[clug] OT: ADSL2+ and AM radio interference

Ben Nizette bn at niasdigital.com
Mon Apr 26 03:15:40 MDT 2010

On 26/04/2010, at 6:34 PM, steve jenkin wrote:
> Ben - any comments on noise from powerful inverters?

I think "inverter" in the A/C sense refers to a system which can drive the compressor at something other than full- or zero-speed yea?  As opposed to an "inverter" in the electrical sense which is usually used as a DC/AC converter like getting 240V from your car battery.

The motor controller in the air con which does this by varying the drive speed of the motor, generally by pulse width modulation.  Humans (esp children) can hear somewhere near 40kHz so any PWM frequency less than this will cause the various wires to physically vibrate in such a way as to produce an audible noise.  Frequency much above a few hundred kHz will loose too much in RF radiation.  So, each summer you've got a device pumping hundreds of watts of power in something close to a square wave through a big coil at a frequency between, say, 50 and some-hundred kHz, that certainly sounds like a recipe for interference to me! :-D

(of course the square wave matters because the Fourier transform of a square wave is the infinite sum of odd harmonics so an n-kHz square wave has heaps of different radiating frequency components, at least one of which generally causes pain ;-) )


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