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

[Eric Goodill <ericg@xxxxxxxxxxx>]

Bob,

What is 'close' in your test board?

-Eric

Robert Haller wrote:

> 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


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