[SI-LIST] Re: About Self coupling

Naren,

I would like to clarify a few things for you, as the two responses you 
have received so far are not grounded in theory.  Your question about 
serpentine traces (your term was banding) used for length matching 
really comes down to coupling (crosstalk), whether the aggressor in 
another signal, or, in your situation, the same signal (you obviously 
realized this as the subject line indicates).  A cross-section of your 
serpentine traces would look something like this:

        |<--- w --->|<---- s ---->|<--- w --->|
     ____XXXXXXXXXXXXX            XXXXXXXXXXXXX
     ^
     h
     v____________________________________________ nearest ref. plane

When you draw the electric and magnetic fields surrounding each 
conductor, you quickly come to the conclusion that reducing crosstalk is 
highly dependent upon increasing two ratios:  s/w and s/h, but primarily 
s/h (unless h is very large with respect to s and w).  This is 
especially the case with balanced stripline, as the presence of the 
second reference plane reduces coupling from trace to trace.  So the 
rule of thumb that has developed over the years is that s should be 
greater than 3h.

The ratio w/h is determined by the impedance of the trace.  Now it just 
so happens that in a 50 Ohm system using FR4 for a dielectric, the w/h 
ratio is approximately equal to 1.  This has led a number of people to 
substitute w for h in the rule, which may work fine in many cases but 
could lead you astray in general.

Having an inadequate spacing can, in the worst-case, lead to severe 
signal distortion.  Keep in mind that even with s equal to 3h, while the 
coupling may be small, it is not zero and will result in slightly less 
delay than is predicted by the actual length of copper.  For high 
accuracy, you will need to use a field solver.  I would also recommend 
that you pick up a copy of Dr. Howard Johnson's latest book "High-Speed 
Signal Propagation" and take a look at pp. 610-616.  This section 
discusses serpentine delays and also outlines many other factors that 
can influence delay matching.

I hope this has helped.

Best regards,

     -Bill


       /************************************
      /                billw@xxxxxxxxxxx         /
     /                                  /
    / Advanced Electronic Concepts, LLC /
   /           www.aec-lab.com        /
   ************************************
======================================================
Qazi Arif Iqbal wrote:
> Naren,
> Clearances are usually measured from track width basis. 
> Also try to avoid sharp edges.
> 
> Qazi
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Naren
> Sent: Friday, April 29, 2005 4:09 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] About Self coupling
> 
> 
> Hello
> Recently we are designing PCB @ 700 MHz (Digital) signals.
> In that we are using banding (As shown in attached picture)for length
> matching. As per thumb rule it should be more than 3X .
> 
> But I am confused that X means track width or Track(PCB Copper) height?
> 
> Anybody can clarify it?
> 
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