Spontaneous Construction: The $20 Bill, or Oh No, Not Another 40m DC Receiver.

UPDATE: 22 February 2018 - Corrected Schematic

Thank you to Walter, KA4KXX, for kindly pointing out some errors in the "toroid table" at the top of the "Preliminary 2" schematic previously published here.  This has been corrected in the "Preliminary 3" schematic below.

 

This project started last weekend, on a whim after my Grandson, Austin, came over and asked if we could "Make some circuits".  So, I started putting together a low powered audio amplifier and it blossomed into a pretty competent little direct conversion receiver.

If you follow Bill, N2CQR's SolderSmoke blog, you know that he's working on a simple DC receiver of his own, which, along with his aversion to using the magic chips, served as inspiration for this rig, which is how it got it's name: $20 is my guess at what it'd cost to duplicate, and Bill, of course, is Bill.  The alternate name is based on my belief that the last thing the world needs is another 40m DC receiver.

As with most of my other projects, the chassis is bent from a combination of 20ga and 22ga galvanized steel.  I had just enough in my scrap pile to build the chassis and cabinet, which is currently out in the garage while the paint dries.  The decals were left over from the set I purchased from Radio Daze for the Echophone EC-1B.  Waste not, want not.

This is one of my few projects where I didn't steal any circuits from anyone else; I designed it in my head, refined it using LT Spice, and built it Manhattan style, and the thing worked perfectly right out of the gate - the only tweaking that I had to do was some futzing with the inductor in the VFO to get it to cover the entire band.

The circuitry is pretty simple: Signals first pass through a band-pass filter and then hit an FET preamp before arriving at the input of the diode product detector, where they're converted to baseband audio and applied to the audio amplifier string. At first glance, it might appear that I've got way too much gain, but if you look again, you'll see that the middle two stages are feedback amps and don't provide that much gain, but they're very stable and dead quiet.

Also, the audio passband is tailored to start rolling off frequencies above about 2KHz.  I thought about making it tighter, but wanted to be able to use it to listen to SSB; For some reason, I get a kick out of listening to the guys running their state-of-the-art megabuck rigs on a junkbox rig.  

The output stage is a push-pull 2N3904/2N3906 pair that provides a couple dozen milliwatts of power, plenty for headphones and adequate for a speaker if you're using it in a quiet room.

The VFO is a basic series-tuned Colpitts, lightly coupled to a buffer amp and then to a gain amp.  The capacitors are all NPO, and the inductor is wound on a T50-2 core and "hot glued" to the board.  There's a slight drift at start-up, but after 15-20 minutes, it settles down and is more stable than it has a right to be - watching signals in the waterfall of WSJT-X, I see no drift at all over several minutes.  The only magic chip used in the rig is an LM78L05 to regulate the VFO voltage, only because I don't have any suitable Zeners in my junk box.

What really impresses me, though, is how incredibly quiet the receiver is: Put on phones and crank the audio without an antenna and you'd swear that you forgot to turn it on; there's NO background hiss at all - something that's bothered me about my prior DC receiver efforts.  Connect the antenna and you'll quickly reach for the gain pot - signals seemingly jump out of nowhere!

The rig can be powered either from its internal 9V battery or a 12V bench supply.  It is a bit current hungry, drawing about 70mA, which is why I added the provision for external DC power.

As I mentioned, I started this last weekend and worked on it a couple of hours each evening and another 3-4 today, so yes, one can build a practical rig without giving up all semblance of a life outside of the shack.

There's a major CW contest this weekend, and the wall to wall signals make using a DC receiver challenging, it didn't take long to get the hang of using it and was able to make several DX contacts.

I don't often wear a hat, but if I did, I'd tip the brim toward Bill in Falls Church for giving me the idea to build a (mostly) discreet component, hardware defined radio.  I have a feeling that more will follow, but I really need to get back to the SDR-2018 that's sitting here taunting me.