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AM-PM measurements

Question asked by ben00i on Apr 5, 2007
Latest reply on Apr 11, 2007 by ben00i
I am using Agilent's PNA-L N5230A network analyzer for my AM-AM and AM-PM measurements. I used a power meter for source and receiver calibration and followed through the measurement step accurately from Application Note 1408-7: Amplifier Linear and Gain Compression Measurements. Here is the problem. While the AM-AM data is fantastically accurate, my AM-PM measurements are totally out. I know this for a fact as at low power there will be not much absolute phase change across power. Approaching higher power the phase will slowly degrade and the rule of thumb of 3deg change would occur at the input power where 3% EVM occurs. So one would expect. with reasonable confidence, a flat phase response for a few dB and then a gradual increase in the phase as power is increase. This is not the response I am getting. In fact, I am getting a near linear phase increase starting at low power(-20dBm to 1dBm). This is with the power meter calibration. Now, the funny thing is that when I compare with the uncalibrated power, the AM-PM seems to make more sense. So, my confusion is which is the accurate AM-PM measurement, the uncalibrated or calibrated power? Could someone explain to me why this is happening and how the NA does AM-PM measurements with and without power calibration?