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Transistor Radio Series – The 7 MHz Scratch Synthesizer

Transistor Radio Series –  The 7 MHz Scratch Synthesizer

[Section Four — VXO]

We require a VXO that tunes the VCO from 7.0 to 7.065 MHz. I had some HC49-S style 28.4 MHz crystals and hoped to divided them by 4 — and get enough frequency swing to tune my desired VCO range. I failed here! The crystals on hand proved terrible and would not pull as far as I’d hoped without compromising my desired less than 1 Hertz VXO temperature drift fluctuation.

I tried them in a super VXO fashion + varied the inductance and such but could not get a fully meshed to fully open tuning capacitor swing over 10 KHz with a divide by 4. In my collection, I found an air variable capacitor that offered more range, but alas it suffered backlash and when I mounted it in my enclosure the Bakelite support material cracked and crumbled rendering it garbage.


I hope to get some different 28.8 MHz crystals and derive the tuning range I initially calculated and hoped to achieve. I will also try to find an air variable capacitor with a lower minimum capacitance like the 1 that broke. Perhaps it was foolhardy to chose a divide by 4 scheme? We’ll see.


— VXO schematic. The single hex inverter buffer might not be needed, however, I’ve got lots of CMOS logic to use up and more years behind me than ahead.

Final synthesizer tuning range = 7.007 to 7.065 MHz with some gaps as follows:

The VXO tunes from 7.189152 to 7.19702 MHz. As mentioned, Ill work on the VXO to remove the gaps in ranges 1, 2 and 3 — and hopefully get down close to 7.00 MHz. Limiting the VXO range to only that what’s needed improves the synthesizer tuning resolution.

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