Outstanding explanation.
Gary...wa6fgi
On 11/10/2016 8:07 AM, Jim Brown wrote:
Hi Greg,
Outside of compliance test labs, our concern is NOT to determine
compliance with FCC Rules for emissions, which are not necessarily a
good indicator of the extent to which a given device will radiate
enough noise into our antennas to be problematic. A LISN (Line
Impedance Simulation Network) simulates the DIFFERENTIAL loading of
the noise source by the power system, but most noise is radiated
either by 1) the combination of poor circuit layout and poor
shielding, or 2) as a COMMON MODE signal on ALL wiring connected to it
(that is, not only the power line).
As a point of clarification, what EMC rules call "common mode" is
voltage between neutral and green, while what WE call common mode,
because it's what causes radiation, is the sum of all current on the
cable in question. In most products, that turns out to be the current
on the green wire and on the shields of interconnecting cables. The
most common cause of common mode noise current is what audio
professionals call "the Pin One Problem," where the cable shield or
the AC green wire fail to bond to the shielding enclosure at the point
of entry, but instead go THROUGH the shielding enclosure to the
circuit board. This puts noise current on the Green Wire, which goes
right past a commercial line filter external to the noise source. That
filter CAN be effective only if it bonds the Green Wire to the
shielding enclosure by a "zero length" lead, ideally mounted to the
shielding enclosure.
As I see it, a key benefit of using an SDR in chasing RFI is in
identifying the noise -- that is, is it a power system device like an
SMPS or variable speed motor controller, the clock for some sort of
digital electronics, or impulse noise. An SDR can also help by
letting us monitor more spectrum at once as we switch the potential
noise source on and off.
Bottom line -- as hams, we want to know if a given device will bother
us, not whether it complies with FCC Rules.
73, Jim K9YC
On Thu,11/10/2016 5:26 AM, Greg Troxel wrote:
Jim Brown <jim@audiosystemsgroup.com> writes:
Thanks for posting this work. Several comments. First, the use of an
SDR to look at spectra of the noise is a really good idea. I'm
currently working on updates to the ARRL Handbook, and this is
something I'm suggesting. The SDRPlay ($130) is a particularly good
choice, because it has a very wide tuning range and can display a
large chunk of spectrum at one time.
I have also been trying to make measurements, but so far just looking at
a PX3 hooked up to either an attic dipole or a proper outside antenna.
Operationally for me, if I can't hear the difference between on/off on
any band on either antenna, with a desk lamp in the shack, it's good
enough. But I'm starting to try to do this listening on closed bands;
my 80m noise level is vastly lower midday than evening, which I suspect
is typical for relatively quiet locations.
It seems that for repeatable quantitative measuremnts, one wants a Line
Impedance Stabilization Network and a spectrum analyzer, but this is at
best $2K. I have borrowed a LISN (which is large and heavy) and intend
to hook it up (with at least a 20 dB pad) to a KX3/PX3. That's not a
proper SA, but should be pretty good within the ham bands.
So I wonder if you are able to suggest (for the Handbook) how to
approach repeatability without buying or homebrewing a proper LISN.
Also, it would be good to discuss rough calibration of things like the
SDrplay to get measurements that are plausibly close to using real lab
equipment.
I say all this realizing that the audience includes people with varying
technical backgrounds and budgets.
But overall, with some repeatability and rough calibration, it should
then be reasonable to build up a web database of results. It seems
like the hard part is the LISN equivalent. Perhaps that would make a
good kit, or perhaps I'm overestimating the parts cost/hassle.
73 de n1dam
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