[RFI] EMI/RFI AC line filter design info?
jim at audiosystemsgroup.com
Mon Sep 23 11:51:00 EDT 2013
On 9/23/2013 7:39 AM, Ray, W4BYG wrote:
> Looking to minimize conducted and radiated noise from switching supply power
> cords and my double conversion (full time) UPS in the shack. Snap on
> ferrites aren't helping enough.
In my experience, most electronic noise is conducted as a common mode
signal, not as a differential signal, so conventional filters won't
solve it. A common mode choke formed by WINDING TURNS of the conducting
cable through a suitable ferrite core is the best solution when this is
the mechanism. Study http://k9yc.com/RFI-Ham.pdf then use guidelines
in the Choke Cookbook for the size of coax that corresponds to the size
of cable that you are trying to choke. You're right that simply snapping
ferrite cores onto those cables won't make a dent -- if you study the
data sheets, you will see that they are VHF suppressors.
It's also worth taking the system approach -- replace as many as
possible of these noise sources with equivalent product that is not
noisy. I recently went through the exercise of replacing all of the
newer switch mode wall warts that came with newer equipment with old
linear supplies from my junk box. When I ran out, I went shopping at an
electronic flea market for more and came home with several dozen of
various voltage and current ratings. Average cost under a buck.
You might also want to re-think that full time UPS. For at least ten
years, all the 12V gear in my shack has been running on one of the
biggest 12V deep cycle batteries that Costco sells for about $100,
float-charged by a clean, well regulated switch-mode power supply. The
PSU I'm using is sold by PowerWerx. I use a laptop for logging, and have
a good standby UPS on the business computer across the room.
I recently augmented the Powerwerx box with four solar panels that a
neighbor ham rescued from the dumpster at work. And on the advice of
other local hams, I upgraded from the Costco battery to a pair of 6V
golf cart batteries with more than twice the Ah rating. I'm using a
simple hysteresis regulator between the panels and the batteries, so no
noise from that either.
73, Jim K9YC
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