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

[Bob kb8tq]

Hi

The stability depends on a long list of things. Since you can get higher
Q with an AT than you can with an SC, if Q was all that mattered, the AT
would be the king of the hill. 

The oscillator circuit matters, but different parts of it matter in different ways.
The things you might do for low phase noise at a 100KHz offset might be 
a bad idea if very good ADEV at 100 seconds was the target. 

Tuning any high Q circuit very far off frequency probably is not a great idea.
Keeping all of the “optimizations” on target over a wide pull range is not at
all simple. 

If you are designing an OCXO from scratch, there is a lot to learn and hundreds
of papers out there to get you started. If you are buying one, things are a bit
more simple. You look at the spec sheet and decide if it’s going to do the job
or not. Worst case, you buy a couple and test them. 

Bob

> On Jun 8, 2022, at 10:01 AM, Ross P via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi,So, highest short term stability depends on the Q of the crystal and quality of the feedback circuit. In that case, an AT-cut with a low noise feedback amplifier will be as good as an SC-cut with the same amp. Does pulling the oscillator affect the short term walk?rp
> 
>    On Wednesday, June 8, 2022 at 10:44:24 AM PDT, Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:  
> 
> Hi,
> 
> I agree in general. However, I do see that other work to get good 
> resulst have been done when SC-cut is considered, so rather than SC-cut 
> as a cut is better, it becomes somewhat of a tell-tale of that other 
> work being done properly. I.e. it is meaningless to take the step to 
> SC-cut when other defects dominate so the SC-cut properties only makes 
> things more expensive than the AT-cut.
> 
> As far as I remember and know, you can achieve about the same 
> phase-noise properties as you hit about the same bandwidth from the Q, 
> and noise contribution is about the same. So, it boils down to do the 
> supporting amplifier well.
> 
> Cheers,
> Magnus
> 
> On 2022-06-08 06:27, Bob kb8tq via time-nuts wrote:
>> Hi
>> 
>> Simple answer is: no.
>> 
>> More complete answer is: no
>> 
>> There is a lot more to stability than just the crystal cut. Having this or that cut is
>> in no way a guarantee that the result is “better” than some other cut. Indeed there
>> are more exotic cuts than the SC that improve on this or that. There are also mounting
>> / fabrication techniques that improve on this or that, regardless of cut.
>> 
>> All that said, the “typical” SC cut based OCXO is likely newer than an AT or BT cut
>> alternative. Various improvements here or there are likely to make it a bit better than
>> the other examples …. ( but not always )
>> 
>> Bob
>> 
>>> On Jun 7, 2022, at 6:04 PM, Ross P via time-nuts <time-nuts at lists.febo.com> wrote:
>>> 
>>> Hello,My first post.I have created a 64-bit frequency counter, 15.9 digits after converting to floating point.
>>> Oscillator random walk is +- 0.01 ppm with an SC cut crystal at 10 Hz filtered, and 0.1 ppm with at cut.Is it the crystal or the oscillator electronics (inside a can) that determines the noise?The oscillators I am using are 1 double oven SC 10 MHz vs 1 single oven AT cut 10 MHz in one test,and 2 generic crystal oscillators (on a Terasic DE1 cyclone II FPGA board) for the other test.I assume the single oven oscillator will have better stability than commodity oscillators.I am able to chart random walk at up to a few thousand samples per second at full double precisionresolution, and FFT shows some alien tones in the walk pattern that come and go suddenly, I thinkdue to oscillating mode changes in the oscillator itself, mostly show in the commodity crystals.My question is: is the SC quartz the most stable for random walk.I would like to know if such a frequency counter / alien to detector is useful enough to be producedfor sale? It would require at least 3 separate frequencies of refer
>>> ence time standards and > 50Klogic elements in the FPGA for 3 cross coupled monitors to cover a range of 0 to 50 MHz.
>>> Quite a risk if no one needs it. 3 separate high stability reference oscillators are expensive.rp
>>> 
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