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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Transmission line question
Date: Fri, 13 Sep 2002 19:16:11 GMT
NNTP-Posting-Date: Fri, 13 Sep 2002 12:16:11 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
"Christopher R. Carlen" wrote:
> This question resulted from another guy's post about grounding ribbon
> cables at both ends or just one end.
> Let's say we have the following transmission line arrangement:
> +--Zg--+===============+---+ <--+ cond.#1
> | | |
> Vg Zo1 | |
> | | |
> +------+===============+ | Zo2 cond.#2
> | | |
> | Zl |
> | | |
> +------+===================+ <--+ cond.#3
> Where Vg is an AC voltage generator,
> Zg is the generator impedance,
> Zl is the load impedance,
> Zo1 is the characteristic impedance of the transmission line formed by
> the top two conductors (1 and 2), ie the conductor from the hot end of
> the generator and the conductor that is grounded at one end but open at
> the other end.
> Zo2 is the characteristic impedance of the transmission line formed by
> the top (hot) conductor 1 and the bottom conductor 3, the one grounding
> the load.
> We assume that Zo1 << Zo2, which is likely to be the case if Zo1 is
> formed by two adjacent ribbon cable conductors, and Zo2 is formed by the
> hot ribbon conductor in relation to a ground wire far away.
> What happens?
> It seems to me that a wave propagates forward on the line with Zo1,
> since that is a lower characteristic impedance than Zo2. But some
> proportion will also propagate on the Zo2 line. What happens when the
> wave reaches the end of the line? Hmm. It could be said that since the
> Zo1 line is effectively unterminated, that there is total reflection of
> the proportion of the energy that is propagating in the wave on the Zo1
> line. Thus, only the energy carried by the Zo2 line would bear on the
> load, which depending on how it is matched to Zo2 will produce a
> reflected wave related to the reflection coefficient for that Zl, Zo2
> What happens when the two reflected waves get back to the generator?
> Obviously they will react with the Zg and absorb in Zg or reflect
> according to the reflection coefficient formed there between Zg and the
> two Zo values. But the proportioning also happens again.
> Well, I have formed a picture in my head of what is going on here, and
> it is quite complicated. The end result though is that it has conviced
> me that it is likely to be the best choice to ground the ribbon cable at
> both ends, and send any signals with reasonably fast components over a
> ribbon with interspersed ground conductors.
> Comments appreciated.
> Christopher R. Carlen
> Principal Laser/Optical Technologist
> Sandia National Laboratories CA USA
I do not knw how a ribbon cable is made or what it liiks like.
In general, a 2-conductor cable should be terminated at the far end
across its two wires for best pulse response.
Now, let us assume the ribbon cable is made with flat strips of copper
with an insulator between, and then more insulation on the outside
surfaces; the result looking like a flat wire.
In that case, it does not matter which side is signal and which side
But, now dess it flat against a copper chassis or ground plane.
The flat wire closest to ground now has a much lower impedance than
that specified for the cable (which is normally done in free air, sor of
like TV leadin).
Now one should tie the top foil to the ground plane at both ends and
terminate in the new characteristic impedance, and perhaps use a T pad
to match the generator as well.
Does this help?
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