What is the phase difference between the I and Q RF signals that software-defined radio (SDR) equipment uses for modulation and demodulation?
Why I and Q stand for In-phase and Quadrature, and "quadrature" means a quarter of a cycle, or 90 degrees. In an SDR the incoming signal is mixed with two copies of the local oscillator that are 90 degrees apart, producing two baseband streams whose relative phase lets the software determine both amplitude and phase (and therefore which side of the carrier a signal falls on). That extra phase information is what allows one pair of mixers to handle SSB, CW, FM and digital modes in software.
Watch out 180 degrees is the phase relationship in a push-pull or balanced circuit, where the two signals are simply inverted; that would give no new information about the signal, since a 180 degree copy is just the negative of the original.
Q = quadrature = a quarter of a cycle = 90 degrees.