waveform 2.1 basic digital waveforms 2 measurement paul godin updated october 2009
TRANSCRIPT
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