2001-11-11
HF TRX with PTO based VFO
While surfing around the 'net I came across WA6OTP's website. Now I've seen PTO based VFOs before, but his implementation is very simple and elegant. I just couldn't resist the urge to build one myself and try it out.
I built the circuit straight from his 40 metre radio design, but wound many fewer turns for the initial coil, just being a test I was in a rush and being lazy. It worked perfectly, with a strong and stable signal around 14 MHz. His design has no buffer so it pulls quite easily, the drain of the oscillator being quite a high impedance and my operation of it much higher up the band makes the coupling cap too large.
I figured what the hell, and started building a mixer on the spare side of the ground plane. That is as far as I have gotten. The mixer seems to work, I quickly shorted the two ends of the diodes together with a 1k resistor to ground and an alligator clip lead for an antenna. Playing my Walkman through the AF port I was able to hear a good quality DSB signal on an adjacent HF receiver. The carrier suppression was actually very good considering the mixer was totally untrimmed and the diodes weren't matched in any way. I may dispense with a carrier nulling pot completely.
I'll probably need to add a buffer to provide a low drive impedance for the mixer, it is loading down the VFO to about 600mV p-p while into 10M it puts out about 8V p-p. I am also running out of board space fast, so I'll need to put the AF circuits and RF front/back end on a separate board. I can probably squeeze a post-mixer amp on the same board.
The brass threaded rod moves around a bit in the nuts, they aren't spaced too far apart and are of very poor quality, so there is a bit of play which shifts the frequency somewhat. On 80m this probably would be acceptable, but on 20m it is very annoying. Using a smaller ID coil former and/or adding a bit of tape to the end of the threaded rod to centre/hold it in the former would fix this. I'll try that later, for now the remaining AF and RF bits are the focus.
My intention is to allow the TRX to share the same mixer, kind of a minimalist, maximum reuse, design. The RF and AF ports will be bidirectional, with TRX switching. In fact for the AF side I may be able to get away with no signal level switching, just turn off the MIC amp on RX and turn off the AF preamp on TX.
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