Active Lowpass Filter and AGC Noodling – QSD Receiver

14 Apr

Click Here to see the project from the beginning

Now that I have the signal chain functioning from antenna input to speaker output, I’m starting to work on the final missing function block – the steep-skirted audio lowpass filter. One of the PCBs I developed is an 8-pole active lowpass filter.


The 8-poie lowpass filter PCB

This will go just before the audio power amp which is presently an EBay LM386 module. I have a PCB for a TDA7052A which is an audio power amp with 80dB of voltage controlled gain reduction.

The TDA7052A Audio Amp PCB

One of the things I want to try is sampling the audio amplitude with a Nano A-D input and a software peak detector and using that number to index through a lookup table which gives a control voltage value to keep the final audio output constant. This “feedforward” method, if I can make it work. has several advantages which I’ve written about.

I hope to have the lowpass filter built up and in the signal chain this week.

Stay tuned


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Dian Kurniawan

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