Massimo,
1) I have a small amount of experience with this. Symmetry will
play an important part. In some bus designs, designing in
added lengths to various traces may be necessary to keep an
equal prop delay among the traces of the bus.
2) EMI behavior depending on unequal signal paths is dependent
upon the frequency. Unequal signal paths will form simple
loops. Loop antennas respond directly to the square of the
frequency. Wires on the other hand respond directly to the
frequency.
3) I always advise shielding cables. But, this can also cause
more problems if a few things aren't considered:
a) Make sure the shield is making 360 degree contact
when connecting to the body of the connector.
b) At high frequencies, do not use small drain wires
for ground connection. Especially long lengths
of small ground wire.
c) Use braid style shielding for wires that may be
subjected to a moderate amount of movement during
their use. Use foil as a last choice and when
it's fairly certain it will be subjected to almost
no movement during it's use.
4) Discontinuities in transmission lines cause reflections.
This will "smear" digital signals. How much is theoretically
calculated by the different reflection coefficients:
a) For the one that is coming back at you from the
discontinuity:
Tau(reflected) = E(reflected)/E(incidence)
= (RLoad - RSource)/(RLoad + RSource)
b) For the one that is traveling on beyond the
discontinuity:
Tau(transmitted) = E(transmitted)/E(incidence)
= (RLoad^2)/(RLoad + RSource)
Interesting to note:
1) When RLoad < RSource, the reflection back to
the source is inverted in polarity.
2) When RLoad = 0, theoretically, the reflection
back to the source is 100% and inverted.
3) When RLoad >> RSource, theoretically, the
reflection is 100% and not inverted.
And the transmitted wave is proportional
to the load.
Just my two cents worth, Doug
At 08:39 AM 10/14/98 +0100, Massimo Polignano wrote:
>
>We are about using LVDS (Low Voltage Differential Signaling) technology to
>transfer high speed video data to a XGA display 1 m from the driver board.
>Differential signaling is very good to improve EMC characteristics of the
>link, but in my opinion is also very critical in actual layout: any lack of
>symmetry can jeopardize its performances.
>
>Has anybody esperience with these circuits?
>How much EMI behaviour depends on equal signal paths?
>Is it advisable to shield cables?
>What does a discontinuity on the transmission line (change in
>characteristic impedence) produce?
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