From: Ron Miller (rmiller@Brocade.COM)
Date: Tue Dec 21 1999 - 11:36:24 PST
Please consider that shielding is in fact " shielding". Whether it is shorted
at the ends, terminated at the ends or open at the ends of the signal trace
which it mirrors providing the return path. The treatment of the ends of
the path changes the mode through which it may radiate.
CANCELLATION OF COMMON MODE:
What cancellation of common mode do you expect to get from a microstrip?
If you think that ground current radiation is going to cancel trace radiation
you can extend this logic to a broadside coupled line in a vertical stackup by
analogy and it becomes again a differential line with no shielding.
Radiation from coax is far less than differential open line, so that theory falls
Shielding is far more effective than depending of ground radiation to offset
microstrip conductor radiation
For maximum isolation, float the ground shield at one end so that currents do not
flow in the shield.
"Ingraham, Andrew" wrote:
> >Chris, if you are comparing differential microstrips vs differential
> >striplines it should be obvious to you that stripline, being surrounded
> >by grounds would radiate less.
> Wait a minute ... maybe it's just me, or the phase of the moon, but this
> isn't obvious.
> For the differential component, I agree. For the common-mode component, I'm
> not so sure. In microstrip, both paths of the common-mode component are
> exposed to the side of the board, both might radiate, out-of-phase, and
> cancel one another in the far field to some degree.
> In stripline, the common-mode component on the signal traces is shielded
> from the outside world. At high frequencies, the return path takes the
> inner surface of the return plane, and is shielded by the thickness of the
> return plane from the outside world. But at lower frequencies, the return
> path, and ONLY the return path, might be exposed to the outside world, into
> which it alone can radiate. No cancellation of radiation components as you
> could get in microstrip.
> There may be some leaps of faith in these assumptions, but it seems that the
> potential could exist for stripline to maybe cause more radiation than
> microstrip. This seems flawed, but I'm not sure how.
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