colin wrote:
"Anthony Fremont" wrote in message
...
colin wrote:

Its only a pull of ~100ppm, this should be easily pullable for most
fundamental xtals.

I'd like to get close to 500ppm if possible.

do you need that much ?

I "need" an increase of about .0546%. Isn't that about 546ppm?

what freq you need ?

3581.5kHz to zero beat with the desired signal.

whats the colourburst crystal freq ?

3579.545kHz.


ofc if the tolerenace all add up against you you might find it hard.

I would also try reduce the 100pf caps on the sa602 too.
100pf is higher than most crystals I use would like,
50pf or 20pf or if youve got some spare trimmers ...
you can adjust the ratio too, say just reduce the one accross pin
6-7

I figured that they were voltage dividers to set the amount of
feedback, but I can certainly see how they could have an effect. Since
I'm close to where I need to be, I will try a couple of 33pF
caps to see what happens.

no they are involved in setting the frequency too,
in order for you circuit to work it needs to resonate,
with the 2 100pf the input where the crystal is looks like a
capacitor with some negative impedance,
the circuit with your crystal, inductor, and trimmer must be
inductive, it then forms a resonant ciruit with the capacitance of the
input.
usualy the crystal would just be operated so that it looks inductive.

Ok, I had to read that a few times to get it. A period between ".....100pf"
and "the input..." there would have been quite helpful. ;-)

the negative part of the input impedance must be stronger than the
loss in the tuned circuit.
this is affected by the ratio of the 2 100pf capacitors.

So they do act like a voltage divider of sorts and shunt some of the
oscillator output to ground and some back to the input.

a crystal can apear to be a very high inductance at resonance,
at the point where you want to operate it probably has very high
inductance indeed.
you can determnine the eqv inductance by using the equivalent internal
inductance and capacitance. you need to find the mutual capacitance
of the crystal wich is hard to find man specs wich tel you this but
it is often something like 14ff for example.
(0.014pf) you can then work out the eqv series inductance for it to
resonate with the std load wich may be 20pf.

you can then work out what inductance the crystal will apear to have
at the frequency you want. and hence the series capacitance you need.
you might find the inductance is so high that you need less than 1pf
or it has become capacitive.

Cool, a way to figure out just how high you can pull it and how to attain a
certain frequency. I'll probably stick to tinkering though. ;-) Thanks
allot for the detailed explanation. :-)