[SI-LIST] Re: Serpentine Traces
- From: Robert Haller <rhaller@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 10 Sep 2002 08:59:16 -0400
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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