[Milsurplus] EMI from LED lighting

[Dennis Monticelli]

John,

There are all kinds of schemes from frequency hopping to dithering.  Dither
is the easiest.  Dithering (i.e. FM) a PWM clock spreads it out modestly,
just enough to knock down the carrier below the FCC limit while pushing the
excess energy into new sidebands.

Dennis


On Sat, Mar 8, 2014 at 6:43 AM, J. Forster <jfor at quikus.com> wrote:

> Apparently, dilution is the solution to pollution, in electronics too. Do
> you know how much they vary the frequency/period?
>
> -John
>
> ==============
>
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> > My company makes IC's for electronic ballasts so I can provide a little
> > insight.  Be forewarned that the story isn't a comforting one.
> >
> > First of all the LED itself is as benign as an incandescent lamp.  The
> > problem lies with the switching power supplies that feed a constant
> > current
> > from a high voltage to the low voltage of the LEDs.   All these products
> > must pass FCC Part 15.  They are not cut any slack with regard to meeting
> > the spectral mask just because they are "green" and represent a
> > multi-billlion dollar industry.  Unfortunately, the FCC's spectral mask
> in
> > the HF spectrum is insufficient to protect a sensitive receiver from
> > noise.
> >  Lights are everywhere so unless your antenna is far removed from your
> > home
> > and fed with well-shielded coax, you will likely experience some noise
> > proportional to that proximity.  Most of the noise will emanate from the
> > AC
> > line feed not the small circuit board.  So choking off the currents on
> the
> > AC line will help more than extra shielding around the lamp.  The IC
> > makers
> > are under pressure to make the chips cheap and to require minimal
> external
> > components for meeting EMI.  This translates into various PWM schemes
> that
> > smear out the noise spectrum so as to avoid high amplitude fixed
> frequency
> > spikes.  So instead of getting a few strong raspy tones in our radios we
> > get a general raising of the noise floor.  The total noise energy is the
> > same, just spread out.  The more consumer PWM stuff near your antennas,
> > the
> > higher the noise floor becomes.
> >
> > I wish I had a happier story to tell.  Unfortunately, PWM devices are
> > appearing everywhere in the home while the FCC is all about "let the
> > market
> > decide" and opening up spectrum (including your home power line) for
> > various digital data formats.  The FCC doesn't really care about AM radio
> > and the next generation of consumers doesn't either. The BCB is so
> > "yesterday."  Hams are just kind of caught up in the tide.
> >
> > Dennis AE6C
> >
> >
> > *A friend asks:*
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> > *My biggest question about LED lighting is how much radio interference
> > itgenerates at frequencies up to about 30 MHz (HF).  Other users of
> > theradio spectrum are more concerned by interference at higher
> > frequencies,through VHF and UHF.AFAIK, an LED by itself does not generate
> > radio interference; it is theelectronics between the LEDs and the AC
> power
> > line that concerns me.Fluorescent lights (both CFLs and long straight
> > tubes) with electronicballasts are bad because an electronic ballast
> > switches AC line power withvery short rise & fall times, so the ballast
> > strongly excites the linewith radio-frequency power at whole-number
> > multiples of the switchingfrequency up to 30 MHz and beyond.It is
> possible
> > to reduce this radio ?noise? or interference to a tolerablelevel by means
> > of shields, L-C filters, and ferrite chokes; but very fewmanufacturers of
> > fluorescent lighting products do.  If they do anything,it is nowhere near
> > enough.I?m waiting for the prices of LED-lighting products to drop
> further
> > beforeI buy some for evaluation.  Meanwhile, can anyone direct us to
> > published,quantitative information about the radio noise generated by LED
> > lightingproducts?Best,-John*
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