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.

73, Ed. EI9GQ.



--
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Remove 'X' to reply by E-Mail

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.

"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.

Uncle Peter wrote:
"Anthony Fremont" wrote in message


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.

Ok, just like an antenna

A capacitor in series will raise the frequency.

Same thing again, just like an antenna. However, when I increase the series
capacitance, the frequency drops instead of going higher. This I don't
really get.

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.

Did this, it helps, but when capacitance is below about 10pF, it seems to
get unstable and I still can't achieve the frequency I desire.

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.

Can you suggest a value to start with?

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.

That's what I'm thinking now, it just won't pull that far high. I can sure
pull it low though. Maybe I should try a 4MHz crystal and pull it down to
3990, this is where the voice transmissions are anyway.

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.


--
JF

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. :-)

"John Fields" wrote in message
...
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.

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

Colin =^.^=

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.

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.

"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 =^.^=

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. :-)