The differences between these two signals I & Q is 90� at all frequencies. There are two ways you can look this, the first is a base-band signal and second is an IF signal. My HF transceiver uses the base-band approach. For SSB one of the inputs is shifted by 90 degrees compared to the other. Over a frequency range of 300 Hz to 3 kHz, this is done with in a polyphase network.
Most software programs take an IF approach. The IF frequency is normally adjustable around the centre frequency of 12 kHz. The IF frequency range is fix by the PC sound card sampling frequency.
The modulator works by using two double balance mixers off set by 90�. The outputs are then combined into one output. The unwanted side band is cancelled out by its phase. For example:
I in is 0� + 0� = 0� where Q in is 90� + 90� = 180�
I becomes in phase and Q is out of phase.
By swapping over I & Q to Q & I you change from upper side band to lower side band for SSB. Will also change the IF from the hi side, to the lower side of the carrier frequency.