[time-nuts] Bye-Bye Crystals

[Richard (Rick) Karlquist]

On 3/15/2017 4:45 AM, Bob Camp wrote:
> Hi
>
> Where do you plan on getting an OCXO grade crystal at an odd frequency like
> that? Much of the performance of a good OCXO is in the crystal. Doing a proper
> design on one is a lot of work. You *might* think that having a design for 5.000000
> MHz would give you a good design for 5.000050 MHz. I have empirical evidence that
> this isn’t the case. Many years later, I’m still utterly amazed that this is the way things
> work in the crystal business ….( = it’s not just a design issue, it’s also a business decision)
>

Some 30 years ago when GPS was in its infancy, the hardware utilized
OCXO's at 10.23 MHz.  Somehow, HP got suckered into trying to make
modified 10811's that ran at that frequency vs 10 MHz.  Jack Kusters
tried to explain to anyone who would listen that this was a major
redesign of the crystal, because he would have to deal with a new
set of anharmonic spurious modes.  Although only a 2.3% frequency 
change, everything is different.  In terms of business decisions,
only something with as much "juice" as GPS could have gotten Jack
to make a custom frequency.  As it turned out, a few dozen crystals
were made, and that was the end of it.  I managed to snag them
before they were thrown out, in case they might be useful for
something.

The original poster wanted not only an odd frequency (which I don't
recommend for the reasons above) but also wanted to varactor tune
the oscillator.  I also don't recommend doing that because of the
difficulty of generating a clean enough DC voltage.  Against my
advice, the HP smart clocks were tuned with DAC's driving varactors.
They never really got that to work up to their expectations.
Synthesis is so advanced now, 20 years later, that there is
no reason IMHO to voltage tune an OXCO.

Rick