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RE: Dumb Question



I found that the best way to visualize SSB operation is to think in terms 
of generating or receiving a signal using a balanced mixer. In this case, a 
signal is received and "downconverted" to audio or baseband by eventually 
mixing it with a local oscillator. Selecting upper or lower sideband just 
determines the sense or polarity of the demodulated signal. In general, a 
LSB signal is produced when the modulating information subtracts from the 
carrier frequency and USB is produced when the modulating information adds 
to the carrier frequency. In order to properly demodulate a SSB signal, the 
demodulator must reproduce the modulating information in the same sense. If 
the modulation information caused the signal to increase in frequency, then 
when that signal is demodulated, the output of the demod must also increase 
in frequency. If the opposite sideband were selected at the demod 
(receiver), then an increase in frequency at the modulator (transmitter) 
would cause a decrease of frequency in the demodulator (receiver).  When 
using SSB to transmit and receive digital information a "1" or "0" is 
determined by the change of frequency or phase of a carrier.   If the 
transmitter is set to USB, let's assume that a "1" causes an increase in 
frequency and a "0" causes a decrease in frequency. On the receiving end, 
if the receiver is also set to  USB, an increase in frequency will cause a 
"1" to be output and a decrease in frequency will output a "0". If the same 
signal is received using LSB, an increase in frequency from the transmitter 
will cause a DERCREASE in frequency at the receiver demod and therefore 
output a "0" instead of a one. Therefore selecting USB or LSB allows you to 
decide whether you want your data NORMAL or INVERTED. This is very 
convenient because it is very easy for a modem designer to add an odd 
number of flip-flops to a circuit or an extra transistor driver that may 
invert the data somewhere along the path.

This is a condensed explanation of a subject not so easy to condense.  I'm 
sure it will spur comments. I hope I haven't confused you even more.

Howie
AB2S

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