[jacquesm]

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.

[pkolaczk]

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.

[jacquesm]

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.

[pkolaczk]

What do you mean it would destroy the valuable part of the signal?

If the signal has a carrier (like in AM or FM) or some other regular pattern (e.g. digital signal) then the typical the way to recover it from under the noise floor is to use a narrow-band PLL.

I’m experimenting with it right now and just found that a quartz/ceramic oscillator pulled by a varicap controlled by a PLL gives pretty good results - low phase noise, good noise rejection and ability to recover the carrier from a very noisy signal. But for that to work, the signal has to be shifted to a constant frequency through heterodyning first.

[jacquesm]

The exact phase of the input is what I'm after. Using a LO would cause you to read the mixture of the LO and the input signal rather than just the input signal, and that means that you will never see the phase with the same precision as if you were to observe it directly because the two will have slightly different frequencies.

If you were to put both on a scope in XY mode you'd see the phase change directly over time.

But I like your two step approach, use the het to get close and then lock on to the phase.

So far I've not been successful without first bringing the signal up to the point where I can do it directly and then I don't need to add the complication of a PLL.

Injecting various levels of noise into the signal is a good test to see if the system would still work in less than ideal conditions and so far the answer is a hard 'no', it may well be that I'm past my level of competence on this but it's only been a couple of weeks so I will keep trying.

One possible approach is just to use a flash AD and move the whole thing into the digital domain. That has a bunch of other advantages as well, the signal is not particularly high frequency so that should be possible without breaking the bank.