[time-nuts] Re: Custom Quartz crystals

[Bob kb8tq]

Hi

One “interesting” way to look at this is a “delta F” test.
It’s based on a system that (essentially) always measures
things at series. You put the (maybe) 20 pf  load cap in series 
with the crystal and zip off it goes to measure the net result 
of the crystal at a 20 pf load.

Typically you did this by pushing a paddle on a fixture. Yes
that’s very specific to one rig up. Turns out that Motorola 
decided it was a fun thing and we tested a *lot* of crystals
that way. It’s been “a while” … but .. fundamental crystals
came in at > 400 ppm. Third overtones might be around 
30 ppm. Fifth overtones might be around 3 to 5 ppm. 

No this isn’t for the same crystal or even the same crystal
package. It also isn’t for SC cuts, these all where AT’s. 

… and yes, if I could remember the name if the UK outfit
that made the test heads, I’d give them credit for their cute
little gizmo. 

One interesting outcome of testing this way was to spec
crystals as some number of PPM off series rather than 
with a load. Changing the load cap in the fixture was “not
done”.  AFIK that was very specific to internal Motorola
documents. The only time I saw it used elsewhere was
on crystals that operated below series. 

Bob

> On Aug 17, 2022, at 5:28 AM, Richard (Rick) Karlquist via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> It is hard to answer your question.  The tuning range
> depends on the varactor(s) used in the respective
> oscillators, and the range of the tuning voltages.
> I think I could roughly say from memory that putting
> in a crystal meant for the other oscillator would
> probably result in not being able to reach 10.000000
> MHz.  Or just barely reaching it and having no
> aging margin.  I had to apply a lot of arm twisting
> to Jack Kusters to get him to make the series
> resonant (at 10 MHz) crystals.  I made sure that
> this was really necessary before going to war on it.
> 
> Rick N6RK
> 
> On 8/14/2022 3:35 PM, Hal Murray via time-nuts wrote:
>> Richard (Rick) Karlquist said:
>>> We actually used the upper turnover because the E1938 had to operate at
>>> ambient up 85  degrees.  BTW, the E1938 crystals had to be calibrated for
>>> 10.000000 MHz at series resonance, unlike the 10811 crystals which were
>>> calibrated for 20 pF (IIRC).
>> How big is the difference between series and 20pF compared to the tuning range?
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