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A low-power CMOS modulator for carrier-based ultra-wideband (UWB) transmitters is presented. The core of the circuit consists of an oscillator and two control switches. The oscillator sets the center frequency of the UWB signal. The spectrum of the baseband data pulse is frequency translated to the desired band without using an explicit mixer. Since in many pulse-based UWB systems, in particular those used in radar and low-data-rate communication applications, a short duration pulse with a low pulse repetition frequency (PRF) is used, the power consumption of the modulator can be reduced significantly by turning off the oscillator between the pulses. This is accomplished using a combination of two MOS switches. By manipulating the pulse width and phase delay of the pulse signals applied to these switches, the time and frequency domain specifications of the transmitted UWB signal are adjusted. A prototype UWB modulator is designed and simulated in a 0.18mum CMOS technology. Simulation results for operation in the UWB low band, i.e., 3.1 to 5 GHz are presented.