China Electronics Technology Instruments AV3672 Series, User Manual

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8 Basis of Network Measurement
8.2 Phase Measurement
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Similar to the amplitude measurement, the S-parameter is also applied in phase measurement. Phase measurement
is a kind of relative (ratio) measurement instead of absolute measurement. The phase of the signal into the device
(incident signal) and the response signal are compared in phase measurement. The response signal may be reflected
signal or transmitted signal. Given that the analyzer is accurately calibrated, the phase difference (i.e. phase shift)
of two signals is the measurement result of phase characteristics of the DUT. The following figure shows the phase
deviation between the incident signal and transmitted signal observed through the oscilloscope.
Fig. 8.7 Phase Shift between Signals
8.2.2 Why to perform phase measurement?
Phase measurement is an important function of the vector network analyzer. Fig. 8.8 shows the reasons for accurate
measurement of the amplitude and phase.
Fig. 8.8 Reasons for Accurate Amplitude and Phase Measurement
If the element and circuit are used to transmit signals in the communication system, signal distortion must be within
the normal limits. The signal distortion can be divided into two types.
1) Linear distortion: refer to the failure to keep the flat amplitude and linear phase shift within the relevant
frequency range as a result of frequency changes.
2) Nonlinear distortion: refer to the new spectrum component of the circuit, such as AM-PM transformation.
It is important to accurately measure the amplitude and phase characteristics of the element or circuit, so as to
ensure the efficient transmission or adsorption of energy in the circuit and prevent the distortion in signal
transmission. A good example to prevent distortion is to measure the complex impedance of the antenna.
8.2.3 Use of Phase Format of Analyzer
The phase format of the analyzer is used to display the phase changes over the frequency or power. The phase
difference beyond ±180° between the reference signal and response signal cannot be measured by the analyzer. If
the phase value varies from +180° to -180°, the sawtooth type phase measurement trace will be displayed on the
analyzer. The sawtooth waveform may not reach +180° or -180° sometimes in measurement at the discrete
frequency points. The data points of +180° and -180° may not be the points of sweep measurement.