[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Bob kb8tq]

Hi

Lower turning point has been done, both with AT’s (back in ~ the 1950’s) and
with SC’s. Neither one showed any significant benefit. 

Taking a crystal down to sub 20K sort of temps does ramp up the Q. The gotcha
is that the frequency vs temp curve is so steep that very minor temperature variations
utterly trash the stability of the device. 

Bob

> On Jun 8, 2022, at 1:49 PM, Gerhard Hoffmann via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Am 2022-06-08 21:53, schrieb Tom Van Baak:
>> Would it be advantageous, then, to run a high-performance laboratory
>> oscillator at its lower turnover point? Or at -78 C (CO2) or 77 K
>> (liquid Nitrogen)?
> 
> I have no idea about the crystal itself. Maybe Bernd or the SC (SantaClara)
> veterans can help?
> 
> When I measured the Q of the recovered SC crystal from that Morion MV89A,
> there was not much of a difference in the wanted resonance between room
> temperature and +89°C. I think I have published the data here a year ago.
> My deep freezer in the basement can do -36°C, but the VNA is so heavy...
> 
> Infineon boasted that their SIGET transistors work nicely at a few Kelvin,
> so it would probably not fail for semiconductor availability (BFP640 & friends).
> OTOH, Ulrich Rohde wrote that the noise figure of the sustaining amplifier would
> take a hit under large signal conditions, but I don't know hard numbers.
> That would not disappear.
> 
> But then, in a Driscoll for example, you can give the 2 transistors enough
> current so they run class A and do the little bit of limiting on the output side
> with Schottkys. For the amplifier, that is not large signal.
> 
> That might be different for an amplifier in Lee-Hajimiri style.
> This is Dirac pulse excitation at the peak of the cycle to avoid phase modulation,
> that is optimized for mixing up 1/f noise.  :-)
> 
> Anyway, with a noise figure of the sustaining amplifier of a dB or even a few,
> there is no game changer to be expected from cooling.
> 
> Whispering gallery saphire, anyone? I was at the precious stones museum
> in Idar-Oberstein here in the 'hood and saw all these huge saphires.
> I left with the head full of ideas...
> 
> Cheers, Gerhard
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