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

Sat Jun 4 08:05:28 EDT 2016

A vector voltmeter can be replaced by a spectrum analyser or even a 14 bit scope with or without a preamp. In lieu of phase information one can measure the 3dB bandwidth to elicit the Q, combined with measurement of the low frequency shunt capacitance and the attenuation at resonance, the crystal parameters can be extracted without knowledge of the phase shift.
Bruce

    On Saturday, 4 June 2016 11:01 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

 On reflection, building a jig to measure the crystal transmission as a function of frequency may be much easier than attempting to build an oscillator for an unknown crystal. However a suitable signal generator and detector are required.
Once the crystal parameters are known its much easier to design a suitable oscillator circuit. Measuring the location of spurious resonances may also be useful.

Bruce

    On Saturday, 4 June 2016 9:52 PM, Bruce Griffiths <bruce.griffiths at xtra..co.nz> wrote:

 Some idea of the crystal equivalent circuit at the oscillation frequency would help considerably. Usually lower frequency crystals have a considerably higher series resistance than those operated at 1MHz or above.
At frequencies below 100kHz or so a Meacham bridge using something likke a wideband FET opamp may be feasible.
Bruce

    On Saturday, 4 June 2016 9:01 PM, Mike Cook <michael.cook at sfr..fr> wrote:

 Hi,

I have a number of crystals either in glass, bakelite, ceramic or metal housings that I would like to get resonating . They are of three basic types.
 Square, or rectangular flat  
 Round flat 
 Bar  square section
Sizes range from 2-10cm or more in the longest face.

Some have frequency markings. ranging from IKHz 5MHz. 
Others have none.
Some are of  Military origin, probably radios and as they have markings I can probably find a schematic from the radios to see how to proceed.  There may be dedicated testers still around. I am not so interested in this bunch at the moment. 
Others have no known origin so I have no idea what oscillator circuits were used with them. 
In terms of vintage, I would guess pre 1940  to late 50s 

I have built a little Pierce circuit an tried a few. Some of the later 1-5MHz crystals will oscillate but there are a lot of parasitic signals as well as the supposed fundamental. I cannot make any of the low frequency / big crystals to react.

So my question:
If you had a crystal with unknown frequency and drive requirements that you wanted to investigate. How would you go about it?

If I can get them going I will share the Adevs. I don’t have a spectrum analyser so I can’t do phase noise.

Regards

"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

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