"Anthony Fremont" wrote in message
...
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.


If you want a VXCO, you use the capacitor and inductor to achieve
maximum range of the crystal frequency.

An inductor in series will lower the frequency.
A capacitor in series will raise the frequency.

Eliminate the coil and just use the series trimmer to see how
how high the crystal frequency can be raised before it becomes
unstable or quits oscillating.

As Ian suggested, you can add an inductor ACROSS the
crystal to raise the frequency. Think of the crystal as being
a parallel LC circuit (over simplification) and you'll
be better able to visualize how this works.

Try a few other color burst xtals in the circuit for the heck
of it. You may not be able to pull the crystal that far.