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Why no fixed phase difference with doppler RDF observed?


From: Robert Heerekop
Subject: Why no fixed phase difference with doppler RDF observed?
Date: Sun, 22 Dec 2024 14:49:15 +0100

Hello Radio Friends,
I wonder why I don't observe a fixed phase difference between a demodulated 500Hz doppler FM-signal and a fixed GNR 500Hz sine source.
The setup is a USB-RTLv3 in a RaspberryPi. The RTL-oscillator is divided by a (hardware!) synthesizer chip and drives the antenna rotation which results in a 500Hz  audio-tone.
Due to the fact that the rotation of the antenna is based on the SDR-clock I expected that both 500Hz signals would have a fixed phase difference.
I think however I make a fundamental mistake and the 500Hz GNR sine-source is not related to the sample rate of the incoming IQ-stream originating from the SDR...

Question: What is your opinion on this and can you provide guidance how to sync the 500Hz GNR sine-source to the incoming IQ-stream?


Background information:

- The GNRsetup is running on the RaspberryPi4:
  In the flowgraph the received 500Hz audio signal from the NB-FM receiver is displayed in a Time Sink together with a second fixed 500Hz source.
  video: https://youtu.be/OxQMvt5ILUY (grc-code and GNR-status messages in the shared folder)

- Validation of the doppler effect occurring:
  The pseudo doppler effect is validated and demonstrated with a SDR++ recording (below the video of the test and the related base band IQ recording)
  In this recording the RPi runs RTL_FM. An ethernet connected Windows computer runs SDR++ and processes and records  the IQ-stream.
  video: https://youtu.be/kwGM5PabdAA (recorded IQ-stream is in the shared folder)

Shared folder with recorded IQ-stream, GRC-code and high-resolution videos: https://tinyurl.com/34zv37x4

Thanks for your guidance!

Robert

ps. As a rookie (still in the steep gnr learning curve) I did read the tutorial pages.

Attachment: Flowgraph.PNG
Description: PNG image


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