[Amps] IMD and spurious measurements

[Fuqua, Bill L]

  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
bulb. 
73
Bill wa4lav