I've got to make a very clean 10.05944444... MHz VCXO for a redo of one
of my old circuits. I previously used a 10 MHz ceramic resonator, which
was easy enough to push around in frequency. Of course, I have a couple
dozen of those somewhere, but can't find them now that I need them
again. I figured I'd just pull the ones out of the old circuit, but
since I did find a whole bunch of 10 MHz quartz crystals, I'd like to
revisit whether I can push one of those that far with decent results. As
I recall, the results of my previous experiments in doing this were less
than satisfactory, which is why I went with the ceramics.

This would be a change of 60 kHz out of 10 MHz, or 0.6 percent - a
helluva lot for a crystal. The frequency will be exactly phase locked to
a reference. It doesn't need to have extremely high in-circuit Q or
long-term stability - just tunable to that magic number - the PLL will
do the rest. A conventional varicap circuit will provide the VCO-ness,
while the tuning range just needs to be enough to accommodate drift and
the initial setting. The power gain element will be a 74HC04 or 74HC86
section. The PLL reference will be 59.44444...  kHz - way above the
necessary loop BW.

Has anyone successfully pushed a quartz crystal that far off, with
reliable (still sort of a sharp resonance) operation and no spurious
modes? Any ideas? If this isn't practical, I'll just go back to the
ceramic resonator (which worked just fine), but I'd like to settle it
once and for all.

Ed

You'll never pull a crystal that far without grinding :).

Regards,
Tom

----- Original Message -----
From: "Ed Breya" eb@telight.com
To: time-nuts@febo.com
Sent: Thursday, January 17, 2013 6:38 PM
Subject: [time-nuts] How far can I push a crystal?

I've got to make a very clean 10.05944444... MHz VCXO for a redo of one
of my old circuits. I previously used a 10 MHz ceramic resonator, which
was easy enough to push around in frequency. Of course, I have a couple
dozen of those somewhere, but can't find them now that I need them
again. I figured I'd just pull the ones out of the old circuit, but
since I did find a whole bunch of 10 MHz quartz crystals, I'd like to
revisit whether I can push one of those that far with decent results. As
I recall, the results of my previous experiments in doing this were less
than satisfactory, which is why I went with the ceramics.

This would be a change of 60 kHz out of 10 MHz, or 0.6 percent - a
helluva lot for a crystal. The frequency will be exactly phase locked to
a reference. It doesn't need to have extremely high in-circuit Q or
long-term stability - just tunable to that magic number - the PLL will
do the rest. A conventional varicap circuit will provide the VCO-ness,
while the tuning range just needs to be enough to accommodate drift and
the initial setting. The power gain element will be a 74HC04 or 74HC86
section. The PLL reference will be 59.44444...  kHz - way above the
necessary loop BW.

Has anyone successfully pushed a quartz crystal that far off, with
reliable (still sort of a sharp resonance) operation and no spurious
modes? Any ideas? If this isn't practical, I'll just go back to the
ceramic resonator (which worked just fine), but I'd like to settle it
once and for all.

Ed

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Maybe I should clarify what I meant by pushing the crystal frequency. I
meant only using various topologies and electronic components in the
associated circuitry, that would detune it from its natural resonance
far enough to reach the new frequency, and still have it be sort of a
narrow-bandwidth crystal oscillator - not doing any mechanical changes
to the crystal element itself.

Since the ceramic resonators seem to work well, and can be pushed (or
pulled?) fairly far away by proper selection of the associated component
values, I was wondering how far quartz crystals can reasonably go. I
encounter this situation often - needing an oddball frequency, but
preferring to use common or standard parts. The nominal choices in
ceramic are quite limited, while in crystals, there are many more - but
few ever seem to land at or near enough to a frequency I need.

The only thing I have thought of so far is to maybe add some series R to
drop the crystal Q, so broadening the resonance, and just dragging it up
by extra series C, but at some point there's no point to even having the
crystal there at all. I'm just trying to figure out what's possible and
reasonable.

I know that I can get any custom frequency by spending enough money, but
that takes the challenge and fun out it sometimes.

Ed

Having just adjusted a crystal oscillator at 10.715 MHz, I would hazard
a guess that the most one can easily pull a crystal of nominal frequency
around 10 MHz would be of the order of +/-1 kHz. Certainly not 60 kHz.

dr

Hi Ed,

Only now I found this thread , as I was out for several days.
The essentials were already said by others:
The only way to pull the crystal frequency by significantly more than say
100 ppm is to use a series inductor, which will shift the resonance
frequency of the combination L-Xtal downwards. To be accurate: The resonance
frequency of the crystal itself is not "pulled", it is unchanged. Only the
combination with a reactance in series results in a new resonance frequency
of the overall circuit, which is below the series resonance of the crystal
(and above fs if a series capacitor is used).
There are two risks and limitations by "pulling" the frequency with a series
inductor:
1- the combination creates an additional series resonance above the parallel
resonance. This undesired resonance can sometimes have a lower resistance
than the desired one. Its excitation can be attenuated by adding a resistor
in the 10 kOhm range across (parallel to) the crystal
2- The larger the series inductance is, i.e. the wider the frequency is
shifted down, the more contributes the inductor's (low) Q and stability to
the overall performance. For a fundamental mode AT crystal a reasonable
limit is in the range of -500 ppm to -1000 ppm (= 5 to 10 kHz @ 10 MHz). If
you "pull" much wider, finally he crystal is only a decoration, and the coil
governs the performance.

A parallel inductor for compensation of the static capacitance C0 does not
help much at 10 MHz, because such a coil, which resonates out a 6 pF
capacitance has an internal winding capacitance, which is larger than 6 pF.
So you would need a coil which has a self-resonance of slightly above 10
MHz.

BTW: The whole subject of frequency pulling was covered by me in detail in a
publication in VHF Communications (UKW-Berichte) in 1979 . You can download
the paper under www.axtal.com/data/publ/ukw1979_e.pdf

Another remark:
Higher pulling ranges up to +-5000 ppm with still maintaining crystal class
stability can be realized with Langasite (LGS) resonators. LGS is a
piezoelectric crystal of the same crystallographic class as quartz with much
higher piezoelectric coupling factor, and thus higher motional capacitance
C1 at the same C0 as quartz.
More informations can be found on www.axtal.com under Technical Notes -
Technical Articles and publications.

Best regards

Bernd
DK1AG

-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] Im
Auftrag von Ed Breya
Gesendet: Freitag, 18. Januar 2013 00:39
An: time-nuts@febo.com
Betreff: [time-nuts] How far can I push a crystal?

I've got to make a very clean 10.05944444... MHz VCXO for a redo of one of
my old circuits. I previously used a 10 MHz ceramic resonator, which was
easy enough to push around in frequency. Of course, I have a couple dozen of
those somewhere, but can't find them now that I need them again. I figured
I'd just pull the ones out of the old circuit, but since I did find a whole
bunch of 10 MHz quartz crystals, I'd like to revisit whether I can push one
of those that far with decent results. As I recall, the results of my
previous experiments in doing this were less than satisfactory, wh
ich is why I went with the ceramics.

This would be a change of 60 kHz out of 10 MHz, or 0.6 percent - a helluva
lot for a crystal. The frequency will be exactly phase locked to a
reference. It doesn't need to have extremely high in-circuit Q or long-term
stability - just tunable to that magic number - the PLL will do the rest. A
conventional varicap circuit will provide the VCO-ness, while the tuning
range just needs to be enough to accommodate drift and the initial setting.
The power gain element will be a 74HC04 or 74HC86 section. The PLL reference
will be 59.44444...  kHz - way above the necessary loop BW.

Has anyone successfully pushed a quartz crystal that far off, with reliable
(still sort of a sharp resonance) operation and no spurious modes? Any
ideas? If this isn't practical, I'll just go back to the ceramic resonator
(which worked just fine), but I'd like to settle it once and for all.

Ed

time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Thanks all, for the feedback on this issue. In summary, I got these
points out of the discussion on crystals:

1. The correct terminology is "pulling" the frequency.
2. Getting beyond about a few hundred ppm from the nominal frequency
   ranges from very difficult to pointless.
3. It's easier to pull down than up.

It looks like it would not be worth fooling around with crystals, so
I'll just use the ceramic resonators. By the way, I just tonight managed
to reach the correct geological layer of stuff out in the garage, and
found the missing 10 MHz resonators, and a whole tray of other parts
that were in reserve for completing this project from a couple of years ago.

For the curious: The 10.0594444... MHz is made by a PLL using the
59.4444... kHz reference, which is 10.7 MHz divided by 180. The 10.7 MHz
is a from another VCXO (which can use a standard crystal, ceramic
resonator, or ceramic IF filter - easy) that's phase locked to a 10 or 1
MHz reference, using two fixed dividers. The 10.0594444... MHz is used
as the reference for a phase locked microwave brick oscillator, using
n=120, to make 1207.1333... MHz, which is exactly one-third of 3621.4
MHz, the low-band upconversion IF of the HP8566B spectrum analyzer. The
1207.1333... MHz is harmonically mixed (m=3) with the first LO of the SA
to produce the tracking signal centered in the passband of the SA. All
of this is built into the modified carcass of an HP8443A tracking
generator, originally built for older SA models. Using the new stuff,
plus parts of the 8443A, the net result is a 50 kHz to 250 MHz tracking
generator, with power up to +10 dBm, leveled within about 1 dB, and with
130 dB step attenuator range - very nice for low RF and baseband work.

The 10.0594444... MHz is only one of many frequencies that could be
multiplied by various n-values to give the same result, but it was
chosen because it was very close to a standard frequency available in
ceramic resonators, high enough that n didn't need to be too large, and
it could be synthesized with a very simple PLL system.

I had all of this built and running, but I had made the fatal
engineering mistake of putting way too much stuff in too small a space.
Space was tight, so I squeezed the entire LF control system and
synthesizers into one small can, and necessarily optimized for minimum
IC package count. Then I found that there was too much crosstalk between
virtually all the signals in the box, so there was too much phase noise
to work at 300 Hz and less IFBW. The problems were irreversible -
sharing IC packages for multiple signal processing was an especially bad
move. After many hours of rearranging signal paths, adding shielding and
grounds, and changing topologies, I concluded that I had to rebuild it
the right way. So here I am. The two main frequencies will be generated
in separate boxes, and no ICs will contain multiple signals that aren't
being processed together.

This time I'll get it right, and finally wrap it up.

Ed

Hi

If you are going to multiply this up to microwaves, you may have some issues
with phase noise....

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Ed Breya
Sent: Friday, January 18, 2013 1:33 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] How far can I push a crystal?

Thanks all, for the feedback on this issue. In summary, I got these
points out of the discussion on crystals:

1. The correct terminology is "pulling" the frequency.
2. Getting beyond about a few hundred ppm from the nominal frequency
   ranges from very difficult to pointless.
3. It's easier to pull down than up.

It looks like it would not be worth fooling around with crystals, so
I'll just use the ceramic resonators. By the way, I just tonight managed
to reach the correct geological layer of stuff out in the garage, and
found the missing 10 MHz resonators, and a whole tray of other parts
that were in reserve for completing this project from a couple of years ago.

For the curious: The 10.0594444... MHz is made by a PLL using the
59.4444... kHz reference, which is 10.7 MHz divided by 180. The 10.7 MHz
is a from another VCXO (which can use a standard crystal, ceramic
resonator, or ceramic IF filter - easy) that's phase locked to a 10 or 1
MHz reference, using two fixed dividers. The 10.0594444... MHz is used
as the reference for a phase locked microwave brick oscillator, using
n=120, to make 1207.1333... MHz, which is exactly one-third of 3621.4
MHz, the low-band upconversion IF of the HP8566B spectrum analyzer. The
1207.1333... MHz is harmonically mixed (m=3) with the first LO of the SA
to produce the tracking signal centered in the passband of the SA. All
of this is built into the modified carcass of an HP8443A tracking
generator, originally built for older SA models. Using the new stuff,
plus parts of the 8443A, the net result is a 50 kHz to 250 MHz tracking
generator, with power up to +10 dBm, leveled within about 1 dB, and with
130 dB step attenuator range - very nice for low RF and baseband work.

The 10.0594444... MHz is only one of many frequencies that could be
multiplied by various n-values to give the same result, but it was
chosen because it was very close to a standard frequency available in
ceramic resonators, high enough that n didn't need to be too large, and
it could be synthesized with a very simple PLL system.

I had all of this built and running, but I had made the fatal
engineering mistake of putting way too much stuff in too small a space.
Space was tight, so I squeezed the entire LF control system and
synthesizers into one small can, and necessarily optimized for minimum
IC package count. Then I found that there was too much crosstalk between
virtually all the signals in the box, so there was too much phase noise
to work at 300 Hz and less IFBW. The problems were irreversible -
sharing IC packages for multiple signal processing was an especially bad
move. After many hours of rearranging signal paths, adding shielding and
grounds, and changing topologies, I concluded that I had to rebuild it
the right way. So here I am. The two main frequencies will be generated
in separate boxes, and no ICs will contain multiple signals that aren't
being processed together.

This time I'll get it right, and finally wrap it up.

Ed

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Yes Robert, the 59.4444 kHz is effectively added to the 10.0000 MHz, but
not by direct mixing. The 1 or 10 MHz reference drives a D-flop flop,
which samples the 10.059444 MHz, leaving the difference frequency
59.4444 kHz, the feedback signal in the second PLL. The direct way to do
it would be with very accurate, full I-Q mixing to get only one
sideband, but that gets very complicated. The current scheme is simpler,
and works quite well. The main pieces are decade dividers (74HC390),
dividers for 107 and 180 (74HC393 or 74HC4040 each), a D-FF (74HC74), a
CD4046B for each PLL, and gates for the oscillators (74HC04 or 74HC86).
Using an '86 allows for getting push-pull output with equal prop delays,
in case I need to run it through some differential LAN LPF modules that
I have on hand.

It took some effort to come up with workable numbers that all fit within
the constraints, but I'm sure there are many other undiscovered sets
that would do it. I like your single-PLL 300/953 idea - it may be doable
within 74HC speeds, and I think ceramic resonators are available at 32.0
MHz. The PLO would like the much higher reference frequency - I think
any n from 8 to 150 or so will work.

Scaling that by two to 600/953 , making 15.88333 MHz, with a 16 MHz
resonator (I have some), fc=16.6666 kHz, and n=76 should work too. It
would be OK with 74HC for sure, and it would just fit through the LPFs,
which cut off at 17 MHz. The comparison frequency fc is getting kind of
low, but may be OK, depending on how much near-in phase noise I have to
contend with. That was one of the reasons I opted for the two-stage
approach - to avoid having a very small fc, or dealing with fractional-n
ripple.

I will investigate these possibilities and put together some
experiments. I can directly drive the microwave section from an external
synthesizer to try various reference frequencies.

Ed