Re: gEDA-user: trace calculation

[sdb@xxxxxxxxxx (Stuart Brorson)]

> > Why? Via doesn't have much more inductance than a piece of trace,
> > does it?
> 
> At high enough frequencies, traces are waveguides, not just
> conductors.  Vias have nontrivial geometry relative to the signal.

Indeed.  Folks doing > 1GHz design (to name a rough cut-off point)
spend a lot of time worrying about the waveguiding properties of their
traces.  Vias represent impedance discontinuities.  Modeling vias is a
big industry for signal integrity engineers.  Here's a paper about a
commerical product used to model a via (amongst other things):

http://www.cadence.com/community/allegro/resources/resources_pcbsi/mgh/tp_CDN_Bayside_SingleViaPaper.pdf

> That reminds me of another optimization I'd like to implement in the
> future.  Instead of corners, treat all traces like elastic bands, so
> that they end up as sweeping curves (lines+arcs) with no corners at
> all (except for 3-trace nodes).  I want this myself just because it
> makes the board look pretty, but I wonder if gentle curves provide
> better waveguide performance than corners?

This is also a subject of inquiry amongst SI engineers.  At one time,
it was believed that you needed semi-circular sections to make a bend
for high-speed signals.  Nowadays, however, the consensus
is that it is enough to just miter the corners of a 90degree bend, or
(better) use two 45 degree bends to transition between a 90 degree
bend for all but the very highest frequencies.  IN the past, microwave
engineers took great pains to create semi-circular traces for
microwave tracks, but that apparently is not needed.

For those interested in SI, you can sign up to read a good mailing
list devoted to the subject here: 

http://www.freelists.org/archives/si-list/

There are a number of real experts reading and posting to that list.

Stuart