[time-nuts] femtosecond jitter anyone?

[Bruce Griffiths]

Chris Mack / N1SKY wrote:
> On Apr 8, 2009, at 8:02 PM, Bruce Griffiths wrote:
>
>
>   
>> Unless you are prepared to place the crystals in an oven with the
>> temperature regulated tightly and carefully tune the filter  
>> periodically
>> then using a crystal filter (or any passive filter with a sufficiently
>> narrow bandwidth to cleanup the skirts) will not be particularly  
>> useful.
>> It would be much easier to use a low bandwidth analog PLL with a low
>> noise VCXO to cleanup the 38.88MHz signal.
>>
>>     
>
> http://www.national.com/pf/LM/LMK04000B.html
>
> Their app note uses a very low noise VCXO...  The simulator software  
> gets pretty close...
>
> Cheers,
> -chris
>
>
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>   
Chris

If you divide the output down to ~38MHz using a noiseless divider then
the performance is 20dB or more worse than can be achieved with a good
~38MHz crystal oscillator.
This seems to be a complex method of degrading the performance over that
possible with a simpler design.

Bruce