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.
