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.
;-)

In message .com,
writes
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


I'll try and draw what I have in mind, and post it in
alt.binaries.schematics.electronic. However, in the meantime, let me try
and explain. The explanation may not be absolutely 100% complete, or
even 100% correct, but it may help in moving a crystal more HF than it
wants to go. Sorry that it's a bit rambling!

In Anthony's circuit (http://newenglandqrp.org/files/w1aw-receiver.jpg),
the crystal will probably be functioning as a series-tuned circuit.

As Steve has stated, a crystal suddenly goes into parallel resonance
just HF of its series resonance. This limits how far the series
resonance can be pulled HF by the addition of a series trimmer
capacitor. However, if this parallel resonance can be removed (or moved
further HF), it should be possible to move the crystal further HF. The
technique described certainly does work with VHF overtone crystals
(between 50 and 200MHz), but should also work with HF crystals working
on their fundamental frequencies.

A crystal is a mechanical device, but can be represented as being a
series-tuned L-C circuit. (Call these L1 and C1.) Also, across the two
is a parallel C (C2). Forget about losses (represented by a resistor).
[Note: L1 and C1 are not actual electrical components, and only appear
to have these values at or near to the L1-C1 resonant frequency.
However, C2 essentially is a physical electrical capacitor consisting of
the plating on each face of the crystal, with the crystal as the
dielectric between.]

L1 is very large (possibly 1H or more, depending on the frequency). C1
is very small (say only a few pF or even a fraction of a pF - again
depending on the crystal frequency). [So adding a relatively large
series trimmer capacitor has very little effect on the frequency.] C2 is
typically around 5pF, regardless of frequency.

Imagine doing a test where you look at the resonant frequency of a
crystal, using a signal generator. This feeds an RF signal through a
crystal, into a 50 ohm load. You measure throughput of the crystal by
measuring the voltage across the load.

Swing the sig gen frequency slowly from LF to HF, through the resonant
frequency of L1-C1. [Let's forget about C2 for the moment.] Below the
resonant frequency of L1-C1, the L1-C1 circuit acts like a small
capacitor, so there is very little throughput. Above the series resonant
frequency of L1-C1, the L1-C1 circuit acts like a large inductor, so
again there is very little throughput. However, when you hit the series
resonance of L1 and C1 (F1), reactance of L1 and C1 cancel. The crystal
acts like a short-circuit (or nearly so) and there is a large
throughput. Because the L-C ratio is very high, the resonance peak is
very sharp.

The effect of C2 across the L1-C1 circuit is to produce a second
(parallel) resonant circuit. VERY slightly HF of the L1-C1 resonance, C2
resonates with effective inductance of the L1-C1 circuit. This produces
a parallel resonant circuit (F2). Another way of looking at it is that
L1 resonates with the series combination of C1 and C2 (so F2 must be
higher than F1). The parallel resonance is, of course, a high impedance,
where there is almost no throughput through the crystal.

As a result of this double resonance, the crystal acts as a series-tuned
circuit at F1 (one you want), and a parallel-tuned circuit at F2. The
transition between the two is very sudden. The frequency response peak
of the throughput is very lopsided, and gets chopped off suddenly on the
HF side.

The difference between F1 and F2 is very small (a few Hz to a few kHz,
depending on the frequency and type of the crystal). If F1 is lower than
you want, and you add an external series trimmer capacitor to try and
pull the crystal L1-C1 series resonance HF, you effectively hit a brick
wall with the parallel resonance at F2. The parallel resonance will
block any throughput at (or near) this frequency.

A possible solution is to neutralize C2.
[Note: Neutralization is a technique sometimes required when using VHF
crystals, as C2 may be large enough to allow the oscillator to free-run,
instead of being locked to the frequency of L1-C1. However, it may also
be used with advantage, as described below.]
You can neutralize C2 by adding an inductor across the crystal (ie in
parallel with C2). The value required is that which parallel-resonates
with C2 at the crystal frequency. In effect, C2 no longer exists. With
C2 neutralized, there is no longer a sudden transition from the wanted
series resonance F1 to the unwanted parallel resonance F2. The peak the
response curve of the throughput of the crystal (at F1) is now nice and
symmetrical, without the sudden cutoff at F2. In practice, the actual F1
peak will probably be somewhat more HF than before, and the crystal
should be more pullable with a series capacitor.

Finally, if you reduce the value of the inductor so that its resonance
with C2 is somewhat higher than the crystal frequency, this tends to
pull the F1 resonance peak even higher in frequency. However, if you
overdo this, the oscillation will probably unlock from the crystal, and
start to free-run.

As I said, sorry for the ramble.
Ian.
--

In message , Anthony Fremont
writes
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?



If you do try an inductor across the crystal, make sure that you still
do have a DC blocking capacitor somewhere in the path to ground (as
provided by the existing C2 trimmer).
Ian.
--

Ian Jackson wrote:
In message .com,
writes
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


I'll try and draw what I have in mind, and post it in
alt.binaries.schematics.electronic. However, in the meantime, let me
try and explain. The explanation may not be absolutely 100% complete,
or even 100% correct, but it may help in moving a crystal more HF
than it wants to go. Sorry that it's a bit rambling!

In Anthony's circuit
(http://newenglandqrp.org/files/w1aw-receiver.jpg), the crystal will
probably be functioning as a series-tuned circuit.
As Steve has stated, a crystal suddenly goes into parallel resonance
just HF of its series resonance. This limits how far the series
resonance can be pulled HF by the addition of a series trimmer
capacitor. However, if this parallel resonance can be removed (or
moved further HF), it should be possible to move the crystal further
HF. The technique described certainly does work with VHF overtone
crystals (between 50 and 200MHz), but should also work with HF
crystals working on their fundamental frequencies.

A crystal is a mechanical device, but can be represented as being a
series-tuned L-C circuit. (Call these L1 and C1.) Also, across the two
is a parallel C (C2). Forget about losses (represented by a resistor).
[Note: L1 and C1 are not actual electrical components, and only appear
to have these values at or near to the L1-C1 resonant frequency.
However, C2 essentially is a physical electrical capacitor consisting
of the plating on each face of the crystal, with the crystal as the
dielectric between.]

L1 is very large (possibly 1H or more, depending on the frequency). C1
is very small (say only a few pF or even a fraction of a pF - again
depending on the crystal frequency). [So adding a relatively large
series trimmer capacitor has very little effect on the frequency.] C2
is typically around 5pF, regardless of frequency.

Imagine doing a test where you look at the resonant frequency of a
crystal, using a signal generator. This feeds an RF signal through a
crystal, into a 50 ohm load. You measure throughput of the crystal by
measuring the voltage across the load.

Swing the sig gen frequency slowly from LF to HF, through the resonant
frequency of L1-C1. [Let's forget about C2 for the moment.] Below the
resonant frequency of L1-C1, the L1-C1 circuit acts like a small
capacitor, so there is very little throughput. Above the series
resonant frequency of L1-C1, the L1-C1 circuit acts like a large
inductor, so again there is very little throughput. However, when you
hit the series resonance of L1 and C1 (F1), reactance of L1 and C1
cancel. The crystal acts like a short-circuit (or nearly so) and
there is a large throughput. Because the L-C ratio is very high, the
resonance peak is very sharp.

The effect of C2 across the L1-C1 circuit is to produce a second
(parallel) resonant circuit. VERY slightly HF of the L1-C1 resonance,
C2 resonates with effective inductance of the L1-C1 circuit. This
produces a parallel resonant circuit (F2). Another way of looking at
it is that L1 resonates with the series combination of C1 and C2 (so
F2 must be higher than F1). The parallel resonance is, of course, a
high impedance, where there is almost no throughput through the
crystal.
As a result of this double resonance, the crystal acts as a
series-tuned circuit at F1 (one you want), and a parallel-tuned
circuit at F2. The transition between the two is very sudden. The
frequency response peak of the throughput is very lopsided, and gets
chopped off suddenly on the HF side.

The difference between F1 and F2 is very small (a few Hz to a few kHz,
depending on the frequency and type of the crystal). If F1 is lower
than you want, and you add an external series trimmer capacitor to
try and pull the crystal L1-C1 series resonance HF, you effectively
hit a brick wall with the parallel resonance at F2. The parallel
resonance will block any throughput at (or near) this frequency.

A possible solution is to neutralize C2.
[Note: Neutralization is a technique sometimes required when using VHF
crystals, as C2 may be large enough to allow the oscillator to
free-run, instead of being locked to the frequency of L1-C1. However,
it may also be used with advantage, as described below.]
You can neutralize C2 by adding an inductor across the crystal (ie in
parallel with C2). The value required is that which parallel-resonates
with C2 at the crystal frequency. In effect, C2 no longer exists. With
C2 neutralized, there is no longer a sudden transition from the wanted
series resonance F1 to the unwanted parallel resonance F2. The peak
the response curve of the throughput of the crystal (at F1) is now
nice and symmetrical, without the sudden cutoff at F2. In practice,
the actual F1 peak will probably be somewhat more HF than before, and
the crystal should be more pullable with a series capacitor.

Finally, if you reduce the value of the inductor so that its resonance
with C2 is somewhat higher than the crystal frequency, this tends to
pull the F1 resonance peak even higher in frequency. However, if you
overdo this, the oscillation will probably unlock from the crystal,
and start to free-run.

As I said, sorry for the ramble.
Ian.

OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You
should set your clock to way in the future and repost that message so it
sticks around for a while. ;-)

So to make a long story short I need to put an inductor of roughly 400uH
across the crystal to cancel the 5pF of C2. Wow that's a ton of
inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it.
I'll let you know how that works out. I found another crystal,
unfortunately it's identical and possibly from the same batch. I haven't
tried it yet, but I'm not expecting any miracles. I'm tired of burning my
fingers unsoldering parts, so I'm goint to tinker on the breadboard with
another 602 set up just for the oscillator testing with capacitor changes.
I will apply the new coil to the soldered up version though.

The receiver hears, as we just had a storm earlier and I could hear
lightning crashes in the distance. In my narrow tuning range, I can hear
what is likely the carrier of a broadcaster too, or maybe my TV. Later
tonight when the band opens up some more, I should hear something from W1AW
hopefully.

Thanks again

wrote:
On Mar 14, 4:23 pm, "Anthony Fremont" wrote:

If you can get to 1 KHz below the transmitter, you should be able to
copy CW as 1KHz audio. If you can only get to 1.5 KHz below, you'll
get 1.5 KHz audio... not ideal listening, but probably workable.

Right. I was hoping to be able to "get on the other side" of the
signal to have a second chance at avoiding possible QRM.

My guess is the project's original author hadn't planned on that...
most likely, they'd planned on rubbering the crystal only far enough
to get a reasonable audio tone from the RF-LO combination.

Yeah, I probably expect too much from it. On a side note, IT WORKS. I can
hear stations, but the noise (man made) is something awful. It really needs
a narrow audio filter.

Obviously you can get more range out of a VFO, though building nice
VFO's isn't simple.

Even the tuning caps can be a pain. I spent a while as a teenager
knocking alternate plates out of old 365 pf AM broadcast caps to try
to make some suitable for 40 meters - or you can use a series
capacitor. Beware hand capacitance when you go to tune it. Today,
varacter tuning is another option - stable regulated supply and a
multi-turn pot.

Another thing you might do is google the "poundshop" (dollar store)
receiver projects. Those are little KHz-IF varactor tuned auto-
scanning FM radios that people have been modifying into direct
conversion ham band receivers.

I found some stuff on those, the look pretty neat. I may have to head to
the dollar store and see what I can find. ;-)

In message , Anthony Fremont
writes
Ian Jackson wrote:


As I said, sorry for the ramble.
Ian.

OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You
should set your clock to way in the future and repost that message so it
sticks around for a while. ;-)

So to make a long story short I need to put an inductor of roughly 400uH
across the crystal to cancel the 5pF of C2. Wow that's a ton of
inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it.
I'll let you know how that works out. I found another crystal,
unfortunately it's identical and possibly from the same batch. I haven't
tried it yet, but I'm not expecting any miracles. I'm tired of burning my
fingers unsoldering parts, so I'm goint to tinker on the breadboard with
another 602 set up just for the oscillator testing with capacitor changes.
I will apply the new coil to the soldered up version though.

The receiver hears, as we just had a storm earlier and I could hear
lightning crashes in the distance. In my narrow tuning range, I can hear
what is likely the carrier of a broadcaster too, or maybe my TV. Later
tonight when the band opens up some more, I should hear something from W1AW
hopefully.

Thanks again



As you say, at around 3.5MHz, you will need a fairly large inductor to
resonate with 5pF. An alternative might be to make a bridge circuit,
where you actually use another (5pF) capacitor to balance out the
unwanted 5pF. I used to use an extremely simple balancing circuit to
make accurate measurements of the resonant frequencies and ESRs
(equivalent series resistance) of VHF crystals, and it should be
possible to use something similar in an oscillator. However, maybe
someone out there can advise on a tried-and-tested circuit which will
definitely work. [There's no point in re-inventing the wheel!]
Ian.
--

On Wed, 14 Mar 2007 06:26:39 -0500, "Anthony Fremont"
wrote:

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.

---
Here's a good tutorial:

http://www.foxonline.com/techdata.htm
--
JF

John Fields wrote:
On Wed, 14 Mar 2007 06:26:39 -0500, "Anthony Fremont"
wrote:

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.

---
Here's a good tutorial:

http://www.foxonline.com/techdata.htm

Thanks John. :-) I was referring to core saturation and when to suspect
it/materials/etc, but I can sure stand to learn a few things more about
crystals too. That's pretty good information in the link you posted. If
anyone knows about crystals it should be Fox. ;-) I had never tried
pulling one high before, only tweaking them down a little to get them on
frequency. I can pull this one low several kcs without much of a problem
other than stability, but it sure doesn't want to go any higher than about
500Hz above spec. I'm going try the parallel inductance trick to see if I
can get the frequency higher, that should prove interesting. I like doing
reality vs. theory experiments. ;-)

Just to set the record straight....

Someone suggested that you need to 'neutralize' the parallel resonance so
the series resonance can be tuned toward it. This is completely wrong! The
series resonance is, for practical purposes, invariant. The motional
parameters (L and C) of the series resonance are such high reactances (small
capacitance; high inductance) that external components have only a tiny
influence on the series resonance.

The series resonant frequency is the lower of the two crystal 'resonances'.
The parallel resonance is above it. When you make a VCXO with any
substantial tuneability, you're probably operating the crystal at its
parallel resonance. This leads to the common observation that you can 'pull'
a crystal up in frequency more than you you can pull it down. You can only
pull the parallel resonance to approach the series resonant frequency, but
you can't pass it because the crystal is effectively a short-circuit at that
frequency.

Also for the record, the crystal's quartz only has one fundamental and
significant natural resonance - the series resonance. The so-called
'parallel resonance' is actually a controlled spurious resonance caused by
the holder capacitance. At frequencies above series resonanve, the crystal's
RLC equivalent looks inductive, and at some frequency the holder capacitance
will resonate that net inductance.

Joe
W3JDR


"Ian Jackson" wrote in message
...
In message , Anthony Fremont
writes
Ian Jackson wrote:


As I said, sorry for the ramble.
Ian.

OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You
should set your clock to way in the future and repost that message so it
sticks around for a while. ;-)

So to make a long story short I need to put an inductor of roughly 400uH
across the crystal to cancel the 5pF of C2. Wow that's a ton of
inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do
it.
I'll let you know how that works out. I found another crystal,
unfortunately it's identical and possibly from the same batch. I haven't
tried it yet, but I'm not expecting any miracles. I'm tired of burning my
fingers unsoldering parts, so I'm goint to tinker on the breadboard with
another 602 set up just for the oscillator testing with capacitor changes.
I will apply the new coil to the soldered up version though.

The receiver hears, as we just had a storm earlier and I could hear
lightning crashes in the distance. In my narrow tuning range, I can hear
what is likely the carrier of a broadcaster too, or maybe my TV. Later
tonight when the band opens up some more, I should hear something from
W1AW
hopefully.

Thanks again



As you say, at around 3.5MHz, you will need a fairly large inductor to
resonate with 5pF. An alternative might be to make a bridge circuit, where
you actually use another (5pF) capacitor to balance out the unwanted 5pF.
I used to use an extremely simple balancing circuit to make accurate
measurements of the resonant frequencies and ESRs (equivalent series
resistance) of VHF crystals, and it should be possible to use something
similar in an oscillator. However, maybe someone out there can advise on a
tried-and-tested circuit which will definitely work. [There's no point in
re-inventing the wheel!]
Ian.
--

On Mar 16, 5:49 am, "W3JDR" wrote:

The series resonance is, for practical purposes, invariant. The motional
parameters (L and C) of the series resonance are such high reactances (small
capacitance; high inductance) that external components have only a tiny
influence on the series resonance.

Yes....this is the point of a VCXO...to allow an almost
infinitessimally small, but still useful, variation about the crystal
frequency while maintaining most of the crystal's stability.

The series resonant frequency is the lower of the two crystal 'resonances'.
The parallel resonance is above it. When you make a VCXO with any
substantial tuneability, you're probably operating the crystal at its
parallel resonance. This leads to the common observation that you can 'pull'
a crystal up in frequency more than you you can pull it down.

Nearly all VCXO's I've run across work the other way. You can pull
the frequency down substantially while maintaining good stability
(typically on the order of 0.1%), but not up. This certainly applies
to the circuit for which the original poster provided a link.

Do you have any examples of practical circuit schematics which use
parallel resonance and which can be pulled substantially up in
frequency ? I assume it should be possible to do with a parallel
inductor, for example in a Franklin oscillator circuit, but as was
pointed out the inductor values can be inconveniently large.

Steve