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Subject: Skew calculations
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valeriew
Posts: 1
Online: User is Offline
7/27/2007 9:45 AM  
I am doing reflection simulations of a differential driver and differential receiver thru a 3-board system.  My extracted topology shows segments of coupled t-lines and segments of uncoupled t-lines, as expected for fanouts, etc.  However the PropDelay results in the SigXplorer table for the + path, the - path, and the diff path all give the same number.  I believe they should be different.  Also this calculated PropDelay is considerably lower than what my waveforms show.  (1.9 ns versus 2.7 ns)

How is PropDelay calculated within SigXplorer? 

Timing skew within an LVDS diff pair is critical.  Is there a way to calculate the difference in prop delay between + and -? 

Thanks,
vw
Kalevi2
Moderator
Posts: 69
Online: User is Offline
7/27/2007 9:51 AM  
The easiest way to obtain the difference in P and N net lengths is to use the constraint manager. You can use it for a single board or through design links. Prop delay is calculated as the net length/prop velocity where prop velocity is c/root(Dk) for each dielectric layer the signal goes through.
Kai Keskinen
chrishalford
Posts: 6
Online: User is Offline
8/13/2007 8:05 AM  
The PropDelay is purely that, length x prop velocity.

When comparing these CALCULATED figures to the results of SIMULATIONS, they sometimes do not appear to correlate as the waveforms also show rise/fall times, measurement thresholds, turn-on delays, etc. Are any of these masking the PropDelay?


Chris Halford
Kalevi2
Moderator
Posts: 69
Online: User is Offline
8/13/2007 8:39 AM  
Prop delay is not masked by any of the items you mention. The easiest way to obtain net length or prop delay is to use constraint manager. The report from probe with diff pairs can be confusing if there is a parallel termination.
Kai Keskinen
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