QRSS on a breadboard... figuring out the FSK shift

At the workbench......

I have a nice VXO for that will become a QRSS rig at some point in the future. One of the things that I needed to figure out was how much capacitance was needed to move about 5 hertz for my desired FSK shift.

So tonight I started experimenting with some different capacitors. Initially it took about seven 3.3pF capacitors in series to get the 4-7 hertz shift that I was looking for. I was not happy with that approach (too many parts) but it gave me some ideas and an approximate range to work with.

I did some more testing and confirmed that I was going to need something smaller than 1pF. I was figuring about .5pF. Where do I get that kind of part? (Yeah... could make a gimmick capacitor but it lacks reproducibility.)

I start looking through my parts boxes without much luck. I don't have any 1pF caps on hand. So I started looking through the variable capacitors. Eventually I found some .5 - 1.5pF variable mica capacitors. I hooked up two in series and that was the magic. (The capacitors are probably older than I am.)

It looks like 0.54pF while move the circuit about 5 hertz. (Sweet!)

I need to think about how to approach this from a commodity component point of view so that it is reproducible for other builders. I have some ideas that I will try as I do some more prototyping in the near future.  My approach might get tweaked as I start to work on the PIC 16F628A integration in the near future.

I am still amazed at that the stability. I am seeing 1-2 hertz of of shift in the couple of hours on the work bench tonight. That is pretty good in my book.

73 de NG0R