This may be true for RF applications, but it isn't universally true. Three-phase power is also modeled this way and both positive and negative frequencies defi

This may be true for RF applications, but it isn't universally true. Three-phase power is also modeled this way and both positive and negative frequencies definitely exist. Three-phase/three-wire electric power doesn't have three linearly independent currents or line-to-line voltages. Rather than fight the constraint that Ia+Ib+Ic == 0, we transform the system into one in which the constraint doesn't exist. The three (redundant!) 120-degree three-phase signals are transformed into a virtual two-phase reference system and we operate on the two phases that way. Sometimes we even "frequency shift" the signal into the reference frame of a physical (or virtual) rotor. There are some interesting side-effects. In the alpha-beta (stator) frame, unbalanced three-phase load looks like the sum of a dominant positive 60 Hz and smaller negative 60Hz signal. Diode-rectified loads are dominated by the negative fifth and positive seventh harmonics in the stator reference frame, but are frequency-shifted to negative and positive sixth in the rotor's reference frame. Three-phase four-wire (A, B, C, and Neutral) does have three linearly independent currents and voltages. But their disturbances are so different from each other that it is frequently convenient to model this system as the sum of a complex alpha-beta line-to-line current plus a virtual common-mode phase (the zero phase). See also the Clarke and Park transforms.