Jack,

trace impedance isn't the decisive factro in  decoupling anymore. The largest
improvements can be made when lowerinmg the diatnce between the power and
ground plane, especially at high frequencies. I.e. use 100um cores between
power and ground. That will really help. Next, decoupling cap values have to be
adapted to the impedance (and its frequency dependence)  of the power ground
system. Yes, the power/gnd planes form a transmission line as well.
There's a lot of magic around decoupling, but: Mr. Maxwell has the solution.
Physics are always right.

regards,

Peter Marek



Peter Marek
MarekMicro GmbH
General Manager
Phone + 49 - 9661 - 908-210
Fax + 49 - 9661 - 908-100
      E-Mail [log in to unmask]

> -----Ursprüngliche Nachricht-----
> Von:  Jack Olson [SMTP:[log in to unmask]]
> Gesendet am:  Freitag, 16. April 1999 18:28
> An:   [log in to unmask]
> Betreff:      [DC] Trace Inductance
>
> I'm curious about something I read in an article on trace inductance. We
> design boards that are 1.9GHz, and the engineers are so worried about
> inductance that every decoupling cap (other components too) has two vias per
> pad. It seems to me that its not the two vias that make any improvement, but
> the fact that there are now two traces instead of one connecting the pad to
> the plane.
> But these traces are SO SHORT... is the inductance of a 20 mil trace that
> travels a short distance (maybe 10 to 100mils) something to worry about? How
> do you know when a trace has enough inductance to be a problem? (is there a
> short answer? grin...)
>
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