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.

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. :-)

Arv wrote:

My W1AW receiver uses the crystal with just a 5-47 pf variable
capacitor...no inductor, and it nets right on frequency. Try shorting

Story of my life. ;-)

the inductor and see if this gets you closer to the required
frequency. Not all color burst crystals were created equal. If you
have another crystal you might want to try it.

That seems to be the common consensus. I suspect my crystal is just too
good. ;-)

Your ferrite will not be saturating at the small amount of signal you
are sending through it as part of an SA-602 oscillator.

Thanks, I know very little about these things.

Arv - K7HKL
_._

"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 =^.^=

On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
Hello all,

I was playing around and saw that my junk box had all the parts so I started
tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg
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

The inductor will pull the crystal down in frequency, as others have
suggested.

In fact it is extremely difficult to pull a crystal's series resonance
up in frequency more than a few Hz. This is because the crystal's
parallel resonance is just above its series resonance. If you put a
capacitor in series with the crystal the series resonant frequency
goes up...BUT...if you approach the parallel resonant frequency you
can no longer get a low impedance resonance condition since the
crystal's parallel resonance makes the crystal look like an open
circuit, regardless of what you put in series with it.

If you want to understand this better, try to find a reference with a
good discussion of the equivalent circuit of the quartz crystal
resonator. the only one I know of at present is Kenneth K. Clarke and
Donald T. Hess, Communication Circuits: Analysis and Design, Addison-
Wesley Publishing Co., 1971. It may be a bit hard to find outside a
good university library.

As Ian Jackson suggested a parallel inductor might work....this
modifies the parallel resonance. Ian..is there a schematic available
for that ? I'd be interested in what actually worked.

Steve

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.

wrote:
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
Hello all,

I was playing around and saw that my junk box had all the parts so I
started tossing this
together:http://newenglandqrp.org/files/w1aw-receiver.jpg
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

The inductor will pull the crystal down in frequency, as others have
suggested.

I understand now. I knew that adding capacitance tended to slow xtals down.
I was unfamiliar with how an inductor would act in series. I knew that this
circuit was trying to pull the crystal higher than spec so I, quite wrongly,
assumed that the inductor would have the opposite effect of capacitance.
And it seems that it does when used in parallel and that seems sensible to
my feeble mind. But in series, it has the opposite effect. What still
throws me is that raising series capacitance doesn't have the opposite
effect of adding parallel capacitance.


In fact it is extremely difficult to pull a crystal's series resonance
up in frequency more than a few Hz. This is because the crystal's
parallel resonance is just above its series resonance. If you put a
capacitor in series with the crystal the series resonant frequency
goes up...BUT...if you approach the parallel resonant frequency you
can no longer get a low impedance resonance condition since the
crystal's parallel resonance makes the crystal look like an open
circuit, regardless of what you put in series with it.

Hmm....food for thought. :-)

If you want to understand this better, try to find a reference with a
good discussion of the equivalent circuit of the quartz crystal
resonator. the only one I know of at present is Kenneth K. Clarke and
Donald T. Hess, Communication Circuits: Analysis and Design, Addison-
Wesley Publishing Co., 1971. It may be a bit hard to find outside a
good university library.

As Ian Jackson suggested a parallel inductor might work....this
modifies the parallel resonance. Ian..is there a schematic available
for that ? I'd be interested in what actually worked.

Doodling with reactance formulas, it appears that 20uH (coincidence?) would
offset 100pF of capacitance fairly well by having a an opposing reactance
(well resistance at this point) of about 450 Ohms at 3581kHz, the same as
100pF. I'll try putting the coil I made in parallel and see what happens.
Hopefully it won't short the oscillator and kill my 15 year old NE602, I
only have two spares.

Anthony Fremont wrote:

Doodling with reactance formulas, it appears that 20uH (coincidence?)
would offset 100pF of capacitance fairly well by having a an opposing
reactance (well resistance at this point) of about 450 Ohms at

s/.well resistance at this point.//
It's just inductive reactance, I need more coffee. ;-)

3581kHz, the same as 100pF. I'll try putting the coil I made in
parallel and see what happens. Hopefully it won't short the
oscillator and kill my 15 year old NE602, I only have two spares.

Should I be afraid to do this? Does it need something to block DC current?

"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 =^.^=

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

Doodling with reactance formulas, it appears that 20uH (coincidence?)
would offset 100pF of capacitance fairly well by having a an opposing
reactance (well resistance at this point) of about 450 Ohms at

s/.well resistance at this point.//
It's just inductive reactance, I need more coffee. ;-)

3581kHz, the same as 100pF. I'll try putting the coil I made in
parallel and see what happens. Hopefully it won't short the
oscillator and kill my 15 year old NE602, I only have two spares.

Should I be afraid to do this? Does it need something to block DC
current?


the trimmer should block all the dc.
im not convinced about the inductor in parallel with the crystal
it might work though.