Sideband Inversion

Joel Hallas, W1ZR, (aka "The Doctor") has an especially good column in QST this month.   He takes on a topic that has confused (and re-confused!) many of us: sideband inversion.  Simply put, if you have a single sideband signal, and you put it through a mixer, depending on the frequencies involved and on whether you take the sum or the difference product of the mixer,  the sideband may or may not get INVERTED!   You could start out with an UPPER sideband signal coming out of your sideband generator, then, after you mix it with your VFO (or Si5351!) you end up with a LOWER sideband signal.  This can be quite an unpleasant surprise.

Joel gives us a good rule for remembering when this will happen: 

"Sideband reversal occurs in mixing only  if the signal with the modulation is subtracted from the signal that isn't modulated."   

Words to live by my friends.  Words to live by.     

The confusion on this topic often arises in discussions of the old scheme of using a 5 MHz, 9 MHz filter/VFO combination to generate LSB on 75 meters and USB on 20 meters.  This is very convenient, but you need to remember Joel's rule to get this scheme right!  If you start out with a sideband generator putting out UPPER sideband at 5 MHz. and mix it with a VFO running at 8.5 -- 9.5 MHz,  for 20 meters you will take the SUM of the two frequencies.  So no sideband inversion.  You will be happily on 20 meter USB (the mode used on that band).   For 75 meters you will be SUBTRACTING the SIGNAL WITH THE MODULATION (5 MHz) from the SIGNAL WITHOUT THE MODULATION (8.5-9.5 MHz).  So, following Joel's rule you WILL get sideband inversion.  Here you will be on 75 meter LOWER sideband (the mode used on that band). 

It is easy to get confused on this.  I got confused when Steve Smith sent me a 9 MHz filter out of an old Yaesu.  I had visions of using the old 9 MHz 5 MHz scheme.  But no.... With a 9 MHz sideband generator, you can get on both 75 and 20 with a VFO running at 5 to 5.5 MHz, but you won't get the nice sideband inversion situation described above because with neither band will you be subtracting the signal with modulation (9 MHz) FROM the signal without modulation (5-5.5 MHz).

It was very nice that Joel admitted to falling victim to this kind of confusion himself in a column he wrote years ago. 

Thanks Joel!  


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