A quality factor of switching diodes for digital modulation is proposed. It is defined by Q^= √(r1- r2)2+ (x1- x2)2/√r1r2where r1+ jx1and r2+ jx2represent the diode impedances in the two switching states. It is invariant to lossless transformations and permits direct computation of the loss produced by the diode in a given switching circuit.