BLACK MASK: Here's some code for a project to modify old Mobira FM transceivers for FSK-based packet data or digital voice. Instead of using their FM discriminator to demodulate FSK, their 455 kHz intermediate frequency is digitized to use a DSP-based demodulator.

Wednesday, October 10, 2018

Here's some code for a project to modify old Mobira FM transceivers for FSK-based packet data or digital voice. Instead of using their FM discriminator to demodulate FSK, their 455 kHz intermediate frequency is digitized to use a DSP-based demodulator.

Sufficiently old models such as the Mobira RC25 have a varactor-modulated
crystal oscillator as the FM modulator. Fairly clean and simple FSK modulation
should be possible by feeding the baseband signal straight to the varactor.
More details on modifying the transceiver for amateur band:
http://oh3tr.ele.tut.fi/suomi/moppeakatemia/25.shtml#rc25
We will try to implement FreeDV 2400A to provide a simple alternative
to an SDR built completely from scratch.
More details on the mode:
http://www.rowetel.com/?p=5119
Some files here
digimoppe/digimoppe.ino: software running on an atmega328 Arduino board.
It transmits samples from ADC to UART and controls the modulator.
grc/: Some gnuradio companion flowgraphs for spectrum analysis.
pc/: Software that runs on Linux, communicates with the Arduino
and runs the modems.
https://github.com/tejeez/digimoppe