AM demodulation and carrier phase measurement of multiple signals simultaneously

[Ramsey_]

Hello All,

I need to extract and compare the carrier phases from a number of medium wave AM signals using a 5600 downconverter and a 5122e digitizer. The signals are all real off air broadcasts, and so are in the band 500kHz to 1500kHz. Since the transmitter frequency references may all drift I guess the most sensible way to do this is to filter and mix each signal down to the same IF and track their relative phases. However I'm not sure what clever tricks can be done with current Labview vis. I've had a dig through what I have (I have the modulation toolkit amongst others) but can't use the features based around the 5660 due to my hardware. I can see the signals where they should be on a spectral plot, but have not had much luck getting sensible looking results using mt_niscope_demodulate_am.vi. I am currently tuning the 5600 to 15MHz rather than attempting to tune it to central frequencies around 1MHz, I presmue this is the right thing to do? Could someone point me in the direction of some good starting point? Any suggestions welcome 🙂

Thanks
Ramsey

[Andy_Hinde]

Hello Ramsey,
I assume you are trying to capture these AM signals and compare their relative phases all against each other. Sort of like saying the AM 1300 carrier is X degress in phase different from the AM 1530 carrier.

This is what I would do. I would use the full PXI-5661 RF analyzer and capture the entire AM band in the acquisition. This is roughly a 1 MHz bandwidth. At this low bandwidth, a low IQ sample rate can be used with the PXI-5661 (1.25 MSamples/sec gives the PXI-5661 a 1 MHz IQ data BW). With this low IQ rate, you could actually stream the entire AM band to hard drive for some time.

Since you have recorded the AM band to disk for a period of time, you can extract all the relative phase information you want. And the receiver reference is the same for all acquired data so the only variation will be due to the Tx side.

Look at this example:
RFSA Acquire Continuous IQ.vi

located at:
C:\...\<LabVIEW>\examples\instr\niRFSA

Regards,
Andy Hinde
National Instruments

[Ramsey_]

Hi Andy

Thanks for your response, unfortunately I can't use any of the RFSA stuff because I have a 5122 digitizer, not a 5142 and so cannot combine it with my 5600 to create the pseudo 5661 - the drivers simply don't exist to my knowledge (if they do please let me know where they are!!). I can stream to disk with my current harwdare and process everything later in matlab but I was hoping to know something up that would work on the fly in labview - at least calculate all the relative phases and then perhaps output them to Matlab for further processing. I have had a look at using vis like SMT mag-phase for niScope but am not sure I'll be able to use that directly - I guess I'll have to build something that downconverts a few frequencies in parallel to the same IF and then compare the phases, I was just wondering if there were any vis already along the lines of "niscope extract and look at a few signals from the same capture simulaneously.vi"(!)

Cheers
Ramsey