Hi all,

The following page has a number of links about an interesting method of
stabilizing VFOs.

http://www.hanssummers.com/radio/huffpuff/contents.htm

The basic idea is to divide down the output of a crystal reference
oscillator to 10 Hz, and then mix this with the output of your VFO, using a
D flip-flop (acting more like a sample and hold circuit than a regular
mixer). The result will be a highly aliased square wave, and will go up and
down in frequency from 0 to 5Hz and back down to 0Hz as you tune the VFO.
You make a frequency locked loop which tries to hold the mixed output at
2.5Hz by using an integrator to compare the output of the mixer with a
2.5Hz reference, and which has an output feeding a varactor diode in the VFO.
        So, any slow drift of the VFO will be be prevented and it will sit right
at one frequency if you don't touch the dial. However, if you tune fast,
the slow freq. locked loop has no effect until you stop tuning, at which
point it brings the frequency to the nearest multiple of 10Hz + 2.5Hz and
holds it there.

Just thought I pass along what I thought was a really simple, ingenious idea.

Sean


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