[time-nuts] Thunderbolt versus Home made

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Mar 1 08:46:06 EST 2007


 
In a message dated 2/27/2007 15:01:44 Pacific Standard Time,  
not.again at btinternet.com writes:

Sorry,  my original post was not very clear - I had only wondered if
faster PPS  signals might ever help with the measurement each second; I
didn't mean to  suggest that they might affect the required overall
loop time much. (  Thinking back to all the posts a couple of months
ago, I should probably  resist any urge to comment on the effect of
sawteeth on stability for the  moment...  )
Anyway, I was just curious. It's one of the many things I  don't have
much time to mess with, so wondered if anyone else had -  but
apparently not.

Angus.


Hi Angus,
 
in a traditional PLL where the low-noise reference source is basically  
"disciplining" a noise oscillator (VCO in this context), the phase comparator  
frequency is related to the achievable phase noise. Higher  comparison frequencies 
result in lower phase noise.
 
Take a look at National Semiconductor's application notes about this, they  
give you the underlying math. Analog Devices has great app notes, a PLL  
simulator etc on their website as well.
 
For a GPSDO, the tables are exactly turned: the reference is much more  noisy 
(short term ADEV) and the VCO is the provider of low short-term phase  noise, 
while the GPS reference provides low long-term phase noise.
 
bye,
Said
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