Re: SI-Termination Comments

Joachim Mueller (go@helm.cadlab.de)
Mon, 22 Apr 1996 16:36:50 +0200 (MET DST)

hallo,

>Norman Wong wrote:
>
> 1. When using uncontrolled-impedance FR4 PCB, based on my calculation on =
> a 6 layer board, the micro-strip (8 mil) Z0 is about 75 Ohm and the =
> micro-strip Z0 is about 45 Ohm. That means a mismatch every time I =
> switch layers. Does anyone has experience on this? Does it matter for 5 =
> ns rise time? How about 1ns rise time?
>
Due to the uncontrolled impedance you get a reflection coefficient of
|r| = 0.25 which yeald to a considerable reflected wave. But if the
electrical lenght (t_d/t_r) of your transmission line section is short
enough the impact of this reflected wave on your signal shape is com-
pensated by multi-reflections between two layer changes. In our experience
'short enough' means t_d < 10 * t_r. This border is also dependend on
the value of your reflection coefficient r . As larger your r as more
restricted is your t_d/t_r value.

example:
========

section: 1 2 3

D-------------o---o------------------R
^ ^ ^
| | |
75 45 75 Ohms

Additional you should consider that the impact of the first discontinuity
on your signal shape is the biggest. If you have allready several layer
changes, the contribution of one more is not so significant in this view
of ideal switches between two characteristic impedances.
But in the case of many layer changes you have to consider also the
capacitive load of each via in high speed design.

>
> 3. At want point could I use T instead of Daisy-Chain so that the stub =
> look like capacitance, no transmission lines? A lot of time daisy chain =
> line is longer than a treed line.
>

It is correct that your T layout structure yield to a 'capacitive effect' if
you terminate the stub with a high ohmic resistance. The reason for this is
the reflection and transmission coefficient at the connection point of the
transmission lines and the propagation delay of the multi-reflected waves
in the stubs. Nevertheless the stub is a transmission line but if the
electrical length of your stub is short, the simulation model can be
substituded by a lumped capacitance.

More details about this problems can be found in the following paper:

TRANSFORMATION METHODS FOR A FAST ANALYSIS OF REFLECTION EFFECTS ON
PRINTED CIRCUIT BOARDS

J. Müller

3rd International Conference on Computation in Electromagnetics, Bath (UK)
April 1996

which can be also reached via www:

http://www.cadlab.de/~aatools/PaperWork/papers/Author-F.cgi

Joachim

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