[RFI] tips for finding the source of broadband mixing products

Jim Brown 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
> diodes?

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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