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Coherent Phase Measurements of Individual VNA Receivers (N5242A)

Question asked by euler314 on Sep 27, 2012
Latest reply on Sep 28, 2012 by euler314
In my application I would ideally ratio several "simultaneous" receiver phase measurements, but I am unable to do so because the signals whose phases I am interested in are at different frequencies and I do not have the ability to independently tune the receivers in the N5242A (Opt. 10/80/83/400/419/423) during a single sweep. After reading that this particular model of PNA has the ability to make coherent single-channel phase measurements I thought it might be possible to make unratioed receiver phase measurements at each frequency of interest over several sweeps and then ratio them afterwards. As a way to verify that these unratioed measurements behave as I expected them to or thought they should I decided to compare the results from measuring the group delay of a non-frequency translating device using ratioed and unratioed measurements.

The first measurement just consisted of deriving the device's group delay from an S21 measurement across some band of interest. In the second measurement I used the N5242A to generate a two tone stimulus with a fixed frequency separation (much larger than the IF BW of the N5242A) at the device's input that was then swept across the same band of interest. The phase of each tone was measured at the output of the device with independent sweeps using unratioed receiver measurements. Group delay was then approximated using the phase difference of the unratioed measurements and the frequency separation of the two tones. The resulting group delays did not match. Should these measurements have yielded similar results or do I have some misunderstanding as to what is meant by coherent single-channel phase measurements? Thank you for your time.