Friday, September 3, 2010

Magic super-VXO Frequency?

Years ago, I ebayed a Heathkit HW-8 direct conversion CW-transceiver. It was not in the best shape, but it did not cost the world either. Now, for good measures, finally I want to put some life in it again. So, lets look at the schematics first...
Ahh, mhhhh, aha..... four bands, four crystals and a 250kHz wide VFO.
Lets have a look at the combination for 80m:
The converter-crystal is mentioned to be 12.395MHz. That would determine the VFO oscillating at 8.895MHz at its maximum frequency.

Hmm, 8.645MHz to 8.895MHz, that rings a bell! Yes, you guessed it, there is a widely available standard crystal at 8.86724MHz, resulting a very interesting operating frequency at about 3.528MHz... Interesting, a super-VXO at 8.867MHz, will get us 28kHz off from the lower edges of the four bands.

Now lets see how well a 8.867 super-VXO will do with other standard crystals...
  • 8.867 - 9.000 = (-) 0.133
  • 8.867 - 7.000 = 1.867
  • 8.867 - 12.406 = (-) 3.539
  • 8.867 - 1.843 = 7.024
  • 8.867 + 5.185 = 14.052
  • 8.867 + 9.216 = 18.083
 You're missing 30m? OK, here's your 30m, it's a tricky one however, in particular for filtering!
  • 8.867 + (5.000/4) = 8.867 + 1.25 = 10.117
 Or with a not so common 38.0MHz oscillator:
  • 8.867 - (38.000/2) = 8.867 - 19.000 = (-) 10.133

I leave it to the reader to find more, in particular for the bands 15m and higher...

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