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.

--

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

On Mar 13, 10:02 am, "Anthony Fremont" wrote:
Hello all,

... supposed to pull the colorburst crystal
high in frequency then the adjustable cap should be able to tweak it back
down.

It's only possible to pull a crystal a little way from its center
frequency
by external components. If you put a resistor in parallel with the
crystal (to lower its Q) you can increase the pull range.

"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 , Joop
writes
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


I've eventually found some good plots of the frequency responses of
crystals (see below).

http://g4oep.atspace.com/crystal_fil...rystal_filters
..htm#4)%20Single-Crystal%20Ladder.

This info from G4OEP is about filters, rather than VXOs, but it
graphically illustrates how a crystal can be neutralised in a bridge
circuit, using a small capacitor rather than large inductor. The
capacitor will be the same value as the parallel capacity of the
crystal.

The plots in Figs 9, 10 and 12 also show how the throughput peak moves
around to some extent as the neutralization is adjusted. The circuit is
essentially the same as I used when I was testing a load of crystals to
see if they met spec wrt frequency accuracy and ESR.

You should be able to use the circuit of the single crystal filter
(in-to-out, without the resistive padding etc) as the resonant part in a
VXO, and adjust the neutralization to pull the frequency of oscillation.
It's worth a try.

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.
--

On Tue, 20 Mar 2007 12:08:57 +0000, Ian Jackson
wrote:

You should be able to use the circuit of the single crystal filter
(in-to-out, without the resistive padding etc) as the resonant part in a
VXO, and adjust the neutralization to pull the frequency of oscillation.
It's worth a try.

Ian.
The images primarily showed how the stopband notch moved around.
So I decided to put it into spice.

Compared to L compensation it does seem to have a benefit as higher
frequency peak. It also is not troubled by the side-effect of
L-compensation as passing lower and higher frequencies than those
around the crystal frequency.

Trying to make the most of the balancing compensation I placed a small
capacitor in series with the crystal. This moves the pass band
frequency up a bit more. But the smaller the series cap, the less
pronounced the peak seems to be. Also the circuits starts to attenuate
more and more. This might cause difficulty in an oscillator setup
where the loop gain should stay more than one.

Also with the balanced compensation circuit, the phase is changed
around the peak frequency. Without series cap around -45 degrees,
climbing to -83 with 12pF. This should be accounted for in the
feedback loop of an oscillator. (A properly dimensioned L-compensated
crystal does not change phase.)

But all in all it might be a (complex) method of shifting the working
frequency up.

Joop

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

You need to reduce the inductor.. that is causing a down swing in
your freq.

Thanks, I took it out and the frequency increased by only about 70Hz, but
it did increase. :-) Do you know of anything else I can do to increase
the frequency by about another couple of kcs?


Get the right freq crystal.

Jimmie