[RFI] tips for finding the source of broadband mixing products
jim at audiosystemsgroup.com
Sun Sep 29 23:28:44 EDT 2019
On 9/29/2019 7:48 PM, JW via RFI wrote:
> How are you determining all this - you do realize, in close proximity to
> a transmitting rig, much (albeit low-level) RF comes straight out the
> power leads
If that happens, there are errors in design and/or construction.
and can 'modulate'/be modulated by the power supply
> energy that also escapes, including simple rectifier supplies using only
Ingress/egress is a linear function, and depends strongly on details of
both design and construction. Nearly all modern equipment fails to
terminate cable shields and power green wires properly. They SHOULD go
the the shielding enclosure (chassis), but they nearly always go first
to the circuit board, THEN eventually find the chassis after wandering
around return circuitry for a while. This equipment flaw, first
discovered by a ham working in pro audio, is called "The Pin One
Problem," because the designated shield contact of the connector
commonly used for balanced audio circuits is Pin 1.
The method in which equipment is built usually makes it impractical to
correct these design errors, so the best fix is a serious common mode
choke on the cable(s) involved. And because the ingress/egress is via
the green wire or the cable shield, conventional line filters are
useless UNLESS they are internal, and with their shielding enclosure
bonding the green wire to the equipment shielding enclosure! They treat
only the differential voltage and current between phase and neutral,
phase and ground, and neutral and ground.
The only effect of signal strength is on the strength of the mixing
> We demonstrated this at Heathkit several different ways, including using
> a spectrum analyzer to 'sniff' the stray RF coming back out via the radio's
> power cable WHICH in turn was modulated and showed 120 Hz sidebands ...
So you added an AC line filter with its shielded enclosure bonded to the
chassis, right? THAT would work.
73, Jim K9YC
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