I have a signal that undergoes distinct 90 degree phase shifts but has an unpredictable frequency over a predefined range (anywhere from DC to 300MHz). From my limited knowledge on PFDs, they can only measure a phase difference between a reference frequency and another of the same frequency. Is there an analog way to detect those 90 degree phase shifts (like the ones that occur in BPSK modulation) and/or a method to measure the phase difference between two waves of different frequencies? Phase and frequency are linked hand-in-hand so I don't think the latter is possible.
I'd like to avoid having to use ADCs and digital signal processing if possible.
Electronic – Analog method for measuring distinct phase shifts
phase shiftRFsignal processing
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I've tried a similar setup and I've realized that you're using a very small sweep time (500 us). Your transient, that seems so wide in the picture, last only about 30 us. When using a quasi-peak detector it will be certainly removed.
I can show you my pictures. In my setup I've used a 10 dBm signal, 868 MHz central frequency, and an offset of 200 kHz when looking for the transient. In all the figures RBW is 120 kHz and VBW is 100 kHz.
Here it is the zero-span picture, no PA ramp is used:
My sweep time is 5 s. The pulse is then about 0.8 seconds wide, and the transient is less than a few ms (didn't measure it accurately).
When using a Quasi-Peak detector I get this:
If I repeat both measurement after activating PA ramp in the transmitter, the transient peak disappears:
And using a Quasi-Peak detector:
Conclusions:
Your phase noise is in fact fine, and so are the transients. The fact that the transient doesn't change when enabling PA ramp seems to indicate that there is some kind of implementation error with this feature. Anyway your transient will strongly smooth out under a Quasi-Peak detection.
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Best Answer
Phase is a difference in time between two things. Like voltage it is measured between two things. A phase detector is therefore a device that takes two signals and gives you the phase between them. It will always take two inputs for the same reason a voltmeter takes two inputs.
If two signals have different frequency, then the phase difference between them increases at 2PI times the difference in frequency between them per second. That isn't what you're looking for, it'll just be a constantly increasing signal with occasional extremely tiny changes in slope when the phase of one signal changes.
Usually you have a reference clock or something similar that you compare against to see if phase has changed.