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A 3-5-GHz ultra-wideband front-end for low-data rate wireless personal area networks is presented in this paper. The circuit, fabricated in a 90-nm CMOS technology, includes a Gaussian-envelope carrier-based transmitter, a direct-conversion down-converter, and an LO frequency synthesizer. A highly accurate Gaussian pulse shape is achieved by using a nonlinear pulse-forming approach, which provides a spectral efficiency of 38%. Moreover, a side-lobe rejection higher than 27 dB is reached without external filters. The transmitter achieves a peak pulse repetition frequency of 500 MHz supporting both pulse-position modulation and binary phase-shift-keying modulation schemes. It includes a baseband digital modulator, which implements spreading and time-hopping functions, and can be easily programmed to support different data rates, mean pulse repetition frequencies, and modulation timing parameters. The down-converter exploits a single-ended low-noise amplifier to avoid the off-chip wideband balun and minimize power consumption. It provides a 29-dB conversion gain and 5.2-dB noise figure while drawing only 8 mA. Operating from a 1.2-V supply, the overall front-end dissipates 42 pJ/pulse in RX mode and 56 pJ/pulse in TX mode.