HP HP 8712ES, User Manual, Page: 121 / 507

ES User’s Guide 3-69
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
for and make a basic phase-derived delay (group delay) measurement.
An S
21
measurement is demonstrated in this example, but the same
general concepts apply for an S
12
measurement.This example uses a
two-port calibration, but other calibration choices are also available. For
example, a response calibration could be used for this measurement.
Though not quite as accurate as two-port error correction, a response
calibration offers the advantage of faster measurement speeds than
when using a two-port calibration. In this example, an insertable
bandpass filter like the one that was supplied with your network
analyzer is used.
∆φ
–
∆
f
( )
360
( )
-------------------------
frequency span
number of points 1 –
2 approx. DUT delay
( )
------------------------------------------------------
<