May 9, 2025

Now I had it sort of working I needed to do some refinements…

I made a close copy of  F1AVY? Circuit (see 30THz group.io) and this gave some noticeable improvement in sensitivity, although I didn’t use the same JFET’s as the design.

The receiver was on a bare board and there was lots of other noise getting in, 50Hz etc..

I fitted it inside a diecast box with phono socket for the output, the sensor peeking through a small hole on the bottom.

Noise was significantly reduced and the SNR across the room was improved.

I tried with various modulation rates from 5Hz up to 30Hz and all worked similarly, although there is more noise below 15Hz from the surroundings.

I was wondering how to get some more gain in my system, i don’t have any parabolic mirrors laying about!

I do have various Fresnel lenses though, they are used ibn the handheld thermometers so must work to some extent…

I tried the 25x25cm Fresnel lens i use on my Optical setup but this did not work at all, seems to block it completely.

I then tried some thin ‘credit card’ sized Fresnel lenses I had… Bingo, some gain!

I doubt these are anywhere near as good as a reflector of the same size but it gets me going.

Tests across the room now show the signal about 30dB over the noise so i oved outdoors for a longer test.

15m was the best distance where I could still detect the signal, not far!  but it’s some progress.

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