Greetings. Back in the 1970s, amongst other circuits, I used to build simple crystal radio sets using a tuned LC circuit and a germanium diode based on designs in the 1950s electronics magazines. They were not very good, maybe picking up a couple of stations with poor selectivity and only then with a 20m wire antenna. The good news is they introduced me to a career in RF electronics (electron physics) design which has served me very well these past 40+ years.

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Recently I thought about how I would make a better amplitude modulation crystal radio, in a simple way, using only a few, more modern semiconductor devices and without resorting to exotic spider wound coils and such things. Spider coils, if used correctly, can provide higher Q quality factor and therefore higher frequency selectivity because they have lower parasitic capacitance. However, any kind of low impedance load and their selectivity benefits are immediately lost.

 

Instead, I thought I would use two JFETS, used as voltage followers for high input impedance, in order to not load the LC tank tuned circuit, and the detector. Also, a low power npn bipolar RF transistor for RF Voltage gain. I also used a simple LC circuit with ferrite antenna to concentrate the RF e.m. energy inside the inductor and a low forward voltage drop IN34A type germanium diode for the a.m. detector, similar to those we used to use in crystal sets. I applied a small amount of d.c. bias current to the detector in order to provide a d.c current return and to reduce envelope distortion.
 

This very simple design below is what I ended up and it works reasonably well considering its simplicity, receiving at least a half a dozen stations, with good selectivity. I think this radio sounds better than a typical modern superheterodyne receiver design, but perhaps that is just me. With a power supply current of less than 3mA, its efficiency is hard to beat. No other antenna is needed, and it provides an audio output level high enough to drive an amplified computer speaker. Another benefit is that with no superheterodyne 455KHz local oscillator, the receiver operation cannot be detected. A future upgrade might add auto level adjust, or you can simply use your speaker volume control.

I added the LTspice simulation zip file below the schematic.

A stroll down memory lane and the crystal set perfected?

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Chris Owen

Nov 5th, 2022
 

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