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FM Audio Demodulation

A project log for VUHF Radio

A dual band VHF/UHF module based around two AX5043 radios. Built to work with HAM Radio communications using AFSK and G3RUH modulations.

NotBlackMagicNotBlackMagic 09/02/2020 at 20:040 Comments

Analog FM demodulation, and modulation, is a unique and officially supported special feature of this transceiver. ON Semiconductor even provides an application note on how to set up and use the transceiver as an analog FM transceiver, "AX5043 Use as Analog FM Transceiver". Both the application note and the configuration used are for Wideband FM signals, the type used in FM Radio Broadcast. FM Radio Broadcasts were actually used to test the FM demodulation with good success!

As with the AM Demodulation, the basis of the FM Demodulation is the internal DAC. For FM Demodulation the DAC is setup to output the Frequency Tracking value. With the transceiver setup properly, the FM signal is first down-converted into the IF stage where the frequency change over time is tracked and output by the DAC. This tracking information is also accessible over the SPI interface of the transceiver.

To show the FM modulation in the IF stage, the DAC was configured to output the SAMPLE_ROT_I (or SAMPLE_ROT_Q) and a RF signal generator connected to the module is used to generate the FM signal, using as the modulating source a pure 500 Hz tone. In the figure bellow the FM signal in the IF stage is shown in blue and the pure 500 Hz tone is shown in yellow.

With the DAC setup to output the frequency tracking value, the FM demodulation, the signal observed is shown in the figure bellow. In blue is the FM demodulated signal and in yellow the original audio signal used as the modulation source.

As with the AM demodulation, a lot of noise is visible on the demodulated signal. This noise is also very clearly audible in the audio output of the module, through the audio jack. The main source of this noise is from the not well filtered DAC output.

Samples of the output audio can be listened to on my website, as well as all the configurations used to achieve the FM demodulation.

A little teaser on what I'm testing and working on for this project can be seen on my twitter, very early stages still.

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