Audio AM 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 07/26/2020 at 20:150 Comments

This very unconventional use of the AX5043 transceiver was triggered by a question posed to me by a fellow Hackaday member, if the transceiver could be, somehow, used as an Audio AM receiver. Over the past week I explored this idea and it turns out that it actually can!

The basis for this is the very versatile DAC output of the transceiver which, among others, can output the amplitude tracking information of the signal at the entrance of the demodulator block. With the DAC set up to do this, the transceiver has to be "tricked"/setup to keeping the amplitude modulated signal at the IF stage, where this tracking information is calculated. The signal at the IF stage is visible in the figure bellow in blue (DAC set to output either SAMPLE_ROT_I or SAMPLE_ROT_Q), in yellow is the original audio wave:

With all properly setup, the DAC outputs the demodulated AM signal, the desired Audio signal, as can be seen in the figure bellow. In blue is the demodulated AM signal as output by the DAC and in yellow the original audio wave:

It can be seen that the demodulated signal is very noise. The main source of this noise is the not well filtered DAC output, even after a second order low pass filter. The transceiver uses a Sigma-Delta DAC. Even with this noise the performance is much better then expected, and by plugging in a headphone to the audio jack of the Module one can listen to AM audio transmissions!

More information on the set up used to achieved this as well as an audio sample file output by the Module are available on my website.