[Test-Equipment] SSB on GR 1001a signal generator

[Fuqua, Bill L]

Well,
  It does not look like anyone has taken any interest in this sideband note.
Ends up that the residual FM modulation produced by the internal AM modulator is just enough to 
produce a Narrow Band Modulated signal with just the right modulation index to generate a lower sideband
equal in amplitude to the AM lower sideband.  Since the lower sideband of a NBFM signal is 180 degrees out
of phase with the upper sideband the lower sidebands cancel out. This only happens around 5 MHz on the
1.5 to 5 MHz band. The upper sideband is doubled in amplitude since both are in phase. 
  The hint from the specifications is the incidental FM specification of 30 to 300PPM.
At the low end of the bands where the capacitance is 9 times that of the high end, you get 30 PPM
and at the upper end about 9 time that at minimum capacitance. 
  The output tube behaves something like a reactance tube modulator when it is amplitude modulated
due to the plate to grid capacitance of the tube. 
  You would never notice it unless you observed it with a good spectrum analyzer with resolution of
100 Hz or less. 

73
Bill wa4lav

________________________________________
From: test-equipment-bounces at mailman.qth.net [test-equipment-bounces at mailman.qth.net] On Behalf Of Fuqua, Bill L [wlfuqu00 at uky.edu]
Sent: Monday, December 12, 2011 2:40 PM
To: Discussion of Electronic Test Equipment
Subject: Re: [Test-Equipment] SSB on GR 1001a signal generator

A few extra comments. this was in the early 70s and I had recently had a class in
information theory, which involved modulation and RF techniques. Today informantion
theory classes mostly are about digital and little about radio technology.
  Another  hint is in the specifications in the manual.
73
Bill wa4lacv

________________________________________
From: test-equipment-bounces at mailman.qth.net [test-equipment-bounces at mailman.qth.net] On Behalf Of Fuqua, Bill L [wlfuqu00 at uky.edu]
Sent: Monday, December 12, 2011 12:39 PM
To: Test-Equipment at mailman.qth.net
Subject: [Test-Equipment] SSB on GR 1001a signal generator

   When I got my first spectrum analyzer working I noticed something odd.
I was using a GR1001A signal generator with internal modulation and noticed
that it had a carrier and only the upper sideband.
   Since my  spectrum analyzer's basic IF  was 5 MHz and I did not have the
external local oscillator connected I had tuned the 1001A to 5 MHz on a the
top of that band.
  I  figured something was wrong with the analyzer. But I realized if I switched to
the next band up where 5 MHz was at the bottom of the range it produced the
signal that I had expected, carrier with upper and lower sidebands.
  It took me a while to figure out what was going on and once I analyzed the
circuit I it made sense.
  Here is a trivia question for everyone.
What happened to the lower sideband?
   By the way I noticed the same effect on more than one of these signal generators.

73
Bill wa4lav
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