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

aha ok, thats a fair bit, but maybe within range, as as been said at a
certain frequency the crystal becomes a complete open circuit.

I didn't design it, but allot of folks seem to have had success with getting
it working. Like one poster said, I'm already within range to copy the
signal, but it would be about 1.5kHz. That's a tad high in pitch for my
taste. I was hoping to be able to get to the other side of zero beat, but
perhaps that was wishful thinking on my part. I've managed to get it 500Hz
higher than the marked frequency so I should probably be happy with that.
After all it's a crystal and it was designed to be operated well within
50ppm of its marked 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.

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.
;-)

Yeah I kinda got lost in my own explanation myself. didnt have much
time to explain.

Don't get me wrong, I appreciate the information.

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.

this circuit is an emiter folower wich has no voltage gain,
so actually they operate in the opposite way wich is kinda confusing
but let me explain ...
consider a typical tuned circuit with LC and a tap in the L,
driving the circuit at the tap acts as a step up,
but the crystal is the inductor wich make it difficult to put a tap
here, but at resonance the 2 capacitors can act in the same way and
provide a voltage step up.

but you can see there is a curent loop involving all these components
in series,
this is what sets the frequency.


If you decrease the capacitor accross the 2 pins of the ic this will
increase the voltage accros it and so give more drive, as wel as
increase the frequency.

another way to look at it is if you consider that ground is at the
base then the capacitors are in fact a voltage divider wich feed into
the emiter of a comon base amplifier.

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. :-)

The pulling range is usually equal to the motional capcitance over the
crystal self capacitance, so for 14ff and 20pf this gives 700ppm, but
actually at this extreme its unusable in this circuit as its required
to be inductive.

as said by some1 else the farther you pull it the worse the
performance.

Thanks again for the info. I think I'll go tinker with it a bit. My new
scope shippped yesterday and it will be here tomorrow, yaaaayyyyyy.
;-)