09 April, 2012

Whatever happened to the 1 Volt QRP Transceivers?

Output stage of the 1 Volt DL2AVH transmitter
(from http://www.lichtnetzwerk.de/1volttxvr_dkumentation.pdf)
I am intrigued by minimalist transceiver designs like the Pixie 2 which I built some years ago. Therefore the "1 VOLT Challenge" from Dayton 2000 is also something I wish I knew more about. It had these winners:

1. Duncan Walters, G4DFV - The HAMEOBA - A 100 mW Single Cell CW Transceiver (30m)
2. (tied) Helmut Siefert, DL2AVH - A 30m 1V QRP transceiver
2. (tied) Charles Fletcher, G3DXZ - An 80m CW QSK Transceiver
4. Jim Roberts, NC9H - A 20m 1.5V Transceiver

Now, what happened to these designs? I'll try to figure it out from sources on the web.

The DL2AVH 30 m 1V transceiver is documented with a schematic and a photo of the breadboard. The key feature is an 80 mW output at 1.1 Volt supply voltage rising to 180 mW at 1.5 Volts. It used a push-pull pair of BFP196 low noise, low distortion broadband amplifier transistors in the output stage (see figure above). I consider the output stage to be the most challenging part at this low supply voltage. Other parts like audio amplifiers are frequently made for 1.5V supply voltage in for instance powered headphones, so that kind of circuity is more straightforward. The DL2AVH receiver is direct conversion and there are separate oscillators to drive the receiver and transmitter. It seems to have been corrected in 2007 and a printed circuit board seems to have been designed also. But I don't have the final documentation, it may however have been published in the German QRP-Report at the time.

I couldn't find any information on the G4DFV and G3DXZ designs, but found that the NC9H design had a superhet receiver. The rest is probably documented in the QRP-ARCI reports from July 2000 which I unfortunately don't have. 

My low voltage transmitter:
74LVC240 output stage (to the right), oscillator with 74LVC04 (lower left),
LM317-based variable power supply (upper left)
Some years ago I was inspired by these low voltage designs. I also saw several 74HC240 logic gate transmitter designs, originally coming from N7KSB in QST some time in the 90's it seems. That chip runs at up to 8 Volts, so in order to use that concept I had to use a different 74-series logic family.

I made a push-pull output stage with 4+4 gates from an 74LVC240 octal inverting buffer (supply voltage 1.2 - 3.6 Volts), see image to the right. All I was able to achieve was a mere 23 mW at 14 MHz and 10 mW at 3.5 MHz with 1.5 Volts supply voltage. This is really poor compared to the 180 mW of the DL2AVH design. I couldn't really figure out why this was all the power I was able to get.

Now, the question is if there is more information somewhere, and also more inspiration to get from the designs from the Dayton competition. Perhaps also there are newer concepts that have been developed in the mean time which are worth trying?