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This paper presents the design of an I/Q modulator with integrated phase shifter for beamforming. It is targeted at 802.11a WLAN. An efficient beamforming architecture with a linearly controlled 360deg phase shifting range is proposed for linear transmitters. The phase shifting takes place in the local oscillator path at three times lower frequency than the carrier. The phase shifters are implemented as vector modulators in an efficient way by utilizing a current reuse technique. All control currents are provided by a single current steering digital to analog converter by means of time multiplexed sample and hold circuits. The circuit, implemented in 0.25 mum CMOS technology, has a 1 dB output compression point of -7.1 dBm and consumes 61 mA. It has a phase shifting resolution of 10deg with a differential accuracy of 2deg.