[AMRadio] Physical Reality of Sidebands


John Coleman ARS WA5BXO wa5bxo at pctechref.com
Sun Jan 16 23:58:04 EST 2005


Don, I have often pondered the same thing here is another example.

	The Horizontal sweep rate of a TV is 15750 hz.  Every 1/15000 of
a sec there is a sync pedestal, and on the back porch of it is a burst
of a few cycles (as I remember it was 8 to 10 cycles in length) of the
color sub carrier (3.579545 MHz).  This burst is removed and processed
by an amplifier that is key on by the horizontal retrace pulse which has
been synced to the horizontal sync pulse that rides atop the sync
pedestal just in front of the color burst.  The 8 cycle color burst is
phase compared to a crystal oscillator in a phase locked loop.  A good
synchronized scope can look at the full video detected signal and spread
the back porch of the sync pedestal out and view the 8-10 cycles of the
burst.  I often wondered what a spectrum analyzer would look like when
monitoring the output of the burst amplifier with the phase detector
diodes remove.  

	The burst amplifier was a simple tetrode whose plate circuit had
a parallel tank tuned to 3.58 MHZ and where the detected video was
applied to the grid through a small coupling capacitor that would
differentiate and pass the frequencies higher than 3 MHz. The grid leak
was returned to a circuit where a positive pulse from the fly back was
present to trigger the tube on.  The output tank was link coupled to the
phase detector.      


John,
WA5BXO






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