On 4/27/2012 10:10 AM, Wayne Mills wrote:
> I also note that the experiment in the You Tube video is measuring noise on
> the mains line. What about radiated noise? Since the sample is referenced to
> ground, it might be representative of any radiation. The noise that I
> observed was affected by my hand in the vicinity of the lamp, surely
> radiation. Not good.
Some fundamental RFI concepts. Nearly all RFI we hear in our receivers
is radiated by something. It may be radiated by the noisy device, or it
may be radiated by a cable connected to the noisy device. In the EMC
world, noise radiated by the cable is called conducted noise, because
the noise is conducted out of the device to the cable. But it's still
radiated noise.
Noise can be coupled to cables (like the power line) as a differential
mode voltage (that is, between the conductors) or as a common mode
voltage (that is, on all the conductors in the cable). A common mode
voltage causes the cable to carry noise current, and any wire carrying
RF current will act as a transmitting antenna. It is that radiation
that we most often hear in our radios.
Ferrite common mode chokes are an effective way to kill common mode
current, but they must be tuned for the frequency(ies) where the noise
is present. A capacitor that shorts out the differential voltage can
kill that component.
Several mechanisms (beyond the scope of a simple email) convert
differential voltage to common mode current, and common mode current to
differential voltage. Thus, a serious ferrite common mode choke is
always a good thing.
http://audiosystemsgroup.com/RFI-Ham.pdf
LED lights work on low voltage DC, and the trash from products like this
is usually from the switching power supply that converts 120VAC to that
low voltage DC. We know about switching power supply noise -- it can be
quite frequency-dependent, it is generally drifty, it usually appears as
a hump of noise in a spectrum display, and the relative strength in any
frequency range will depend both on the waveshape of the noise in the
device and the effectiveness of both internal and external wiring as an
antenna.
Finally, we as hams must do our part to cause vendors to clean up their
RF noise. If you buy a product that's noisy, return it as defective and
demand your money back. Tell them why you're returning it -- that it
makes your radio and TV unusable, and that it violates FCC Rules.
73, Jim K9YC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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