VHF FM Crstal Set Radios

  
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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. 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
VHF FM Crstal Set Radios - schematic

internal resistance. 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. 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. 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...



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