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Controller and FM demodulation

A project log for X-UHF

X-UHF is a project to build a low cost RF prototyping system for experimenting in VHF and UHF bands.

Colin AlstonColin Alston 09/06/2018 at 20:080 Comments

With the modules I have working currently it was enough for a simple FM direct conversion receiver.


As with every other amateur out there this is Yet Another Teensy SDR, and I put the code here https://github.com/calston/x-uhf/tree/master/controller Most of the code is driving the display and user interface.

I haven't had a chance to put some pictures together, but thought I'd talk briefly through this because a lot of the existing SDR code out there is outdated and doesn't make use of the new Teensy Audio library features.

Basically all I did was mix down to a 10khz IF, then use a Biquad filter with a 15Khz highpass stage and 0.7 Q factor. This effectively converts the FM to AM. Then to convert the AM I simply run an abs function on the stream. This does still need further demodulation that I haven't figured out a simple way yet. It should be noted that I have no idea what I'm doing, this entire process was me learning digital filters for the first time. 

Ultimately I do NOT want to use I/Q demodulation like everyone else does because this complicates the build not being able to use single ended unbalanced signal paths, and the whole point of this was as a simple demonstration tool that matches the block diagrams in text books.

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