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
alpha-charlie-zero-charlie
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