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Re: [Amps] IMD and spurious measurements

To: Manfred Mornhinweg <>, "" <>
Subject: Re: [Amps] IMD and spurious measurements
From: "Fuqua, Bill L" <>
Date: Tue, 11 Aug 2015 16:02:44 +0000
List-post: <">>
  I had an interesting problem once while working on a 2 Meter transmitter.
I was using a small capacitive probe, a piece of coax with about 1/2 inch 
unshielded on the end going to my tektronix 494AP spectrum analyzer.
I observed sidebands on the signal 20 to 100 kHz away from the carrier. This 
was a real concern for me. I moved the probe around on the boards and
saw the same thing. I then lifted the probe from the board and noticed the 
sidebands increased as I got it  further from the transmitter. It was then that
I realized that it was the compact florescent bulb I was using. It was 
switching on and off at a very high rate causing the plasma to switch on and off
thus making a perfect modulator for the signal. It was not the noise getting 
into the transmitter but the modulation of the signal by plasma within the
Bill wa4lav
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