A frequency-modulated ultrawideband (FM-UWB) transmitter (TX) is fabricated in 65-nm CMOS, only consisting of simplified digital reuse modules and a radio frequency front-end (RFFE) stacking unit. All-digital reuse scheme of subcarrier generation and frequency calibration is proposed with a reuse ratio of more than 50%, to relax TX design complexity and eliminate the traditional dual calibration loops and high-frequency clock. The RFFE employs the current stacking of a dual-path ring digital-controlled oscillator (DCO) and a wideband push–pull power amplifier (PA), to achieve a significant RF power saving of 30%. The experimental results show that the 3.5–4.0-GHz TX generates a UWB signal and has an energy efficiency of 4.6 nJ/bit, with an active area of 0.13 mm2, a power dissipation of 0.46 mW, and a data rate (DR) of 100 kb/s, and achieves the transmitted power of ?14.3 dBm and the phase noise (PN) of ?86.2 dBc/Hz at the 1-MHz offset frequency. Compared to the reported literature, the prototype FM-UWB TX presents both digital unit reuse and analog module stacking schemes.