probes - Limitations

◊ Logic probes are designed to measure active circuits only.

◊ Logic probes provide the static logic states and cannot measure:◊ Oscillation values (time high / time low)◊ Voltages◊ Transients (short-lived problems)

Waveform 2.6

Oscilloscope

◊ Oscilloscopes are a common electronics instrument used to measure voltage and frequency.

◊ Oscilloscopes can also be used to compare signals. Most oscilloscopes have at least 2 channels.

Waveform 2.7

Oscilloscope

◊ Oscilloscopes work best with periodic signals.◊ Some more advanced oscilloscopes can:

◊ measure aperiodic signals◊ retain measured values in memory◊ have advanced analysis capabilities

Waveform 2.8

Oscilloscope - Limitations

◊ Oscilloscopes measure active circuits only

◊ Expensive

◊ Most are not very portable and require an AC power source

◊ Requires training and practice to use properly

Waveform 2.9

Waveform Measurement

90%

10%

50%

Rise Time

(tR)

Fall Time

(tF)

Pulse Separation

Ps

Pulse Width

Pw

Period (T)

Rise and fall times are typically measured in nanoseconds (ηs)

Amplitude

Waveform 2.10

Oscilloscope Measurement

◊ Measuring with a scope – Static logic state

1

1

Circuit Ground

1

Set input to DC

Pro

be

5V

Waveform 2.11

Oscilloscope Measurement

◊ Measuring with a scope - Oscillation

1

1Set input to DC

Circuit Ground

Pro

be

5V

Waveform 2.12

Oscilloscope Measurement

◊ Several controls need to be adjusted when measuring with the scope:◊ Volts/Division: controls the displayed voltage◊ Time/Division: controls the displayed time◊ Trigger: controls the trigger point for a periodic

signal. Must be set within the signal’s amplitude.◊ Coupling: should be set to DC (Direct Coupled) when

measuring digital values◊ Horizontal and Vertical position: used to set the

position of the ground reference on the display

Waveform 2.13

EWB Oscilloscope

Normal View with 4 connections

Ground must be connected

Semi-expanded view

Connections correspond to Normal View

Waveform 2.14

EWB Oscilloscope – Expanded ViewCursors used for

measurement. Values between cursors in box below

Position of Red Cursor

Position of Blue Cursor

Adjustable Settings

Waveform 2.15

Exercise: EWB Oscilloscope

Class activity:

◊ Open the EWB file titled scope1 on the site.◊ Use the scope in EWB to measure the output

waveforms and complete the worksheet on the next slide.

Waveform 2.16

Oscilloscope Worksheet

Signal A:◊ Period: ___________

◊ TH (time high):_____

◊ TL (time low):_______

◊ Duty Cycle: ________

Signal B:◊ Period: ___________

◊ TH (time high):_____

◊ TL (time low):_______

◊ Duty Cycle: ________

Signal C:◊ Period: ___________

◊ TH (time high):_____

◊ TL (time low):_______

◊ Duty Cycle: ________

◊ TR (rise time): ______

◊ TF (fall time): ______

Waveform 2.17

Logic Analyzer

◊ Logic analyzers display multiple dynamic (changing) logic states in a format that resembles a timing diagram. Very useful for analyzing more complex digital circuits.

◊ Typical analyzers have 8 or 16 channels.

Waveform 2.18

Logic Analyzers - Limitations

◊ Logic analyzers display logic states, not voltage values. They may not display poor edges or other similar electrical faults.

◊ Expensive

◊ Most are not very portable and require an AC power source

◊ Requires training to use properly

Waveform 2.19

EWB Logic Analyzer

Circuit connectionsExpanded view

Internal Clock Set

Red Cursor

Blue Cursor

Cursor Position

Waveform 2.20

Exercise: EWB Logic Analyzer

◊ Class activity:

◊ Open the EWB file titled analyze1 on the site.◊ Use the Logic Analyzer in EWB to measure the

PW of the 3 waveforms and record below.

Signal A PW: _________

Signal B PW: _________

Signal C PW: _________

Waveform 2.21

Function Generators

◊ Function Generators generate AC voltages, with control over output:◊ Frequency◊ AC voltage◊ DC offset voltage◊ Wave shape (typically Sine, Triangle, Square)◊ Duty Cycle

◊ Some function generators have more features or are more specialized. Example includes generating RF frequency and waveforms.

Waveform 2.22

Function Generator Use

◊ Function Generators are used to inject signals into circuits to analyze their function.

◊ Failure to configure the output of the function generator before connecting it to the circuit will likely result in circuit damage. Check:◊ Output AC voltage◊ Output DC offset voltage (no negative voltage)

Most ICs will be damaged if negative voltage is applied.

Waveform 2.23

Ohmmeters

◊ Most ohmmeters produce a high enough voltage to damage logic circuits and should never be used in a digital electronics environment.

Waveform 2.24

END