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8 Basis of Network Measurement
8.7 AM-PM Transformation
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b) Sudden pulse modulation.
AM-PM transformation is generally defined as the output phase change in radian/dB, corresponding to 1dB
increase of the sweep power applied on the input end of the amplifier (at the 1dB gain compression point). The
phase response of the ideal amplifier is not associated with the power level of the input signal.
8.7.2 Why to measure AM-PM transformation?
AM-PM transformation is a key parameter in the following phase (angle) modulation system.
1) FM
2) QPSK
3) 16QAM
The unexpected phase deviation (PM) will result in deterioration of the analog signal performance or increase of
the bit error rate (BER) of the digital communication system. It is easy to measure the bit error rate (BER) of the
digital system, but this is not helpful to understand the root causes of bit errors. AM-PM transformation is one of
the main factors resulting in bit errors. Therefore, it is critical to quantify this parameter in the communication
system. The following I/O figure shows why bit errors are produced in AM-PM transformation.
Fig. 8.18 Bit Errors in AM-PM Transformation
1) The expected state change is from the small solid-line vector into large solid-line vector.
2) As a result of AM-PM transformation, the large solid-line vector may end like that shown by the dotted line in
the actual situation. This is caused by the phase shift arising from changes of the input power level.
3) For the 64QAM signal (only the quadrant is drawn) shown in Fig. 8.18, bit errors occur in the statistics if the
noise circles of each state are overlapped.
8.7.3 Factors Related to Measurement Accuracy
When the network analyzer is used to measure AM-PM transformation, the frequency of the amplitude modulation
signal is approximate to the reciprocal of the sweep time. Even if the highest sweep power is set, the modulation
frequency is low (generally less than 10Hz). This may result in slight temperature changes of the DUT in the sweep
process, especially the thermal mass of the amplifier is small (typically for the unsealed device). If the nonlinear
characteristic of the amplifier is sensitive to thermal changes, the measurement result obtained in this method may
have errors. In order to improve the measurement accuracy, considerations should be given to the following factors.
1) The amplifier response may vary with the temperature. The test must be done at the working temperature of the
amplifier.
2) If necessary, fully attenuate the output power of the amplifier. Too high power may lead to the following results:
a) The output power is higher than the input compression level of the receiver of the analyzer, which may lead to
inaccurate measurement results.
b) The receiver of the analyzer may be burnt.
3) Attenuate the output power of the amplifier with the attenuator or coupler.