From: Heiko Dudek ([email protected])
Date: Mon Aug 21 2000 - 22:13:35 PDT
How about a much more simple approach ... I'm assuming a homogenous,
linear system of two conductors: aggressor and victim. Because that'll do
the job to explain saturation.
The actual cause for crosstalk is the electric and magnetic field lines of the
aggressor net striking the victim net. Since there's a change in both fields,
there will be a voltage and a current induced in the victim net. The usual way
to model this in a linear system is a number of 'mutual capacitances'
(capacitive coupling between aggressor and victim) and 'mutual inductances'
(inductive coupling between aggressor and victim). So with this, the amount
of capacitive crosstalk on the victim net is based on the amount of capacitive
coupling and on dV/dt - the changing speed of the voltage on the aggressor.
The amount of inductive crosstalk depends on the amount of inductive
coupling and on dI/dt - the changing speed of the current on the aggressor.
Since a signal is not travelling infinitely fast along the transmission line, but
with a propagation velocity which depends on the trace geometry, it will take
a certain time for the signal from the source to the load. Now imagine the
victim and the aggressor lines as a number of coupled inductors plus capacitors
between the two. Once the signal comes to the first capacitive/inductive
coupling, there's a certain amount of crosstalk introduced to the victim net. Now
this crosstalk will 'travel' in two directions: back to the direction where the signal
came from on the aggressor (backward crosstalk) and in the same direction as
the signal on the aggressor is travelling (forward crosstalk). As the signal comes
to the next capacitive/inductive coupling, there's the same thing happening again.
Just with the 'new' crosstalk added to the 'previous' one which was travelling
along the victim line with the same speed as the aggressor signal (supposed the
propagation velocity is the same - or, in other words, the trace geometries are the
same). It's easy to see that the crosstalk signal on the victim line would add up
more and more the longer the traces are running in parallel. Well, would, because
now there's the saturation effect. Once the lines are long compared to the rise
time of the signal, no more voltage (or current) changes are going on at the
beginning of the tline while the signal is still travelling down the line (actually, the
beginning of the tline already went through all states between low state and high
state and is now stable at either high or low). So, if there's no dI/dt or respectively
no more dV/dt, there's no more crosstalk added.
Saturation depends on both the propagation velocity (the trace geometry) and
the edge rate of the aggressor signal.
P.S.: If you have a chance to be at the Cadence User Group Meeting, you're
welcome to attend the 'Signal Integrity 101' (Sunday evening) which addresses
this and other SI issues in much greater detail.
At 12:29 PM 8/21/00 -0700, Mike Jenkins wrote:
>A question about your inquiry....when you say "after some point",
>are you referring to increasing the input amplitude or increasing
>the parallel conductor length? It isn't clear to me.
>If you mean increasing the input amplitude, then the statement is
>false. This is a LINEAR system, and increasing the input to a
>linear system always results in the output increasing proportionally.
>If you mean increasing the conductor length, then I think the
>reply from Mr. Verkammen is what you were looking for (assuming
>you had in mind a multiconductor, nonhomogeneous system).
>If you had in mind something simpler (two conductors and a
>homogeneous medium), then again the statement is false. Far end
>noise increases as the conductor length increases. Near end noise,
>on the other hand, does flatten as the conductor length increases
>past the risetime of the aggressor signal. The noise pulse gets
>wider as the length gets longer, but not higher. (Thought I'd
>add this, just in case you were referring to this more simple case.)
> > Hi,
> > If you have two very long parallel conductors, the forward cross-talk does
> > not increase in amplitude after some point (saturation). Can anybody
> > enlighten me why this saturation happens?
> > Opamps
> Mike Jenkins Phone: 408.433.7901 _____
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