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I'm trying to measure correlation between two RF noise sources and as usual trying to do it on the cheep. I've purchased 2 SDRPlay RSP2pro radios. These operate from 500kHz or so to 2 GHz with sampling rates from 2 to 10 MHz. Not bad for $200 and they work well as radios. The RSP2s have in and out clock ports allowing the user to operate them off a common clock which is a 1.5Vpp signal at 24MHz. For my application I need the relative phase reference to be good to some tolerance (say ##\pm 5^\circ## I'm guessing). As an initial test I've driven the radios off of a common external 24MHz signal. The center frequency is set to 15MHz. A second signal generator is set to 15.1 MHz at 10mV and passed through ~80dB attenuator and teed into the input port of each. I collect IQ data from each radio. A 2048 sample wide window in FFT'd leading to a peak at 15.1MHz where I extract the amplitude and phase off each radio. With this data I can compute the relative phase between the two identical signals. What I get is a meandering relative phase between the two signals which has a rather wide swing.
My operating theory is this meandering phase is due to the Phase Lock Loop internal to each radio caring more for frequency than phase. I'd love to post the plot but it's not clear how to do this (I seem to be homepageless).
Ah, just tried the same experiment with the radios running internal clocks and the results are quite comparable.
So, I guess my basic problem is I'll need to measure the relative phase by injecting a synchronization signal, remove the synchronization signal from the data, correct the data and then do my correlation?
My operating theory is this meandering phase is due to the Phase Lock Loop internal to each radio caring more for frequency than phase. I'd love to post the plot but it's not clear how to do this (I seem to be homepageless).
Ah, just tried the same experiment with the radios running internal clocks and the results are quite comparable.
So, I guess my basic problem is I'll need to measure the relative phase by injecting a synchronization signal, remove the synchronization signal from the data, correct the data and then do my correlation?