VHF FM Crstal Set Radios
There are a number of crystal set designs on the Internet which use slope detection. I am sure I used to have an article on a crystal set which used a form of "true FM detection", but alas I seem to have lost the article.
Also, does anyone have a view on a good crystal diode to use (I assume the GEX66 is now difficult to find).
There are four examples of FM crystal radios at the "FM-Only" web page that is owned by Andy Mitz:
Another example using simple tuned coils can be found here:
All of these radios use slope detection. I can't help but think that a phase sensitive detector such as a discriminator or ratio detector should be tried.
The Ratio detector should be more lightly loaded than usual, as the frequency response of the tuned circuit should be better behaved.
Phase-sensitive detectors usually take the difference between an input FM signal and the same signal after it has passed through a resonant circuit, or through a delay line.
It would be great to hear from anyone who has tried an FM crystal radio that is dependent on some kind of phase-sensitive detection.
A 1N34 should work fine to detect 100MHz, but not all 1N34 diodes are made alike. One with low internal resistance and low capacity would be best. Some fancy shottky diodes may have too much internal resistance.
Hello Bryce and Joe,
A few months ago I was looking for this same information, and was surprised how difficult it was to find any information on FM crystal sets using a frequency discriminator type detector. I finally managed to locate an article written by Ed Richley. The article was in two parts and appeared in the January and March 1996 editions of The Xtal Set Society Newsletter. Ed's design was quite ingenious, using only a single tuned circuit–a coaxial resonator to achieve the required high Q, and two diodes in a quadrature detector arrangement. Perhaps this was the article Bryce was thinking of?
I had intended to build a copy of the Richley receiver, but concluded that a helical resonator would be more practical than a coaxial resonator. So, that was the approach I took. By the time I finished constructing the prototype, I'd also made some other changes to the circuit.
Here is my present circuit:
I have documented the construction of my FM crystal set on this webpage.
A schematic diagram of Ed Richley's original circuit appears there.
I've also posted a YouTube video showing a short demonstration of the receiver. In the demonstration, the detector output is fed into the microphone input of an audio mixer. However, all of the stations which can be heard in the video are also clearly audible with sensitive headphones.
Regarding the question about suitable diodes, I've found that the following work quite well:
Avago (formerly Agilent) HSMS-2850
EE: Please no external links - the two are dead links by now anyway - and this hurts us (by Google) if we leave them!
Hi Robert and a big thank you
That is indeed the article I was looking for and couldn't find.
I started to make my own design using Lecher lines, but haven't completed the phase discriminator, so all I get is (faint) distorted sound. I purchased the Agilent diodes mentioned and they seem to work way better than anything else - even WW2 radar diodes, like the 1N21 etc (On tests using a signal generator). The completed set may need a beam aerial and possibly an audio amp.
The project has been a bit on the back burner, but your post has reawakened my enthusiasm.