Solder Haze

A project log for PolyMorph Receiver

A minimalist direct conversion radio receiver. 4 general purpose transistors and a Polyakov 2 diode mixer.

morphmorph 05/24/2020 at 11:410 Comments

We have a working prototype! Almost all the components are scavanged from PCBs that were meant for a completely different purpose, it makes me happy that they get a new purpose in this radio receiver.

Initially I built the VFO with salvaged varactor diodes as varicaps. It worked well and I could cover the 20m amateur band completely. I had two 1n4004 diodes back to back, and fed the control voltage at the cathode interconnection. Then I remembered that some years ago I had made this receiver topology with a ceramic resonator and I really didn't want to make this build about fighting VFO drift (which there certainly was). I'm trying to keep the components to a minimum and a trade off is that the VFO doesn't have a buffer stage. No problem though, super VXO to the rescue. Two 3.58 MHz  ceramic resonators in parallel promises to cover the whole 40m amateur band, currently that's what I can see on the scope, I haven't tried this in the wild yet.

The diode mixer RF termination pot also serves as volume control. The amplification is pretty good for using two NPN transistors. In fact all the transistors are from the scrap boards, I had to twist the BJT legs around to match them to my board pads, this wasn't a problem. For anyone building this receiver from scrap, most of the component values are not critical and I'm sure mine aren't a perfect fit. For the transistors you should check online which ones are best for HF gain (RF amp and LO) and the others can be used in the audio amplification.

I tested the receivers 'ears' with a si5351 signal and it feel very promising, next I need to find a scrap enclosure and take the Polymorph receiver for a walk. I will probably add some form of a simple RF bandpass filter, a good chance to show some impedance matching tricks. Below is a sa612 superhet transceiver prototype.