Audio Filter Improvements

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 10/06/2020 at 11:570 Comments

As seen in the last project logs/updates, the module is capable of demodulating and outputting audio FM and AM signals. The audio quality is OK, not very good due to the way the DAC works and the not very good filtering of it. Over the last few days I researched different filter typologies to improve the audio quality and I tested a new one that uses the same circuitry as the previous filter, that is already present in the Module.

The previous filter used was a double RC Low Pass Filter, as can be seen in the figure bellow on the left. The new one tested is a double LC Low Pass Filter where the resistors of the RC filters are exchanged with inductors, as can be seen in the figure bellow on the right.

The double RC filter is a second order low pass and the new tested LC filter is a fourth order low pass, increasing the DAC filtering substantially. Bellow is a figure comparing the frequency response of both filters with ruffly the same cut-off frequencies:

Tests showed that the new LC filter improved the Audio quality as can be seen on the figure bellow where the Audio Output of the Module is shown, when in FM Demodulation mode. This can be compared with the previous project log, "FM Audio Demodulation", figure. The test conditions were the same.

The improved Audio quality is also audible, a sample is available on the website, but it has now a lower pitch/tone.

On the website a more in-depth comparison between the filters used, and two others tested before, was also added.

Besides this I'm working on a better command interface for the Module, inspired by the CAT command interface used by some HAM Radios like the ones from YAESU and Kenwood. It will not be compatible as it is a very different kind of device but the syntax is the same, eg. Setting the VUHF Radio Frequency command is: "FA145895000;"