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FM Demod Flowgraph - Add Low-Pass filter prior to Demod

Mark Jessop requested to merge darksidelemm/gr-satnogs:fm_filter into master

With the move to separate demods for FSK9k6, and soon AFSK1k2, I figured it about time to submit a merge request for this.

The change is simple: Add a complex low-pass filter prior to the Quadrature Demodulator block, which effectively acts as a 16 kHz wide (based on deviation) channel filter around the wanted signal.

What this does:

  • Reduces the de-sensing of the quad-demod block by other carriers within the current receive passband (48 kHz!!). This can be a significant issue in suburban areas.
  • Improves output SNR, due to less noise entering the demod.

Examples:

Downsides:

  • The receiver must now be fairly well on-frequency. There is a 3 kHz transition bandwidth around the 16 kHz filter, so there's still a bit of wiggle room.
  • The DB needs to define the frequency of the transmitter fairly accurately. For the sats this demod would generally be used with (FM transponder sats, ARISS APRS (for now)) this is already the case.

For your consideration.

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