Jerry,
I never understood that piece of Ham folklore, or should it be "Ham Myth" :)
In a mixing arrangement comprising a 9MHz IF and a 5-5.5MHz LO, designed
to cover 20m/75m, there is *NO* sideband inversion between the IF and
the RF. A USB signal generated at the 9MHz IF would become a USB signal
on 20m and also a *USB* signal on 80m.
So, simplification of the sideband generation or detection for that
simple mixing arrangement cannot be an explanation for the sideband
convention we have today.
73,
Steve G3TXQ
On 16/04/2011 17:32, Dr. Gerald N. Johnson wrote:
The swapped side bands on 17m is a product of the 9 MHz IF and 5 MHz VFO
scheme used in Tentecs for nearly all recorded history. That comes from
some of the earliest home brew rigs for SSB. And is why we use mostly
LSB on 80 and 40 and USB on higher bands.
In the days of the phasing rigs, changing sidebands was theoretically
simple, just invert one audio line, but in practice that never worked
because the 90 degree phase shift networks weren't exact and when tuned
for maximum suppression one sideband it wasn't adequate on the other
sideband. It was closer to not detectable unwanted sideband suppression,
so with the 9MHz IF and 5 MHz VFO it was tuned for USB on 20m using the
sum of the two signals and got LSB on 75 using the difference. This was
expanded to other bands by mixing the VFO with suitable crystals. Low
side injection to the signal mixer gets USB, high side gets LSB.
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