The PNP-80 Receiver

A small pile of BC556 transistors

This radio has a bit of a story behind it. At the last ARNSW Trash and Treasure one gentleman brought along a car-boot full of junk that was given away free. Most of us there had a great time searching through the pile for interesting items (check out the photo on Peter VK2TPMs website), (I grabbed some optical assemblies from a scanner which included CCFL tubes, inverters, linear CCDs and lots of mirrors and lenses. I collected a bucket-load of lengths of shielded stereo audio cable from PC CDROM drive connectors - you never know!)

Unbeknown to the rest of us pack-rats, John VK2ASU had found the prize item in the pile, a huge bag of BC556 transistors. At the following Homebrew Group meeting John shared his fortune with us and passed them around. John gave me a /very/ generous pinch of the PNP devices which I gladly accepted into my junkbox. (He also shared some dual-gate FETs which will be put to good use at some stage too.)

I decided on the spot to make *something* from these units, it only seemed right now that I had a surplus of them. Some days of thought passed and I settled on building a receiver initially, with the thought that I may expand it into a transceiver eventually... 80 metres sounded like a good band, with the ARNSW challenge focusing on that band, and the lower frequencies making later transmitter stages easier using only these devices.

Like most RF experimenters I tend to be an NPN-bigot, never giving the humble PNP devices much of a second look, except perhaps in symmetric AF output stages. I think we look at our circuit diagrams as if electrons had a sense of gravity at times. PNPs are very easy to work with, the simplest way to port a NPN circuit to PNP is to just swap the supply voltage around and change out the devices. I've taken this approach. The diagram is not drawn conventionally, down is positive in this circuit, but you get used to that quite quickly. Although you will likely forget at some point during the construction and connect your PSU's -ve rail to the ground plane and smoke something (if you haven't got a limiting supply). I had one close call during construction when I installed the AF amplifier emitter bypass capacitors the wrong way round and was wondering why the circuit sounded awful and was suddenly drawing so much current.

I started with an 10 MHz crystal ladder filter and built an IF amplifier to suit it. This went together well, and tested OK, so I added a product detector and a buffered carrier reinsertion (BFO) crystal oscillator. It wasn't long until I had built an AF strip and was listening to the swishing of my wobbulator sweeping across the IF filter passband.

Over the next few days I added a front-end mixer and listened to some signals off-air using my signal generator as a VFO. Stability and usability of this arrangement was terrible, so a buffered VFO was constructed, using a T50-7 core for good stability. This worked very nicely, I could listen to the VK2RSY beacon for hours without it drifting appreciably. Those people who think a VFO must use a FET should re-evaluate their beliefs! This isn't the best VFO out there, but on 80 metres it works fine.

I next added some filtering at the product detector to cure some instability and AF-quality problems, rolling off the audio above 3.5 kHz with a simple RC filter.

An RF amplifier was built for a post-mixer buffer, but the noise figure wasn't especially good and I was supplying it thought a pot to reduce it gain (the RF gain control) to keep the IF stable, so I moved it in front of the mixer to proceed its losses. There it can be run full-throttle without instability, but I can actually drive it into limiting at the moment because there is no front-end filter. I then built a simple series/parallel resonator diplexers to terminate the mixer and proceed the crystal filter. Here I need improvement, the filter is highly reactive except for a small region, so it pulls the diplexers a bit and reflects through it, clearly a common-base amplifier would be of assistance, perhaps with a pad which would help flatten the IF filter response anyway, terminating it correctly.

Here is a photo of the receiver as it currently stands. I'll post more information, including circuit diagrams as I progress:

PNP-80 Current State

TODO to complete the receiver: Band-pass filter for the front-end. Post-mixer/diplexers buffer amplifier, and perhaps a post-filter buffer as well.

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PNP-80 Huge Picture image/jpeg 278.062 kbytes
The VFO Circuit image/jpeg 57.621 kbytes