[time-nuts] How to get unknown frequency quartz crystals oscillating

Szeker K. szeker.k at gmail.com
Sat Jun 4 15:57:50 EDT 2016


XTALs does work in air, or better: dry gases (i.e. nitrogen) too, but OK;
quality exemplares are (for better, spurious free, oscillating) in vacuum...



2016-06-04 19:41 GMT+02:00 Mike Cook <michael.cook at sfr.fr>:

> Thanks all for your advice, hints, tips and links. Lots to read , do and
> some hardware to check.  I don’t have a frequency generator so I’ll have to
> go another route.
> Oh. One last Q. Has anyone tried repairing the « spring » wire electric
> connections on large quartz plates. In one large unit I have they had
> corroded and dropped the plate, luckily no damage.  I have done one, but I
> have no Idea what the original wire composition was so have certainly
> induced some stray capacitance/resistance. It is possible that it was a
> filter rather than a frequency source as it was not in a vacuum.
> Have a good one.
> > Le 4 juin 2016 à 18:49, Bernd Neubig <BNeubig at t-online.de> a écrit :
> >
> >
> > Tim Shoppa wrote:
> >> The Pierce logic-gate-biased-active oscillator is pretty reliable to
> start and will oscillate somewhere with most crystals from kHz to MHz.
> >> As you found out, it will often come up on one of many overtones.
> >> To reduce chance of coming at an overtone, a series resistor from logic
> gate output to the crystal is often enough. If not, a RC low-pass will cut
> down even further (although of course adding phase shift.)
> >
> > This is certainly the easiest and fastest way for a go/no-go test and to
> find the approximate resonance frequency.
> > In the attached circuit diagram make CX1 and CX2 about 10 pF and RGK
> several MegOhms.
> > The inverter gate should be preferably an unbuffered HCMOS or other fast
> inverter.
> > For crystals in the MHz range you can replace RV by a short, for kHz
> crystals make it a few kOhms. If testing small watch crystals @ 32768 kHz
> or around, RV should be 100 kOhm at least. RV reduces the crystal drive
> level (RF current) to an acceptable level to avoid overloading or even
> damaging of the crystal. For low frequency crystals the RV-CX2 lowpass also
> avoids start-up at the overtone.
> > It is recommended to add a second inverter gate at the output to isolate
> your oscilloscope or counter input from the oscillator stage. Add some >330
> ohm in series to the output of the 2nd inverter, if you connect a coaxial
> cable. Then terminate the coax at the oscilloscope or frequency counter end
> with 50 Ohms, so the square wave form will be roughly maintained.
> >
> > In this circuit the crystal will not operate at its series resonance,
> but at a load resonance with load capacitance of something between 8 pF and
> 10 pF (depending on the inverter input and output capacitance plus the
> stray capacitances of your test fixture).
> > If you want to operate the oscillator at a (low) overtone, such as 3rd
> (or maybe 5th), you must add a series combination of 10 nF plus an inductor
> in parallel to CX2. The 10 nF is to avoid DC short-circuiting of the
> output. The inductor together with CX2 must have a resonance frequency mid
> between fundamental mode and 3rd overtone (not at one of them). So the
> tuned circuit acts like a capacitor at the 3rd OT and is inductive at
> fundamental mode (thus the phase condition for oscillation is not fulfilled
> at the fundamental mode)
> > Have fun
> >
> > Bernd
> > DK1AG
> > <Pierce_Logic_Gate.gif>_______________________________________________
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