20m Superhet Reciever With No ICs

The Challenge:

I gave myself a challenge: could I build a superhet reciever without using any IC’s and fit it on a 2.7 x 4 inch copper-clad board? Yes, I could. I built a CW reciever for the 20m band, featuring a very selective crystal filter at the 6 MHz IF frequency. Here’s how I did it:

Step 1: VFO

I’d already decided that my IF frequency would be 6 MHz. Why 6 MHz? Because 6 MHz crystals on Digikey were the cheapest. That means I’ll have to make a ~14-6=8 MHz variable frequency oscillator, and this VFO will have to cover roughly 8-8.1 MHz since CW signals live on the bottom 100 kHz of the band.

I wanted to use a varactor to tune it, since that would mean I could use a potentiometer to tune it. I came up with this oscillator design, using a SV324 varactor diode (C4 in the diagram).

I used a 10k pot for coarse tuning and a 100k pot for fine tuning the voltage applied to the varactor.

Here it is constructed:

And here’s the output of the oscillator: a nice, stable oscillation adjustable from 8-8.15 MHz.

Step 2: First Mixer

I wired up a very small diode ring mixer that uses four 1N4148 diodes and two small ferrite toroids, and wired it up underneath the VFO. It looks so sketch, but it works!

Then I fed in some 14 MHz signals with my signal generator to make sure the VFO and mixer were working correctly together:

Step 3: Crystal Filter

I’d bought a bunch of 6MHz crystals off of Digikey for fairly cheap, and I found their resonant frequencies using my signal generator and the oscilloscope. I had three crystals with roughly the same frequency and I put those in series to make a crystal filter; between the filters are two 20pF capacitors to ground. Combined they had a bandwidth of ~500Hz. I also made a four-crystal filter but that had problems with ringing and self-oscillation for some reason.

I also built a simple common-emitter amplifier with a 2N3904 and put it after the crystal filter.

Step 4: BFO and Second Mixer

I built the beat frequency oscillator with a transistor and a 6MHz crystal. I put an adjustable capacitor in series with the crystal to “pull” the crystal frequency away from the 6MHz IF frequency by a few hundred Hz, enough to create a comfortable audible tone for CW.

A built a second diode ring mixer and strapped it on the bottom of the BFO, like I did with the VFO.

Step 5: Audio Amps

I designed a simple preamp and Class AB audio amplifier for the final stage of the reciever. The volume control potentiometer (a logarithmic potentiometer) is located between the preamp and the audio amp. I added a headphone jack as the output.

I built all the audio circuitry on the remaining board that I had, on the fly. I was quite proud of myself for building it in such a small space!

Looking back, how did I do that? LOL.

Closing Remarks

The reciever works off of a 9V battery, so I added a barrel jack to the back. I also had to throw in a LM7805 5V regulator IC (all the oscillators work on 5V), so … okay, I lied, I used one IC. But that was because I ran out of space on my board.

I mounted a BNC connector to the back, hooked it up to my 20m vertical, and was very pleased to be greeted by dits and dahs of Morse Code after some fine tuning! The reciever is very selective, and it’s quite sensitive as well.

This project took me exactly 1 week of hard work. I did a lot of testing along the way to make sure each stage was working properly (and I often had to rebuild and rewire stuff, which was not easy to do on such a small circuit).