Mixer VXO circuits

It is not too difficult to design and build a VFO that is adequate at 136kHz for normal hand keyed CW operation. There are several examples on other LF web-sites ( see G3YXM , PA0SE, G3YMC and GW4ALG). Anyone attempting basic operation on QRSS, slow CW, using a soundcard DSP waterfall display for visual decoding, will soon find that they require something a little more stable. Several amateurs have built mixers VXOs around logic gates. These will work but do tend to drive the crystals very hard and can result in less than required stability. Some have tried to employ the standard circuits for Colpits crystal oscillators and found that they will not pull very far so that it is difficult to get full band coverage. The reason for this is that most of the published Colpits crystal oscillator circuits are designed to produce a stable oscillator not one that can easily be pulled off frequency. There is a halfway-house which I stumbled over due to my inate idleness.

I realised that I had a box full of small oscillator modules recovered from old Pye Westminster PMR radios, and quickly knocked up a mixer-driver from a pair of these. I used a differential capacitor to swing one oscillator up in frequency whilst at the same time pulling the second down in frequency. This seemed to work admirably and was sufficiently stable for my requirements. Having done a major exercise to sort out some 2000 old crytals, mainly from old PMR sets, I offered pairs to others, who did not have the same succes with them. I then looked again at the oscillator circuit used by Pye and noticed that the capacitors determining the amount of feedback seemed to be a little unusual. It is normal to see the smaller capacitor connected between the base and emitter and the larger from emitter to ground. In the Pye circuits we found that there was a bigger capacitor between base and emitter than from emitter to ground. This effectively increases the feedback and drives the crystal harder. It is obviously not sufficient to heat the crystal sufficiently for it to drift out of the channnel when multiplied to VHF, but probably means that the transmitter oscillator will start up when the radio has been left in a car on a cold night.

For those that want to copy there is a diagram of the Pye oscillator circuit below. It seems to work well over a range of 2MHz to about 12 MHz ( and maybe higher) The components were intended by Pye for a 12 MHz crystal. The outputs of two oscillators were combined in an analogue mixer of two 3k3 resistores and couple of capacitors. The difference was selected by a simple low-pass filter. See the LF Experimenter's Handbook for more ideas.

One plus point is that the TX can be keyed by keying just one of the oscillators. This is how Pye selected different oscillators in their multi-channel radios.

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