[RFI] Another unidentified noise source

N6KJ kelly at thejohnsons.ws
Mon Jul 14 10:39:23 EDT 2003

Thanks everyone for the info on the low voltage lights.  I'm going to 
buy a new transformer and hope that fixes the problem.

This leaves one more source of noise that I have not identified.  I have
confirmed that this second noise source is not coming from my home.  I
can get a relatively good bearing on it with my HF yagi and my rig tuned
to 20 meters.  I hear it on other bands.  The amplitude drops as frequency

I have recorded this noise as a .wav file for anyone that would like to hear
it and/or analyze it, but be forewarned...the .wav file is 1.25M long.  I
can probably decimate it if necessary.

This noise sounds like typical power line "buzz".  For those DSP buffs out
there, here's the interesting part.  I loaded the .wav file into MATLAB
and ran an FFT on it.  What I see is 60/30 Hz noise spikes that follow
a sync function pattern with the peak at 1KHz.  The 60 Hz noise spikes are 
at least 10db greater than the 30Hz noise spikes, but I can see the
30Hz "sync function" just above the noise.  In other words, it looks like
a 1KHz square wave modulating a 60 Hz sine wave.  Would this be a typical
power line noise signature?  If not, any other theories?  So, anyone know of
common noise sources employing a 1KHz square wave that may be (intentionally or 
unintentionally) modulating a 60Hz sine wave?  

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