Re: PCI Trace Velocity

[Lorne J Levinson <fhlevins@wicc.weizmann.ac.il>]

The correct formula for a microstrip's (i.e. a line on a board's
external face) propagation velocity is:

       T (ns/feet) = 1.017 * sqrt(0.475 * Er + 0.67)
                                  -----
                                        not 0.457
for Er=4.7, T=144 ps/inch, still low.

BUT, more importantly, for traces between gnd/vcc planes, i.e.
strip-lines, the formula is:

       T (ns/feet) = 1.017 * sqrt(Er)

for Er = 4.7, T = 184ns/inch, which is inside 150 to 190 ps/inch

I do not know why the propagation velocity is important. Even the delay
seems quite robust.  I successfully operate PCI cards on an extender
card with bus switches for "hot" insertion.

For boards we build, we do follow the rules for trace length, especially
for the clock line.

A handy reference for transmission line theory is Motorola's "MECL
SystemDesign Handbook".


Yoel lavian wrote:
> 
> Hello PCI experts,
> The PCI specifications ver. 2.1 section 4.4.3.3. talk about the PCI trace impedance and velocity. It says " The trace velocity must be between 150ps/inch and 190 ps/inch".
> Talking to several PCB manufacturers, I understand that the parameter which effect the trance velocity is the material between layers, and the most popular is the epoxy glass.
> The equation for the trance velocity is:
> T (ns/feet) = 1.017 * square_root(0.457 * Er + 0.67)
> Er - The material dielectric constant.
> For epoxy glass:  4.5 <= Er <= 4.9
> Even if we are taking the average value of Er = 4.7, then we have T= 1.7 ns/feet --> 142 ps/inch. i.e. we are under the specifications limit.
> I will appreciate if somebody who faced this problem before can share his experience concerning this issue.
> Do I have to use some other material instead the epoxy glass ? Or maybe there is a mistake in my calculations ?
ºØ	Ç