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

[Magnus Danielson]

Hi,

The ability to stabilize at that temperature is indeed an issue. While 
the resonators Q shift, the loaded Q I would not expect to shift as 
much. Futher, the noise of the supporting amplifier also needs to be 
reduced. Naturally, the design could be adjusted to the different condition.

I've seen people taking "the best" oscillator (SC-cut) and go cryogenic, 
and then have severe temperature issues, because they did not understand 
that the benefit of the cut really manifest itself at some specific 
temperature. Luckily that they was able to get that feedback as the 
benefit of presenting to their peers at a conference.

Cryogenic provides an oppertunity, but as with any such condition, it 
takes a number of considerations to be able to harvest the benefits.

Cheers,
Magnus

On 2022-06-09 02:12, Bob kb8tq via time-nuts wrote:
> 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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