A modulator for quadrature amplitude modulating a carrier signal by a first and a second signal derived from an input analog signal includes an A/D converter for converting the input signal to a digital signal in each clock interval and a circuit for producing an error signal representative of quantization errors of the digital signal in the respective clock intervals. The modulator comprises means for dividing the digital signal into a more and a less significant signal comprising more significant and the remaining digits of the digital signal, means for producing the first signal by arranging the more and less significant signals in succession in each clock interval, and a selectively operable inverter for producing the second signal by arranging the error signal in succession in each clock interval with the error signal left as it is and with its polarity inverted.