Which Sideband?

Two questions, has the use of USB & LSB become a standard and why was there this notional dividing line at 10 MHz?

From the 1940s onward, telephone companies and postal authorities around the world used HF. The transmission mode was ISB (independent sideband reduced carrier) which allowed transmission of two separate phone conversations on each sideband. The signal was assembled by generating separate USB & LSB signals with a common suppressed carrier of 100 kHz, combining the two signals and re-inserting a controlled low level of carrier. The resulting ISB signal was then converted (heterodyned) up to the final output frequency.

In those days before frequency synthesisers, the local oscillator signal for the final frequency conversion was a crystal oscillator, with switched crystals for each of the scheduled operating frequencies. For the lower part of HF it is more convenient to have the local oscillator on the high side of the operating frequency; but at the higher frequencies a high-side local oscillator would have required expensive overtone crystals and possibly a separate oscillator for each crystal.

To avoid this, transmitter designers decided that for certain operating frequencies the local oscillator would change over to the low side but, that would also invert the ISB signal exchanging the two side bands and also exchanging the two separate conversions on channels A and B.

A dividing line was agreed upon and the choice was 10 MHz.
During 1950-51 roughly equal numbers of operators were using the filter type and phasing type of exciters. Most filter types were confined to a choice of either one side band or the other. While the phasing type could be easily switched to the opposite side band.On 80 m the initial standard was USB.

In about April 1952 amateurs made the change to the present convention of LSB below 10 MHz and USB above. Another factor that supported this standard was the publishing of the popular W1DX SSB exciter in 1949 which automatically produced a side band inversion

Source: The South African Radio League

The origin of this news item is the RSGB and it was obtained by SARLnews from WIANews. It is a summary of a feature about the history of which sideband we use on each band and how it was developed.

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