Re: more questions about buried PCB

Al Barber ([email protected])
Mon, 10 Jun 96 13:19:23 PDT

> As someone remarked, it seems reasonable to suppose that the sandwich
> of two power planes behaves as a "radial transmission line". When you
> inject current somewhere, a circular wavefront should move away from
> the point of injection.
> How does the amplitude of the radiated wave "die off" as it travels?
> >From what I can tell, it's not dropping "nicely" like 1/r or log(r),
> but rather as something very, very nasty.
> It would be interesting to know the answer, since this radiated wave
> might be able to upset the supply levels of nearby components
> connected to the same power-ground sandwich.
> I'm curious if anyone has better training/insight/empirical evidence
> here...
> --Eric
It's very capable of upsetting nearby components; this is the heart of
the simultaneous switching noise problem. Another model is one of a
distributed resonant circuit supporting standing waves, since the
boundaries of the plane pair will reflect this traveling wave. The
resonance can be high q if the metal resistance is low relative to
the impedance of the traveling wave (e.g. pcb structures), or low
q (e.g. thin film structures).

See IEEE Trans on CPMT Part B, Vol 18, No 4. "Modeling and Analysis
of Multichip Module Power Supply Planes" for one analysis technique
complete with benchmarks against measurements, frequency and time
domain results, and plots of standing wave patterns.

Alan Barber