FreeLists / si-list / [SI-LIST] Re: Serpentine Traces

[Robert Haller <rhaller@xxxxxxxxxx>]

Praveen,
        This is a good question and has been raised before on the SI-LIST. 
Serpentine etch is often done to match clock lines or source syncronous 
data lines.

I did bench testing of serpentine lines, in conjuction with simulations. 
I ran SPICE simulations using field solved 2d models then measured 
varying serpentine configurations. I examined impedance and propagation 
delay variations as a function of varying spacing, and signal edge rate.

When transmission lines are serpentined 'close' to themselves the 
propagation delay decreases (The lines got faster). For example if I 
compare a straight control line and a sepertined line of the same length 
on the same layer, with all the fixturing nulled the propagation delay 
went from 176ps/in (straight line) to 170 ps/inch (serpentined line). 
You can think of the signal taking a shortcut across the distributed 
self capacitance of the closely serpentined lines.
You might think a propagation delay reduction is good, but I also found 
the variability of prop velocity increased. I believe the variability 
increased because the capacitive coupling varies based on geometry, 
while normally the dielectric constant variations are the only source of 
prop delay variability (for a specific rise time and frequency). The 
variability is a small effect but is important when doing source 
syncronous designs.


Regards,
Bob
-- 
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15



Praveen Soora wrote:
> Hello,
> 
> Does anybody know the design rules of the serpentine traces  on the 
> board ? I am trying to evaluate different types of serpentine traces 
> looking at the group delay caused by the corners. Any information or 
> literature on this would be appreciated.
> 
> Thanks,
> --
> Praveen Soora
> ECAD Engineering,
> Apple Computer Inc.,
> 408 974 0595
> 
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-- 
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15


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