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why 20ppm tolerance for ref. clk ?



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
Fax   : +91-80-8520478
email : krishna.sankar@xxxxxxxxx