
ET User’s Guide
3-53
Making Measurements
Measuring Group Delay
Measuring Group Delay
The phase linearity of many devices is specified in terms of group delay.
This is especially true of telecommunications components and systems
where phase distortion is critical.
Group delay is a measure of transit time through the DUT as a function
of frequency. It is approximated by:
where
∆φ
is the phase difference between two adjacent frequencies
∆
f.
The quantity
∆
f is commonly referred to as the aperture. The minimum
aperture is equal to the analyzer's frequency span divided by the number
of points minus one, and can be entered as a frequency or a percent of
span. To measure group delay correctly, the phase difference at a specific
aperture must be less than 180 degrees, satisfying the following
relationship:
If this relationship is not satisfied, incorrect measurements will occur
since the measurement of the phase difference at adjacent points will be
undersampled.
This section uses an example measurement to describe how to calibrate
and make a basic phase-derived delay (group delay) measurement. In
this example, a bandpass filter like the one that was shipped with your
network analyzer is used.
TIP
An optimum calibration is critical for achieving best measurement
accuracy. Refer to
Chapter 6, “Calibrating for Increased Measurement
for detailed information about all of the various aspects of
calibration.
NOTE
Phase-derived delay cannot be used to measure frequency-translating
devices.
∆φ
–
∆
f
( )
360
(
)
-------------------------
frequency span
number of points
1
–
2 approx. DUT delay
(
)
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Summary of Contents for HP 8712ET
Page 8: ...viii ET User s Guide ...
Page 17: ...ET User s Guide 1 1 1 Installing the Analyzer ...
Page 34: ...1 18 ET User s Guide Installing the Analyzer Preventive Maintenance ...
Page 35: ...ET User s Guide 2 1 2 Getting Started ...
Page 52: ...2 18 ET User s Guide Getting Started Performing the Operator s Check ...
Page 53: ...ET User s Guide 3 1 3 Making Measurements ...
Page 118: ...3 66 ET User s Guide Making Measurements Measuring Impedance Magnitude ...
Page 119: ...ET User s Guide 4 1 4 Using Instrument Functions ...
Page 143: ...ET User s Guide 4 25 Using Instrument Functions Using Markers Figure 4 15 Delta Marker Mode ...
Page 149: ...ET User s Guide 4 31 Using Instrument Functions Using Limit Testing Figure 4 16 Limit Lines ...
Page 207: ...ET User s Guide 5 1 5 Optimizing Measurements ...
Page 226: ...5 20 ET User s Guide Optimizing Measurements Measuring Devices with Long Electrical Delay ...
Page 227: ...ET User s Guide 6 1 6 Calibrating for Increased Measurement Accuracy ...
Page 266: ...6 40 ET User s Guide Calibrating for Increased Measurement Accuracy Checking the Calibration ...
Page 267: ...ET User s Guide 7 1 7 Front Rear Panel ...
Page 292: ...7 26 ET User s Guide Front Rear Panel Line Module ...
Page 293: ...ET User s Guide 8 1 8 Hardkey Softkey Reference ...
Page 395: ...ET User s Guide 9 1 9 Specifications ...
Page 397: ...ET User s Guide 9 3 Specifications System Performance ...
Page 443: ...ET User s Guide 10 1 10 Safety and Regulatory Information ...
Page 449: ...ET User s Guide 10 7 Safety and Regulatory Information Regulatory Information ...
Page 450: ...10 8 ET User s Guide Safety and Regulatory Information Regulatory Information ...
Page 451: ...ET User s Guide 11 1 11 Factory Preset State and Memory Allocation ...