[time-nuts] Locking 100 MHz to 10 MHz

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Dec 19 15:50:25 EST 2007


 
In a message dated 12/19/2007 11:54:37 Pacific Standard Time,  
boyscout at gmail.com writes:

>Yes,  I understand all the math.  I just don't know what to expect  from
>the 10 MHz references people will plug in.  Clearly if they  plug in a
>Fury, I should use a wide bandwidth, but who knows what else  they'll
>use...

>Thanks,
>Matt



Hi Matt,
 
here is an idea: instead of switching the control voltage using a Relay or  
analog switch etc (which may introduce more noise, and instabilities etc), why  
not switch the 10MHz reference input using a low-jitter digital  multiplexer?
 
Then you could use a cheap 10MHz Oscillator with scaled phase-noise every  so 
slightly better than your 100MHz oscillator to discipline the 100MHz  
oscillator in the absence of an external 10MHz source. No analog reference  voltage 
required, and the overall noise performance will be better than the  100MHz 
oscillator by itself.
 
By measuring the AC control energy in the PLL loop, and comparing this when  
using your known 10MHz source versus the external unknown 10MHz source you may 
 be able to make a qualitative judgement about the external 10MHz source 
(similar  to a 3-hat measurement). Then again it may be hard to figure out if the 
noise is  coming from the 10MHz or the 100MHz oscillator..
 
A 10MHz oscillator that exceeds your 100MHz oscillator's (scaled) phase  
noise spec should be easy to find and inexpensive (DIP-14 type oscillator  etc).
 
bye,
Said



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