The first, second, and third pairs of DC voltages which set a modulated signal to a predetermined reference level while the levels of I and Q signals supplied to a quadrature modulator are set at “0” are sequentially obtained. On the basis of the first, second, and third pairs of DC voltages, at least three measurement points based on a combination of pairs of DC voltages which set the level of the modulated signal to the predetermined reference level are decided. The coordinates of the center of a virtual circle having the three measurement points positioned on its circumference are automatically calculated by a geometric analytical computation. Adjustment voltages to be added to the I and Q signals so as to cancel out a carrier leak contained in the modulated signal are obtained from the coordinates of the center.
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