Abstract: Method measures imperfections in digital quadrature modulator and demodulator operation, and to a digital quadrature modulator/quadrature demodulator. The modulator includes means for modulating quadrature-phase I and Q input signals to generate an output signals to generate an output signal including quadrature-phase I and Q components; means for taking momentary amplitude samples from the output signal of the modulator at a rate based on a symbol clock of the modulator; means for classifying a direction angle of the symbol to be modulated at a particular time into a particular direction angle sector; means for linking the amplitude samples of the output signal to the direction angle sector corresponding to the symbol to be transmitted at a particular time, and means for comparing amplitudes of samples belonging to each direction angle sector with other direction angle sectors or with an ideal value for determining imperfections of the output signal of the modulator.