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

Tim Shoppa tshoppa at gmail.com
Sat Jun 4 08:51:08 EDT 2016

With random old crystals in holders, it often helps to disassemble the
holder and clean crystal and holder plates with alcohol (my favorite back
in my youth was carbon tetracholoride but not so easy to find these days.)
I'm guessing most of your round blanks were for FT-243 type holders.

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.)

Many surplus crystals from the 50's will be marked with their series
resonant frequency. The military surplus tank crystals are marked with
channel number. By the 60's and 70's, crystals for VHF radios were often
marked with the channel frequency (that's after multiplication and mixing
with IF if a receive crystal.)

Tim N3QE

On Sat, Jun 4, 2016 at 4:46 AM, 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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