|Two x-tal low freq. reference osc.|
Like so many of my little contraptions, this one was whipped-up to meet a specific need: resurrect an old FM stereo generator whose internal 19Khz crystal had utterly given up the ghost. Since I didn't have another x-tal to plug into the thing, I had to concoct a circuit to heterodyne two higher freq. x-tals together, and give me the 19Khz difference. This ckt does that quite nicely.
No, I didn't take a photo of this one... it's just "ugly construction" atop a piece of copper clad board. Nothing to look at, really. I whipped it up in about an hour, and it looks like it! ;) The point was to get a 19Khz signal. I just figured I'd share the schematic here, in case anyone else needed a nice, stable, low frequency generator... so here it is! :)
In my application, the output of Q3 is pretty close to a square wave, with only a little of the x-tal's operating frequency signal present. The simple LPF takes care of that, and also smooths-out the squarewaves a bit. The 19Khz signal is absolutely stable and works beautifully.
The x-tals I used here were salvaged from dead CB radios. One is 10.040Mhz, the other is 10.060Mhz. Yes, that's 20Khz, but by tweaking each x-tal by only 500Hz, I get 19Khz. :)
This circuit is quite generic, using no really frequency-dependent components. You can plug just about ANY x-tal into it and it will run. Best results are obtained when your desired output frequency is less than 10% of the lowest x-tal freq. You could certainly get 100Khz, or even 1Mhz from this kind of design, but would probably want to use LC filters instead of RC as I did here.
Also remember that for x-tals above about 15Mhz, they are running "overtone" mode! A CB x-tal marked 27.xx5 Mhz will actually oscillate at 1/3 that frequency! Keep this in mind when choosing your x-tals. Experiment! Have fun! That's the name of the game!
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