On 10/8/2013 2:42 PM, Tom W8JI wrote:
the system should not be that sensitive to common mode issues on ports.
Right. But first, we're not talking about common mode, we're talking
about chassis-to-chassis noise coupling into unbalanced interconnects,
like computers feeding radios, with or without various interfaces. And
yes, unbalanced interfaces are a lousy way to do it, but I don't know of
a single ham rig that has balanced interconnects for audio or accessories.
So unless a ham wants to rebuild every rig and all accessory gear with
balanced interfaces, the simple power and bonding concepts I've outlined
are the lowest cost, most reliable, and a very robust solution. Yes,
we could add transformers, but transformers cost more, and shielded
transformers cost a LOT more.
As Vice Chair of the AES Standards Committee Working Group on EMC, I was
a principal author of all AES Standards on the topic. It took a while to
reach a consensus, because some purists were unwilling to write
Standards to work with real world equipment. The path we took, and that
the cool heads worked very hard to achieve, was to write Standards
defining the RIGHT ways to do it, both inside and outside of equipment,
but to define the right way to work with vintage gear that was badly
designed/built. Our first EMC Standard, AES48, attacked the Pin One
Problem, which was the most critical root cause, both at baseband and at
RF. We then wrote the protocols for balanced interconnects, including
the advice that when the cable shield needed to be interrupted to
prevent shield current, the interruption should always be at the
receiving end. This is counter-intuitive, but Bill Whitlock showed that
it is the only right way.
The point of this digression is that there's no way in hell that hams
are going to replace our gear with stuff having balanced I/O for audio
and control, simply because it doesn't exist, and to assume that such
gear will exist in the foreseeable future is wildly unrealistic. Heck --
we can't every get manufacturers to build gear without Pin One Problems.
73, Jim K9YC
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Topband Reflector
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