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

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.
--

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

On 16 Mar 2007 06:12:27 -0700, wrote:

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
I have to agree with Joe. Basically there is no such thing as an
crystal oscillator in "parallel resonance". However there are
oscillators that use the crystal in the feedback path to add
substantial phase shift. Such as in the Pierce oscillator where it
behaves inductive. The phase shift changes so rapidly that it can
still make a low-drift oscillator.

In the book by Matthys where he compares various oscillators there is
one in chapter 10.6 where the deviation from the (series) resonant
point is the highest. It is a circuit where the crystal sees a very
high impedance as opposed to regular circuits where highest Q is
obtained with very low impedance. This in effect makes the crystal
load to be around C0 of the crystal with some output and input
capacitance. And therefore it is probably the smallest effective
physical series capacitance obtainable and thus the highest frequency.

Looking up the Franklin oscillator you mentioned, I notice this also
is providing a high impedance to the resonant elements. So yes, it
seems a valid way of implementing an alternative to Matthys' example.

Now also cancelling the effect of C0 of the crystal by adding parallel
inductance might push it a bit further. Right now I would not be able
to predict the effect on loaded Q of the crystal. Lowering Q is
normally not done, but in this case we are primarily in quest for wide
pulling range right?

In a low impedance Butler (overtone) oscillator I have seen C0
cancellation by using parallel inductance as well. There sometimes is
an L plus series R used to lower the Q of the L/C0 combination. This
seems not appropriate for a high impedance oscillator circuit. I would
expect best effect if the Q of the inductor is high (low Rs).

Sorry this is still theory. I have no examples of VCXO in this
context.

Cheers,

Joop

"Joop" wrote in message
...
On 16 Mar 2007 06:12:27 -0700, wrote:

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
I have to agree with Joe. Basically there is no such thing as an
crystal oscillator in "parallel resonance". However there are
oscillators that use the crystal in the feedback path to add
substantial phase shift. Such as in the Pierce oscillator where it
behaves inductive. The phase shift changes so rapidly that it can
still make a low-drift oscillator.

In the book by Matthys where he compares various oscillators there is
one in chapter 10.6 where the deviation from the (series) resonant
point is the highest. It is a circuit where the crystal sees a very
high impedance as opposed to regular circuits where highest Q is
obtained with very low impedance. This in effect makes the crystal
load to be around C0 of the crystal with some output and input
capacitance. And therefore it is probably the smallest effective
physical series capacitance obtainable and thus the highest frequency.

Looking up the Franklin oscillator you mentioned, I notice this also
is providing a high impedance to the resonant elements. So yes, it
seems a valid way of implementing an alternative to Matthys' example.

Now also cancelling the effect of C0 of the crystal by adding parallel
inductance might push it a bit further. Right now I would not be able
to predict the effect on loaded Q of the crystal. Lowering Q is
normally not done, but in this case we are primarily in quest for wide
pulling range right?

In a low impedance Butler (overtone) oscillator I have seen C0
cancellation by using parallel inductance as well. There sometimes is
an L plus series R used to lower the Q of the L/C0 combination. This
seems not appropriate for a high impedance oscillator circuit. I would
expect best effect if the Q of the inductor is high (low Rs).

Sorry this is still theory. I have no examples of VCXO in this
context.

The reason for the parallel inductance in the overtone mode is the
low impedance of the crystal self capacitance at high frequencies,
this on its own can be quite low, and the series resistance
at overtone can be quite high, so this can allow the tank circuit to
dominate rather than the crystal unless it is cancelled.

Not sure about the resistor not seeing the circuit.

the statement all crystal circuits operate in series mode stems from the
fact that the internal eq circuit of a crystal is a series LC, any
capacitance in parallel with the crystal can only be in series with the
internal motional capacitance.

If your lucky you may be able to find a spurious node, but it would be hard
to make it oscillate at this point.

Colin =^.^=

On Fri, 16 Mar 2007 22:51:34 GMT, "colin"
wrote:
The reason for the parallel inductance in the overtone mode is the
low impedance of the crystal self capacitance at high frequencies,
this on its own can be quite low, and the series resistance
at overtone can be quite high, so this can allow the tank circuit to
dominate rather than the crystal unless it is cancelled.

I know why the inductor is usually necessary. The thing is that for
the purpose of the Butler the effect of C0 can be neutralized.
The question is whether it can be made to have a similar effect in the
"pulling arena" as well. Of course it should not have to much side
effects in normal operation of the oscillator.

In the overtone butler there is also another LC resonance circuit
present that determines the possible operating frequency (read desired
overtone). Such a thing might be necessary in most circuits where an
inductor is placed in parallel with a crystal.

Joop

In message lDuKh.7998$vV3.3900@trndny09, W3JDR writes
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!
It may be 'completely wrong', but my experience with getting out-of-spec
(too LF) VHF overtone crystals up to the required frequency indicates
that it does enable the oscillator to work at a slightly higher
frequency than it 'wants to'. This is because the throughput peak of the
series resonance moves HF when the sudden parallel resonance is removed.
[The assumption is that oscillation occurs at the peak of the series
resonance, which may not be entirely true.]

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.
This is more-or-less what I said. The influence of the relatively large
series trimmer capacitor will be pretty small.

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.
Lots of technical information calls the actual parallel resonance
'anti-resonance', and indicates that there is an 'area of parallel
resonance' between the true series resonance and the spurious parallel
resonance. In this area, the impedance of the crystal rapidly changes
from being zero (at the series resonant frequency) to infinitely
inductive (at the anti-resonant frequency). In many oscillator circuits,
the oscillation occurs neither at the series resonant nor the parallel
(anti-) resonant frequencies. Instead, the actual frequency of
oscillation will be determined by some value of this inductance and the
external capacitors, and also on the phaseshift and amplitude of signal
throughput through the crystal. All very complicated!

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.
And neither can you use external elements to pull the series resonance
very far HF, because it runs into the parallel resonance. From my
experience, a swept frequency response through a crystal shows that the
throughput peak of the series resonant frequency never really reaches
its full amplitude before it starts to get pulled down in parallel
resonance hole. Neutralizing the shunt capacitance prevents the parallel
resonance from occurring so close to the series resonance. As a result,
the frequency response throughput curve becomes symmetrical, and the
actual peak is somewhat further HF. Certainly, my oscillators (which
were supposed to operate at the true series resonance of the crystal)
DID move HF when I neutralized the crystal.

[Note that the full frequency response of a crystal with a parallel
neutralizing inductor, from DC to well above the crystal frequency,
consists of a broad notch centred on the crystal frequency (the parallel
resonance of the parallel capacitance of the crystal and the
neutralizing inductor). In the centre of the notch is a very narrow
bandpass (the series resonance of the crystal).]


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.
Exactly so.

At the parallel (anti-) resonance, the reactance of the crystal suddenly
jumps from being infinitely inductive to being infinitely capacitive
(0p). As you move further HF, it stays capacitive, progressively
decreasing in reactance. The parallel resonance therefore presents a
brick wall, beyond which external capacitors cannot resonate with the
inductive reactance of the crystal. However, if you neutralize the
crystal, you kill the sudden transition from series to parallel
resonance, and the frequency range over which the crystal is inductive
is considerably extended. This should enable the resonance with external
capacitors to extend further HF than when the crystal is not
neutralized.

As I originally said, neutralization of the crystal was a suggestion,
rather than a panacea. I still reckon that should work. It's worth a
try. Unfortunately, the size required for the inductor (which resonates
with the crystal parallel capacitance of appx only 5pF) is rather large.
If neutralization DOES help, a brute force method of allowing a somewhat
smaller inductor to be used would be to deliberately add MORE parallel
capacity, and lower the value of the inductor to suit. A more elegant
method would be to build the crystal into a simple bridge circuit, so
that a neutralizing capacitor could be used instead of an inductor.
However, I appreciate that the object of the exercise is to make a
simple receiver, and it would be somewhat incongruous to need a very
complicated circuit just for the crystal.

Ian.

--

"W3JDR" wrote in message
news:lDuKh.7998$vV3.3900@trndny09...
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.
--