A 1.15-mW 3.75–4.25-GHz frequency-modulated ultrawideband (FM-UWB) transmitter (TX) is fabricated in 65-nm CMOS, with digitally reconfigurable subcarrier frequencies, UWB bandwidth, and radio frequency (RF) frequency band. Low-complexity submodules are utilized with significant power savings. An ultralow-current hybrid module is for bandgap reference (BGR) and power-ON reset (POR), and a single-ended crystal oscillator (XO) generates a 12-MHz clock reference. A dual-path ring voltage-controlled oscillator (VCO) conducts linear RF FM, followed by a push–pull power amplifier (PA) for 100-kb/s wireless data transmission. A differential relaxation oscillator (OSC) is for subcarrier generation, and an intermittent dual-mode frequency calibration loop ensures system robustness. Experimental results show that the presented FM-UWB TX successfully generates an Federal Communications Commission (FCC)-compliant UWB signal, with an energy efficiency of 11.5 nJ/bit and an active area of 0.21 mm2from a 1-V supply. The 2.25-$\mu \text{A}$hybrid BGR and POR achieves a temperature coefficient of 32 ppm/°C, a line regulation of 2.3%/V, a supply-ramp-rate-tolerant power-ON, and brown-out function. The 40-$\mu \text{W}$XO features a low-frequency inaccuracy of 4 ppm/V. The 40-$\mu \text{A}$relaxation OSC provides the reconfigured two-frequency shift keying (FSK) triangular amplitude and frequency. The 2.79–5.18-GHz 0.6-mW ring VCO is capable of providing the phase noise performance of −77 dBc/Hz at the 1-MHz offset, followed by the 0.45-mW PA having the output power higher than −14 dBm and the peak power-added efficiency (PAE) of 29.6%. All these benefit low-power low-cost TX implementation, where the carrier and subcarrier frequencies are calibrated by an intermittent digital loop with omitted power dissipation.