Comment by pkolaczk
22 days ago
If you can afford losing a bit of frequency stability, you can use a varicap instead and control it by voltage. Precise multiturn potentiometers are much cheaper. Or just buy a programmable clock signal generator based on PLL and then make it into a superhet (you can still have analog filtering and detection, but digital frequency synthesis so it can look more like a modern radio with frequency display).
There are so many options and all are very cool to explore.
Yes, varicaps are good and there are even varicaps with very large capacitance swings exactly for this purpose.
But it does require a very steady voltage and many more parts than just a single passive to not accidentally load the resonant circuit to the point that it becomes ineffective. You need to de-couple considerably to make this work, especially without injecting (phase) noise. And frequency stability and phase noise can be extremely annoying in particular applications.
That’s why you probably want to use it in an oscillator rather than a filter. Then some of those problems go away. Although, some new ones appear like having to add a mixer … Some time ago I thought making a double superhet would be too complex but apparently it turned out quite a nice DYI project.
One of the problems I'm struggling with is that I'm trying to recover a signal with a known pattern from under the noise floor and every little bit helps. Varicaps will come into play once there is enough signal that the thermal noise and power supply influence would no longer drown out the signal. Interesting project but it is one of those where when you start you think 'how hard could this be?' only to find out that it is in fact pretty hard. We'll see if I can pull this off or not, I give it 10% chance at the moment.
Oh, and superhet wouldn't work, that would destroy the valuable part of the signal.
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> There are so many options and all are very cool to explore.
Very much agree, controlling a varicap via an MCU is a nice intermediate step of "digital tuning" without going all the way to SDR.