John Fields wrote:
On Tue, 13 Mar 2007 18:00:15 -0500, "Anthony Fremont"
wrote:

Eamon Skelton wrote:
On Tue, 13 Mar 2007 12:02:38 -0500, Anthony Fremont wrote:


The problem is (well I think it's a problem) is that I'm all the
way down to a 10pF cap for the crystal trimmer and the highest
frequency I can get out of it is still less than 3580kHz. Pleae
correct me if I'm wrong, but I'm thinking that the 20uH inductor
is supposed to pull the colorburst crystal high in frequency then
the adjustable cap should be able to tweak it back down.

Use less inductance or replace it with a wire link.

I did this and now I'm up to 3580.050; an increase of about 70Hz or
so.. I don't understand how this was designed to work. What is
making the crystal oscillate above its design frequency? I thought
the inductor had something to do with it, obviously not. Is it
being operated in series mode instead of parallel?

I tried putting small cap in series with the adjustable one and that
just greatly narrowed the tuning range and only slightly raised the
frequency. It seemed to be getting unstable at that point as my old
frequency counter started showing variations of quite a few hertz
each update instead of remaining steady. It could be the amplitude
is too low and the counter is missing ticks.

---
Your color burst crystal is ground to ring at 3.579545 MHz, and if
you try to run it at another frequency you'll run into drive and
tempco problems which will make it worthless.

I know that I'm "violating the specs", but that's the intent here. Most of
the circuits I've encountered tend to pull the xtal down in frequency (just
add capacitance), but this case is odd in that W1AW transmits just above the
colorburst frequency. I can pull it down several kcs pretty easily, I just
can't get it above the specced value by more than about 500Hz. I think I
just need to try a different crystal or maybe two in parallel. The parallel
inductance idea is interesting as well, never done anything like that
before.

As for stability, as long as it's not causing warbling audio, I'm not too
concerned. :-)