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

[Bob Camp]

Hi

The original question related to crystals that ran down into the 10’s of KHz range. For those, a
high resistance is not unreasonable at all. For more “normal” AT cut resonators in “big" packages
it is not unreasonable to expect the resistance on the fundamental to be approximately the same 
as the frequency in MHz. As the overtone goes up, it is not unusual for the resistance to go up
as the overtone number. There are an almost infinite number of qualifiers on all of that. 

The range of crystal resistance between things like 3 MHz AT’s and 32 KHz watch crystals is one
of the things that makes building an one size fits all oscillator difficult. 

Bob


> On Jun 5, 2016, at 9:04 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> Bob -
> 
>  Crystal resistance of 100K probably applies to the large low frequency
> crystals. Modern HF fundamental crystals tend to be around 50 ohms at their
> fundamental and I think higher for overtone crystals. Older crystals in the
> HF range in the FT-247 holders seem to usually be several hundred ohms.
> 
>  I've certainly swept crystals before to look for spurs, but I usually use
> small sweep ranges like a few hundred Hz around where I know the spur to be
> anyway. This is kinda like the drunk light only looking under the
> streetlights for his keys because he knows he'll never find them if they're
> anywhere else. Not to sound too pessimistic but when applying this to
> crystals from the 50's it seemed like it was a lot easier to find the spurs
> than the fundamental!
> 
> Tim N3QE
> 
> On Sat, Jun 4, 2016 at 8:29 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Simple approach is to assume that you have a crystal resistance in the
>> range of 100K ohms. If you guess to
>> high, the oscillator will just work to well :)
>> 
>> Multiple JFETS in cascade for the higher frequency stuff should work. For
>> anything below 20 KHz, an op-amp
>> is likely your best bet.
>> 
>> A spectrum analyzer with a tracking generator will give you some idea of
>> their impedance and *if* they still have
>> any activity at all.
>> 
>> Bob
>> 
>>> On 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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