"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 ?
what freq you need ?
whats the colourburst crystal freq ?


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.

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.

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.


Colin =^.^=