For massive MIMO systems, large number of antennas and transmitter chains are considered. This huge and complex circuitry consume lot of power. For saving the power, the modulator and DAC can be integrated in to one. Therefore, by taking in to account of power consumption, the 30 GHz RF-DAC based I/Q mod-ulator has been investigated. The thesis work summarizes the circuit design and simulation of digital I/Q modulator based on RF-DAC which is implemented by combining digital to analog converter and mixer in to one circuit block. The analog blocks are completely removed that saves chip area and power consumption. Since large portion of analog circuitry is removed, so it gives high efficiency and output power. In this thesis, the research work is to design 4-bit RF-DAC based I/Q modulator in 130nm SiGe, BiCMOS process at 30 GHz. The I/Q modulator consist of several blocks: differential branch line coupler, Wilkinson power combiner, 50-100 and 100-50 Ω input and output BALUN circuit. The circuit is designed for the four bits’ digital input. The problem of glitches arises as the output shifts from one level to next. In order to overcome this problem, the concept of thermometer coding logic has been used. So for the four binary bits, the thermometer coder gives 15 outputs. Thus, 15 conversion cells are designed and arranged in parallel in two quadrant RF-DACs I/Q modulator.