[time-nuts] Re: Custom Quartz crystals

[Bob kb8tq]

Hi

Crystals (of any sort) have whatever turnover the manufacturer decided
they wanted them to have. There is no “standard” target for any crystal
cut. It needs to be specified when the crystal is ordered.  That is true 
of heated crystals as well as ones that go into un-heated applications. 
Unless you have a spec, there is no way to “guess” what this or that 
random crystal will have as a turn point. 

Q at a specific frequency is a function of resistance and motional capacitance. 
Pull ability is a a function of motional capacitance, but not resistance. You 
can have a crystal with high Q and reasonable pull ability. It *will* have a 
low resistance. 

Indeed C0 also gets into the pull ability stuff to a degree. To what degree 
depends more than a bit on the circuit used. You can ( and folks very much 
do ) “resonate out” the C0 to get rid of the effect. The impact of C0 is 
what makes the pull ability appear to change with finish point. 

Bob

> On Aug 14, 2022, at 10:32 AM, Donald E. Pauly via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> All crystals are series resonant.  Some are merely cut for resonance
> with an external capacitor such as 30 pFd.  The crystals in my designs
> operate solely at series resonance and an external capacitor is not
> specified. Q is a function of the care in manufacturing and affects
> pullibility. Angle of cut determines the upper turning point
> temperature.  Standard AT cuts have a 65° turning point.
> That turning point is also affected by the overtone of operation and
> is different for fundamental and 3rd overtone.  5th and higher
> overtones have nearly the same turnover temperature compared to 3rd
> overtone.  See https://gonascent.com/papers/crystals/xtlcurve.jpg
> 
> On Sun, Aug 14, 2022 at 10:27 AM Corby Dawson via time-nuts
> <time-nuts at lists.febo.com> wrote:
>> 
>> Thanks for the advice, I'll see if those companies can help.
>> I am wanting to replicate the oscillator circuit  at
>> http://www.wa1mba.org/PLLVCXO/stephensen.pdf
>> but at 60Mhz. The article specifies...
>> 
>> "Y1 is a
>> fifth-overtone, AT-cut crystal ground
>> for operation in the series-resonant
>> mode with a load capacitance of 30 pF"
>> 
>> I thought that a series resonant crystal did not have a load
>> capacitance??
>> 
>> I'll send this info along to the Crystal manufacturer and see what they
>> say.
>> 
>> I had bought an FE 2216A OCXO oscillator of eBay a couple years ago
>> thinking of this same project however it has a 120Mhz output.
>> I read that a lot of these higher frequency oscillators had a fundamental
>> that was doubled.
>> So I opened it up hoping to find a 60Mhz crystal.
>> Once the potting was removed I could see a Minicircuits doubler on the
>> bottom of two boards and scopeing it showed 60Mhz in!
>> However the oscillator/oven board has a 5Mhz SC cut crystal and a hybrid
>> module that gets it up to the 60Mhz! See PIX.
>> 
>> While trying to find a crystal I'll use this board to test out my scheme!
>> 
>> Cheers,
>> 
>> Corby_______________________________________________
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