In message
<[email protected]>, dated
Fri, 5 Sep 2014, "Pawson, James" <[email protected]> writes:
But John, HDMI uses a differential signalling interface which is known
to provide low emissions! 8-)
Oh, right. So no ferrites are actually necessary, ever.
Seriously, two things:
1. If there is a shield and it's not terminated properly (keeping the
impedance low), it can radiate stuff that comes from the grounds of the
equipment, perhaps nothing to do with the HDMI interface itself.
2. There is/was an astonishing amount of 'don't know' about differential
interfaces, even, apparently in some EMC standards committees, and it
hasn't entirely gone away yet. The main point is that a signal may start
off differential, but any impedance discontinuity in the cable or at its
end converts some of the signal to common mode. It doesn't need much of
a discontinuity to produce enough common-mode current to challenge the
emission limits.
A 'pigtailed' shield can present a sufficient impedance discontinuity.
The cable need to be treated as a 3-wire transmission line. The theory
of 3-wire lines has already been worked out, it's nothing new.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Quid faciamus nisi sit?
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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