[time-nuts] femtosecond jitter anyone?

Chris Mack sometimesyoufeellikeanut at twentylogten.com
Sun Apr 26 14:25:19 UTC 2009

Folks, thanks for your input and sanity checking.  To recap...

Having never worked with crystals before (only 2 and 10 GHz stuff in  
GaAs power amp RFIC design for cell phone and the like using lab RF  
generators or Vitesse / AMCC asics with clock recovery already done by  
someone/something  else back in the 1990s),   I am revamping this  
design moving away from an OCXO and seeing what the design holds for  
TCXOs and the like.

A 4 layer board is going to be several hundred dollars due to the size  
of the board and I would like to get as close to ideal on the first  
shot (yes indeed there will probably be another spin)...  Since I only  
have experience in the 2+ GHZ region I was originally concerned about  
via stubs with reflection induced effects, having no "feel" for this  
low frequency region.

I am having issues trying to get the simulator programme for the  
LMK04000 to synch to 44.1kHz and generate 11 / 12 MHz.  One idea I  
suppose: I am looking at preceding the LMK04000 with the Si5326 which  
is a narrow band part compared to the wideband Si5319 to get 44kHz up  
to 10MHz...  Then the LMK04000 can take 10MHz from this or 10MHz from  
an external source outside the box and get it to the final 11 / 12 MHz  
for distribution internal to the box to the converters.  The Si5326  
can also provide an "internally derived" 10MHz from its reference  
port, so that all design criteria for clocking is satisfied (internal  
clocking mode and external synch to 44.1kHz or 10M external).  I can  
use a 3rd overtone crystal to provide the reference port frequency on  
the Si5326....

Regarding spurs in the near carrier region... Yes. these datasheets  
are a bit of a black box, and every time I look at them they give up  
another layer of the onion... I wonder about the tiny spurs on the  
LMK04000 near 100 Hz on their data (granted different carrier  
frequency, 250MHz).  Does anyone have any experience with these chips  
or have a better suggestion; is there even an issue with these small  


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