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Visitor smillertait
Registered: ‎04-04-2012

Clock Jitter and the Clock Wizard and MCB



I am using a Spartan 6 LX150, ISE 13.1 and I have a question regarding the jitter calculation performed by the Clock Generator Wizard.


I am using the wizard to ensure that the oscillator I wish to use as the input to the PLL will allow the PLL to produce a sufficiently low jitter output to be used as the clock for a MCB configured for DDR3.


However, even when I put in "ideal" settings (i.e. lowest input jitter the tool will allow), I cannot get the output jitter below approx 110 - 130ps, which violates the requirements of the DDR3 IC I am using. However, I know that this IC should work, as it is used on the Xilinx development boards.


Is there something that I am missing or misunderstanding here? Or does the MCB reduce jitter on the clock it receives from the PLL?


Any clarification/assisstance would be greatly appreciated.



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2 Replies
Teacher rcingham
Registered: ‎09-09-2010

Re: Clock Jitter and the Clock Wizard and MCB

What is the jitter spec. for the oscillator on the Xilinx development board(s)? Can you use an oscillator that is at least as good?

"If it don't work in simulation, it won't work on the board."
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Visitor smillertait
Registered: ‎04-04-2012

Re: Clock Jitter and the Clock Wizard and MCB

Thanks for the suggestion. Jitter specifications for the oscillator used on the SP605 board are:


RMS Phase Jitter (random) = 0.7ps typical
RMS Period Jitter = 2.4ps typical, 3.5ps maximum

(From: http://www.sitime.com/products/datasheets/sit9102/SiT9102LF_Rev1.44_05-12-2010.pdf)


I could probably find something similar to that and use it on my board, however it is still a bit frustrating to not be able to understand how to get a proper estimate with the tools. Also, since my PLL settings will be different than on the development board, I assume I might get different jitter performance (although I'm guessing this is a small risk).

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