From: Michael Nudelman ([email protected])
Date: Mon Jan 08 2001 - 07:02:58 PST
I will just try to give you the general idea for placing end termination.
A transmission line behaves as an active resistance. And it looks that way for
everything that is in front of it (driver) or behind it (receiver). Officially
the end of the line is the terminator.
Now, this is what happens:
Case 1) Your load is before the terminator (or anywhere on the tr. line)
There is no principal difference between any place on the transmission line
before the terminator. What matters is this - full value of the signal
travelling down the line will reach the receiver and then die in terminator
(the term "die", if sounds too gloomy, can be replaced by "afterlife" in the
"endless happy travelling transmission line", since this is what a term.
resistor looks like - endless transmission line :-).
At the point of meeting with the load, if the latter exhibits some capacitive
quality (a stub plus load capacitance), the cap. "dip" will occur and travel
back to the driver (and be observed on the TDR).
There is practically no delay occurs between the signal reaching a connection
point of the receiver to the tr. line and the receiver sensing the signal.
case2) A load (receiver) is after terminator.
This is what will happen. As mentioned, the transm. line looks like a
resistance to everything after the terminator. (let it be, say, 100 Ohm)
Now, the signal reaching the terminator, your receiver will see the signal
going through an integrating circuit (RC) with R=100 Ohm and C = your trace
capacitance (trace connecting the terminator to the receiver, hopefully short
one) plus your load capacitance.
This way you 1) lengthen your front duration, 2)possibly attenuate your
If a terminator is placed far from the load and bnefore it, you have another
effect of connecting a second tyransmission line (the trace after terminator
and to the load) to the terminator, thus attenuating the signal going to the
receiver and creating an unmatching termination (your 100 Ohm term. resistor
in parallel to the 100 Ohm of the second tr. line creates 50 Ohm terminator
for the first tr. line.) So, the negative reflection travels back to the
driver and attenuated signal goes to the receiver.
So, connect your load before the terminator.
Goferman Stas wrote:
> We are two undergraduate students working on a signal integrity project.
> The field is new for us. The project is about simple signal integrity
> phenomena, like reflection and crosstalk. We designed a PCB with various
> test cases
> that demonstrate these phenomena.
> Our question is about end termination.
> We've built a simple end termination configuration.
> However, we have place the end-terminating resistor not so close to the
> far end.
> We would like to find how close to the end do we need to connect the
> Since we cannot change the location of the resistor, we change the rise
> time of
> a pulse generator. We are using a HP TDR. As we increase the rise time, we
> get a better
> behaviour of termination, and get a smaller sink on the TDR screen.
> For each sink, TDR measures an excess capacitance created by stub.
> For a perfect termination, there is
> no excess capacitance. Is there any relation (equation) between this
> excess capacitance and
> the rise-time or the stub length ? We need some analytic formulas, if any.
> We will appreciate if anyone could help us.
> Stas and Alex
> [email protected]
> [email protected]
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