China Electronics Technology Instruments AV3672 Series, User Manual

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8 Basis of Network Measurement
8.8 Linear Phase Deviation
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this case, the obvious phase shift will be produced. The phase shift is linear when the phase response of the device
is proportional to the frequency. The trace indicating the relationship between the phase and frequency on the
analyzer is an oblique line. The slope is proportional to the electrical length of the device. The phase shift must be
linear for signal transmission without distortion.
8.8.2 What is linear phase deviation?
In the actual cases, the delay time of many devices corresponding to some frequencies is longer than that
corresponding to the other frequencies so as to form the nonlinear phase shift, including distortion of many
frequency component signals. One method to determine the nonlinear phase shift is to measure the linear phase
deviation.
As phase distortion is caused by linear phase deviation, the linear part of the phase response should be removed in
measurement. This can be realized by the electrical delay function of the analyzer. If the electrical length of the
DUT is removed mathematically, the linear phase deviation or phase distortion will be left.
8.8.3 Why to measure the linear phase deviation?
Linear phase deviation measurement has the following advantages:
a) The measurement result is the phase data, instead of the group delay data in second. The phase data may be more
usable for the device transmitting modulation signals.
b) Provide a measurement method with noise lower than that of group delay measurement.
8.8.4 Use of Electrical Delay Function
The electrical delay characteristic of the analyzer has the following functions:
1) Simulate the transmission line of adjustable length and no loss to facilitate the installation in or removal from the
signal path.
2) Compensate changes of the electrical length of the DUT.
3) Make the phase measurement trace on the analyzer flat, to realize the high-resolution observation of the trace
and find nonlinear details of the phase.
4) Provide a method to easily observe the linear phase deviation of the DUT.
Fig. 8.20 Observation of Linear Phase Deviation by Electrical Delay Function
See electrical delay details in “6.5.1 Electrical Delay”.
8.8.5 Factors Related to Measurement Accuracy
The frequency response is an error which plays a dominant role in linear phase deviation measurement. The
through type response calibration can be applied to significantly reduce the error. In order to achieve higher
accuracy, the full double-port calibration should be done.
8.8.6 Steps of Linear Phase Deviation Measurement
1) Reset the analyzer. Adjust the source power of the analyzer to test the amplifier.
a) Set the source power of the analyzer to make the amplifier working within the linear zone (generally at least
10dB less than the 1dB compression point).