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April 9, 2020 at 8:29 am #875PW_PlackParticipant
Hello all. I’m new to SDR, using a just-acquired RSPdx, and have a question about detecting the difference between the receiver’s set frequency and the actual frequency of an arriving signal. My specific mode of interest is NFM.
On legacy analog FM receivers wide and narrow, a voltage developed in the discriminator (detector circuit for FM) could be displayed on a meter, showing how far off the tuned frequency the actual arriving signal was. On old hi-fi FM broadcast tuners, this was often featured on the front panel as a tuning aid.
In theory, if two GPS-locked receivers located in different parts of a city were receiving the same mobile station, comparing the frequency errors from the two receivers could detect whether the mobile user was in motion. A third receiver could allow accurate positioning and speed computations based on Doppler Effect. This might allow event or emergency managers to have automatic updating of the location and status of assets, all without asking. If combined with transmitter “fingerprinting,” software could even automatically label moving dots on a map!
I’m using SDRuno, and do not see any way to detect or display this frequency error, other than hearing distortion if the error is so bad it crowds the edges of the bandpass filter. This idea would require very precise measurement of the relative errors among locked receivers. Am I the first to ask? Thanks! – 73, Paul AE4KR
April 10, 2020 at 8:44 am #879Andy2Participant
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Hi Paul. Not sure if it’s what you want, but you can click on the S-meter and it will turn into a centre zero meter. I don’t think it actually quantifies the error, but neither did the ones on the old analogue radios.
There’s also the synchronous AM mode which (I think) gives an error reading. I haven’t used my RSP2 or Uno for a little while so forgive me if I’m a bit rusty!April 10, 2020 at 8:58 am #880Andy2Participant
Just a further thought – you can read-off any fr error from the small SP2 spectrum display as well. As long as your RX is properly calibrated/locked, you can see very small errors with this method.April 11, 2020 at 3:36 am #883PW_PlackParticipant
Rick, thanks much. The instruction about clicking on the S-meter to change it to a center-tuning meter was buried in a paragraph about signal strength standards in the manual, and I had missed it on the first pass.
I’ve seen the error displayed in synchronous AM, but I need it for NFM. The center-tuning meter shows it clearly, now that I know how to invoke it, but I’d need a way to actually quantify it with high resolution, and export it as data somehow. Alas, this may have to wait for a plug-in from someone with skills I don’t yet have.
73 – Paul AE4KR
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