Re: gEDA-user: Compiling MMI (electromagnetic simulator)

[Dan McMahill <dan@xxxxxxxxxxxx>]

On Tue, Dec 14, 2004 at 09:56:37PM -0500, Daniel Nilsson wrote:
> On Tue, Dec 14, 2004 at 09:21:28AM -0500, Dan McMahill wrote:
 
> Agreed, you might care about these aspects. What I wanted to point out
> it that there are were few real world problem where you actually can
> use the trace capacitance for design purposes. In the example with the
> trace referencing the vdd plane for example, the return current
> flowing on the vdd plane is a function of the driver impedance and the
> trace impedance. The return current will find its way back to the
> driver through decoupling caps a inter-plane coupling. If you wanted to
> calculate the magnitude of this return current you would use the trace
> impedance, not the total trace capacitance.
> 
> I'm not disagreeing with your examples, what I wanted to highlight is
> that in modern electronics you typically have to treat traces as
> distributed elements that have impedance and delay. There is some
> lingering "old school" thoughts about the "total capacitive loading"
> that we used to use back in the days when the slew-rates were so slow
> that the PCB traces looked like lumped elements. Then the input
> capacitance of the receivers set the performance limits of the
> circuit, so we used to sum up all the receivers load by calculating
> fan-out and such. When slew-rates got faster, I sometimes hear people
> say that we now should add the total trace capacitance to the input
> loading of the receiver in order to estimate the delay. What I want to
> point out is that this is seldom a valid approach (expect for on-chip
> interconnect, but that's a different story). With faster electronics,
> PCB interconnect should be simulated and compared against standard
> load in order to determine delay. With fast in this case I mean
> applications where the electrical length of the interconnect is longer
> then 1/10 of the risetime of the signals. If you slower speed
> applications, I can see some use of the total trace capacitance but we
> seldom worry about delay on those classes of signals any more.
 
I'd say it depends on your definition of "modern electronics".  It seems
like in your case you specifically mean high speed digital lines.

There are plenty of other cases of interest.  A very low noise analog 
line which needs to be high impedance for some reason or another.
Capacitive gauge read electronics for example.  I've also seen cases
where coupling to a plane and bypassing a filter hurt ultimate rejection.

So while I agree that with high speed digital lines, total capacitance is
not what you want, there are other circuits still around!

-Dan

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