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

[Robert Haller <rhaller@xxxxxxxxxx>]

Eric ,
        For the example given, 176 ps/in -> 170 ps/in the difference was 
between 5/50 and 5/20 (Line width, /Line Spacing). The delay variations 
were even more dramatic at tighter spacings.

To answer Andy's question, I click Info in Allegro - which I believe 
measures the center of the line.

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


Eric Goodill wrote:
> 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
> 
> 
> 
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> 
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
> 
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> 
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages 
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
> 

-- 
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


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu