[time-nuts] Advantages of GNSS ???

[Leo Bodnar]

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  
- lowering knee frequency requires high-Q resonators
- for classic 1MHz..100MHz range this means crystals
- high-Q crystals require SC-cut 

On the other hand:
- phase noise density is measured as a ratio referred to carrier level
- increasing carrier level improves phase noise figure
- increasing carrier level necessitates increasing drive level
- maintaining reasonable ageing rate at higher drive levels requires SC-cut crystals

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
- High temperature turning point requires oven

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.