[ETS/PARC List] Why 20M on USB and 75M LSB

[Victor J. Pennetta]

Many people wonder why USB is used on 20m and LSB on 75m...  also why when tuning that the USB seems to be on the lower side and the LSB is on the higher side... It's very simple when one knows why. 

>> The signal is actually inverted after the generation and therefore 
>> what was originally the USB signal becomes LSB and vice versa.  In a 
>> "normal" situation putting the crystal on the low side of the filter 
>> produces an upper sideband signal and putting the crystal on the high 
>> side of the filter produces a lower sideband signal.  However, when 
>> mixing the sideband signal using an injection frequency on the high 
>> side of the signal (which Collins does) the sidebands are inverted.
>>
>> If you notice, the heterodyne frequency crystals are operating above 
>> the band (either on fundamental or on the 2nd harmonic) and therefore 
>> the injection frequency is above the final frequency.  This, in turn, 
>> inverts the sidebands.  The choice of i.f. frequencies in the S-Line 
>> makes use of low side injection difficult and therefore the inverted 
>> sidebands.
>>
>> In a lot of the older SSB equipment the sideband generator was 
>> operating at 9 MHz.  Then by using a VFO frequency at 5 MHz the same 
>> circuit would generate a 75 meter and a 20 meter signal.  However, 
>> the sidebands were reversed.  That is one of the reasons that LSB 
>> became the defacto standard on 75 meters and USB became the standard 
>> on 20 meters.
>>

73,
Victor  KC2NBC

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