[time-nuts] How far can I push a crystal?

[Bernd Neubig]

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 at febo.com [mailto:time-nuts-bounces at febo.com] Im
Auftrag von Ed Breya
Gesendet: Freitag, 18. Januar 2013 00:39
An: time-nuts at 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

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