[RFI] Coupling methods for RFI emission testing on SPS

[Jeff AC0C]

There are a few applications where I have needed to use a SPS and worried 
about RFI.  My solution has been to compare the powered and unpowered 
spectrum on a SA, looking for areas where the floor is raised.  Based on 
what I see on the SA, filtering is added to whack the noise.  I also look 
(again with a wire loop moved over the circuitry) what the radiated level 
seems to be.

I generally couple the SA using a few turns of wire looped around the input 
wires, and later the output wires of the SA.  If I can't see a lift in the 
noise floor, I will try a few more turns, or move the pickup wire along the 
wire a bit.

This test setup lets me judge the before/after effect of various 
combinations of filtering to determine which one actually serves best as the 
silver bullet.

It seems to work fine but what I don't know is "how much is too much" with 
respect to the raised noise floor?

I worry this coupling method is frequency dependent and in an absolute 
sense, I really don't know if what I am seeing is a bit insensitive compared 
to "real" antennas.  Or if it is complete overkill (meaning the test setup 
is too sensitive).

Generally I will let a bit of a change in the nose floor (say 10 dB or so) 
slide, with the thought that a signal 10 dB above the floor sensed with a SA 
probe separated only a couple mm from the radiating conductor will be in the 
mud given real-life separate distances between the SPS location and shack 
antennas of a couple hundred feet, against a background of atmospheric 
noise.  But I don't have any test data to say this is a solid assumption, or 
that 30 dB raise would be the acceptable limit, or that 1 dB is...  The test 
criteria is too subjective.

Hoping one of you EMI/RFI pros out there may have some comments on how the 
big boys do it.  Or maybe some other suggestions based on your experience.

Generally I try to use linear supplies whenever I can.  But in a growing 
number of cases, SPS cannot be eliminated so I'm left to clean up the noise 
profile as best as I can and with the significant cost of ferrites I want to 
make sure I'm being efficient in what the target noise level needs to be.  A 
guy can spend a LOT of money on ferrite and never know if it's adequate, 
inadequate or what.

73/jeff/ac0c
www.ac0c.com
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