An ultrawideband (UWB)-based sensor-to-time transmitter consisting of a remote control ( $RC$ ) time-constant interface and an ultralow-power pulse generator is presented. The sensing information is directly extracted and transmitted in the time domain, exploiting UWB pulses with a high time-domain resolution. This approach eliminates the need for an analog-to-digital converter and baseband blocks of sensor tags; meanwhile, it reduces the number of bits to be transmitted for energy saving. The sensor interface measures the discharging time of the $RC$ time constant proportional to the sensor variation. The UWB pulses are triggered with intervals of the $RC$ discharging time, without any digitizing or modulations. The circuit prototype is implemented in the standard 0.18- $\mu\mbox{m}$ CMOS process. Resistance measurement results show that the proposed system exhibits an effective number of resolution bits (ENOB) of 7.7 bits with an average relative error of 0.42% in the range of 200–1500 $\Omega$ . The overall energy consumption of conversion and transmission per sample is measured to be 0.58 nJ with a 1.27-Vp-p pulse amplitude, which is favorable to radio-frequency-powered wireless sensing applications.