Abstract: Structure and technique, for calibrating a vector modulator that does not require a network analyzer, are disclosed which are incorporated in a vector modulation system comprising a standard vector modulator circuit including means for shifting phase to adjust the relative phases of the I and Q components of the modulated RF carrier. Means for attenuating are included in the I and Q modulation system to adjust the relative amplitude of the modulation signals. The disclosure describes a technique wherein the microwave transmitter's I and Q phase difference is measured by measuring the power levels of the I and the Q signals. The power levels are compared and a correction value is stored in a read-only memory to compensate for the error.