Speaking of "return", Jim: Take the Yaesu VX-5R (and, I presume those
following that design). Yaesu properly treated the + DC line with ferrite,
a common mode choke, and bypass caps, but did nothing for the DC return.
Yep......with an external amp in the mobile, I could make the VX-5R do all
kinds of inappropriate operations when keyed with the amp. Returns are
equally important as the "obvious" ingresses.
Thanks for pointing this out.
Dave - WØLEV
On Sun, Oct 3, 2021 at 11:11 PM Jim Brown <jim@audiosystemsgroup.com> wrote:
> On 10/3/2021 11:03 AM, David Eckhardt wrote:
> > CMCs would on 31 material should be much
> > better at suppressing EMI than the included toroidal chokes.
>
> In addition to Dave's excellent advice, I would add this. First, it's
> important to consider the return path. As Henry Ott taught in his EMC
> workshops, the hidden schematic lurking behind the ground symbols -- to
> study where ALL the current is flowing.
>
> By ALL the current, he meant not only the mains or DC in a system, but
> also the RF. I saw a great demo of this at an IEEE workshop in Chicago,
> about 20 years ago when I was giving a talk about EMC in pro audio. The
> setup connected an HP signal generator to a load separated by a large
> table. Signal was carried by RG58, with a large metallic plate wired in
> parallel with the shield, with current probes in the wires to the plate
> and to the shield. As the generator was swept upward from 30 Hz or so,
> where all the current was in the large plate, around 1 kHz it began
> shifting to the cable shield, and by 20 kHz, nearly all of it was in the
> shield.
>
> A bonding and/or return conductor must be run with the current carrying
> conductors, so that it acts like a transmission line to contain magnetic
> fields created by those currents. It is also good engineering practice
> to make each phase twisted pair, to further limit leakage of the fields.
>
> On multi-layer circuit boards, a continuous "ground" layer runs under
> layers of traces, and a transmission line is formed between the trace
> and the "ground" layer, which limits the strength of fields that can
> crosstalk into other circuits. But this falls apart when the "ground"
> layer is interrupted to fit in a trace that the designer forgot -- it's
> no longer a transmission line, so return current flows wherever there's
> a path, like the chassis, creating a magnetic field for crosstalk.
>
> 73, Jim K9YC
>
>
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--
*Dave - WØLEV*
*Just Let Darwin Work*
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