HP HP 8712ET, User Manual, Page: 105 / 480

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
Accuracy,” 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
( )
------------------------------------------------------
<