Download PPT: Waveforms 2

Basic Digital Waveforms 2
Measurement
Paul Godin
Updated October 2009
Waveform 2.1
Measurement Tools
◊ Several instruments are used to analyze Digital
Electronic circuits, including:
◊
◊
◊
◊
◊
Logic Probes
Oscilloscopes
Logic Analyzers
Tracers
Function generators
Waveform 2.2
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 2.3
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 2.4
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 2.5
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 2.6
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 2.7
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 2.8
Waveform Measurement
Period (T)
Pulse Separation
Pulse Width
Ps
Pw
90%
Amplitude
50%
10%
Rise Time
Fall Time
(tR)
(tF)
Rise and fall times are typically
measured in nanoseconds (ηs)
Waveform 2.9
Oscilloscope Measurement
◊ Measuring with a scope – Static logic state
Probe
5V
Circuit Ground
1
1
1
Set input to DC
Waveform 2.10
Oscilloscope Measurement
◊ Measuring with a scope - Oscillation
Probe
5V
Circuit Ground
1
1
Set input to DC
Waveform 2.11
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 2.12
EWB Oscilloscope
Semi-expanded view
Connections correspond
to Normal View
Ground must
be connected
Normal View with 4
connections
Waveform 2.13
EWB Oscilloscope – Expanded View
Cursors used for
measurement. Values
between cursors in box below
Position of Red Cursor
Position of Blue Cursor
Adjustable Settings
Waveform 2.14
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 2.15
Oscilloscope Worksheet
Signal A:
Signal B:
◊
◊
◊
◊
◊
◊
◊
◊
Period: ___________
TH (time high):_____
TL (time low):_______
Duty Cycle: ________
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 2.16
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 2.17
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 2.18
EWB Logic Analyzer
Circuit connections
Expanded view
Red Cursor
Blue Cursor
Cursor Position
Internal Clock Set
Waveform 2.19
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 2.20
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 2.21
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 2.22
Ohmmeters
◊ Most ohmmeters produce a high enough
voltage to damage logic circuits and should
never be used in a digital electronics
environment.
Waveform 2.23
END
Waveform 2.24