On 9/5/2020 3:38 PM, Karin Johnson wrote:
Now with the new so called “state of the art” system the noise sources
Are much different. Eventually I will get this solved. Just takes a bit
Of time. Especially since I know where the source is.
Hi Karin,
The problem with these products is typically NOT the lack of filtering,
but the failure to treat current paths carrying RF current as
transmission lines, combined with bonding return conductors to grounds,
which tends to divide return current between a proper return (the
transmission line) and the ground path. Example -- decoupling caps
accomplish that division.
If a transmission line is the only path, forward and return fields
cancel; when the return divides, cancellation is not only incomplete,
but the return path is both a large magnetic loop AND and antenna. In
other words, it's a circuit layout problem, NOT a filtering problem.
There are discussions of this in several tutorials on my web page, one
written for hams, others for pro audio system designers. Scroll down for
the pro audio tutorials. I'm retired from designing large audio systems
for public spaces, and still Vice-Chair of the AES Standards Committee
WG on EMC. In that world, we run into sources like motors for public
elevators in performing arts centers. A colleague, also an EMC
specialist, encountered this at NJPAC.
Also -- the vast majority of RFI is radiated common mode, often on the
ground conductor (green wire). If the RFI current driving the motors can
be isolated to a transmission line, a common mode choke tuned to the
operating frequency is the weapon of choice, and Fair-Rite #31 mix is
the only good choice for HF. See the "chasing RX noise" app note on my
website for more specific recommendations.
k9yc.com/publish.htm
73, Jim K9YC
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