[time-nuts] Looking for datasheet for Oscilloquartz 8602

[Magnus Danielson]

Attila,

On 06/01/2013 05:11 PM, Attila Kinali wrote:
> On Wed, 29 May 2013 01:59:12 +0200
> Magnus Danielson<magnus at rubidium.dyndns.org>  wrote:
>
>> On 05/28/2013 07:55 PM, Attila Kinali wrote:
>>
>> http://www.pttimeeting.org/archivemeetings/1984papers/Vol%2016_10.pdf
>>
>> See also
>> http://www.pttimeeting.org/archivemeetings/1979papers/Vol%2011_25.pdf
>
> Interesting stuff. Thanks a lot!
>
>>> Juup. I just went back to Vig's tutorial and read up what he wrote.
>>> Misremembering things is not a good thing...
>>> But then, he explicitly writes that SC cut gives a higher stability
>>> over AT cut due to lower temperature dependence and less dips.
>>
>> That is true, but your discreditation of AT-cut was simply way off the
>> mark, so I wanted to bring it into context. There is a difference, but
>> it is not as huge as it sounded like. I like to think about it as such
>> that you better have done much of your homework in form of good
>> oscillator and oven before considering spending money on going from
>> AT-cut to SC-cut, but it does give that extra performance if you need
>> it. The OSA 8600 shows just how far you can take AT-cut.
>
> Oh.. Ok. Didn't want to sound that way. On the other hand, that conclusion
> is not far from the truth. I still have a lot to learn. Thanks for the
> correction.

The benefits (as I recall it) is a somewhat higher Q and less thermal 
dependence.

> But then, the 8600 has a ~10dB higher noise then the 8607. Ie the noise
> is 10 times higher (it is power-dB, not voltage-dB isn't it?). Which
> makes me wonder what the noise contribution is. I would assume that the
> electronics are very similar if not the same (electronics are cheap compared
> to the crystal) and the mechanical construction seems to be very similar
> as well.

You must be looking at the 1 Hz numbers. You really need to look at the 
phasenoise at different offsets to understand what goes on.

The electronics contribute a white phase noise, as well as a flicker 
noise (1/f, a -10 dB/decade slope). The resonators Q-value and frequency 
will define the break-up point, below witch you have a -20 dB/decade 
slope and above it is flat. Depending on the resonator at hand, you then 
see a 1/f or 1/f² noise between the flat white noise and the 1/f³ noise. 
The output amp can then add white phase noise and flicker phase noise.

The 10 dB improvement I would attribute to the improved Q value with SC 
cut crystal, but... these are not the real values, it's the published 
values which is guaranteed for a product. Actual values is different.

Looking in Enrico Rubiolas book, he measures both OSA 8600 and OSA 8607 
(among others) and then the actual numbers is much closer. The 1 Hz 
values is only 4 dB different, but then the wideband noise between these 
two samples lets the 8600 be 2 dB quieter than the 8607 at -155 vs. -153 dB.

So, we go back to actual measurements, we need to realize that each 
oscillator is unique, and that data-sheets only intends to give some 
form of guarantee of how bad they will be as they exit the factory, but 
not necessarily reflect all aspects of the oscillator.

I've seen AT-cut oscillators behave better than SC-cut within the same 
basic conditions, but where the AT-cut was much better implemented.

So, there is a difference, it's just not very large and the full 
performance depends on so many other parameters of a design. The cost of 
SC-cut blanks is higher, and it is not meaningful to use one unless the 
design have come so far that it starts to become a limiting factor and 
other improvements is more expensive.

> In comparison the stability between 2 and 30s has only a factor of 2 inbetween.
> (maximum instability according to spec).
>
> Any ideas what the reason could be?

#1 Datasheet specing difference.

#2 Actual difference is due to Q-value difference, moving the break-up 
point from the Q value.

So, the Leeson model explains this pretty well, and the difference in 
Q-value of the loaded crystal blank is then reflected in noise 
differences with essentially the same buffer electronics.

Cheers,
Magnus