Sounds like you have a difficult task ahead of you. If you want to compare
phases of the outputs of 2 receivers you must use the same oscillators. Unless
there is a way to sync. the DDS references and the PLL oscillators they may
start up at any phase even though they may be the exact same frequency. You
could use a standard signal to inject to both receiver inputs at the same time
after a frequency change to check the phase shift between the receivers and use
that to correct your measurement.
Also it is best take the measurement at a single frequency, the same as you
reference. If you are measuring wide bandwidth you will need to sweep the
entire bandpass and normalize the whole set of data.
The results are quite unpredictable and a reference sweep may will solve that
problem. But that will require a programable reference source or a harmonic
(comb) generator.
Lots of things can change the phase shift (delay) thru a receiver. Phase
shift often changes with RF or IF gain changes. This is due to the fact that
gain changes are (unless switched attenuators are used) by changes in some
devices impedance. It does not matter if it is pin diodes or semiconductor
devices ( such as differential amplifiers ).
Filters also will change noticeably with temperature as well.
Also you may want to intercept the signal before the digital signal
processing. There is no telling what the DSP IF will do to phase. Fortunately
this is done at a low frequency and you may be able to do in your PC.
A useful tool is available free. Check out ?Spectrum Lab?. I have had a
great deal of fun with it and have used it to demonstrate various RF processing
principles in my classes.
http://www.qsl.net/dl4yhf/spectra1.html
If you have some questions I would be happy to help. I have delt with sort of
problem before.
73
Bill wa4lav
elay) thru a receiver. Phase shift often changes w
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