why 20ppm tolerance for ref. clk ?

["Krishna Sankar" <krishna.sankar@xxxxxxxxx>]

Hello All,

 

I have read that, reference clock in Clock Data Recovery Unit used in SONET systems

needs to have a tolerance of + or - 20 ppm to meet SONET output frequency specifications.

Can any one of you please explain how this figure is attained.

 

thanks in advance,

 

cheers,

krishna

~~~~~~~
Krishna Sankar M
Engineer-VLSI/System Design
Tel   : +91-80-8520408 / 8520416   Ext. 4320
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email : krishna.sankar@xxxxxxxxx