[time-nuts] Advantages of GNSS ???

[Bob kb8tq]

Hi 

Well part of it comes from designing, testing, and manufacturing a few 
thousand OCXO designs over the years. We likely built 10’s of millions of 
OCXO’s over the time I was doing / managing that. 

> On Jul 11, 2019, at 3:56 AM, Leo Bodnar <leo at leobodnar.com> wrote:
> 
> Hello, /
> 
> Why would you not want high drive level for best close-in noise?  This is at odds with general thinking in the industry.
> Close-in in this context means from 0.1Hz to 1/f knee which is 1-100kHz depending on the design of the sustaining amplifier.
> 
> There are few reasons why low phase noise "practical" oscillators are built as OCXOs:
> 
> On one hand:
> - close-in noise depends on 1/f knee frequency  

That is a quick approximation. There always will be a bit more to it. 

> - lowering knee frequency requires high-Q resonators

That does help, but it’s far from the whole story

> - for classic 1MHz..100MHz range this means crystals

There are other exotic things you could do. Cost wise they get a bit crazy. 
There are materials other than quartz that give you higher Q. In some
cases *much* higher Q without a massive cost penalty. 

> - high-Q crystals require SC-cut 

Actually not so much. An SC has a lower Q than an AT of similar size
and design up to the point acoustic Q losses completely take over. 
If you are talking about sub 20 MHz OCXO’s with “doable” crystal 
package sizes, the AT will have the higher Q by a significant margin. 

> 
> On the other hand:
> - phase noise density is measured as a ratio referred to carrier level

True, that’s how it’s defined

> - increasing carrier level improves phase noise figure

Which is often how you get good phase noise far removed, since your circuit
does not have issues as you increase the drive level. (if properly designed) 

> - increasing carrier level necessitates increasing drive level

To the degree that “carrier level” in this case is defined to be exactly the same as drive level. 

> - maintaining reasonable ageing rate at higher drive levels requires SC-cut crystals

Not so much. Indeed there are a number of cuts that do well. The SC’s claim to fame is 
immunity to acceleration / stress effects in a single plane. (SC = Stress Compensated ).
 
A side effect of the design is a shallow turn point at higher temperatures. There are other 
cuts that share this characteristic. Things like the FC do get used in OCXO’s. 

> 
> Having established that SC-cut is preferred:
> - SC-cut has high temperature turning point.  Its room temperature tempco is much worse than AT-cut's one making it mostly unusable as XO or TCXO

Depends on the XO, They get built into down hole gear as an XO. There are other “un heated”
SC based applications. 

> - High temperature turning point requires oven

Depending on the application and temperature spec. There *are* indeed low phase noise / SC based
room temperature oscillators sold for some “interesting” applications. There’s a list member who has
posted about their experience doing so. 

========

The bigger problem is that your approximation that drive always improves phase noise falls
apart as you get closer to carrier. There are a lot of things that drive this. If they didn’t, the design 
task would simply be to have a low noise follower amp. There have been designs published in a 
lot of places for amps that are 50 db better than the noise of any OCXO at 1 Hz offset. 

Bob

> 
> Leo
> 
>>> From: Bob kb8tq <kb8tq at n1k.org>
>> It depends a lot on the offset you are looking at. For close in phase noise, you probably don’t 
>> want high drive. If you are only after phase noise past 10KHz, you may not want / need
>> an OCXO in the first place. Selecting crystals (like one in a hundred) for very high drive /
>> low phase noise setups *is* done. It’s just not very practical. 
>> 
>>> On Jul 10, 2019, at 3:49 AM, Leo Bodnar <leo at leobodnar.com> wrote:
>>> It depends whether OCXO is designed for long term stability and low ageing or low phase noise.
>>> Low ageing requires low drive but low phase noise needs as much drive as humanely possible - often approaching mW levels.
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.