An impulse radio ultra wideband (IR-UWB) transmitter (Tx) using finite impulse response synthesis of the raised-cosine pulse is presented. Symmetric pulse combining technique is proposed to reduce the number of power amplifier elements by half. A novel all-digital delay locked loop (AD-DLL) serves as an 8-array pulse position modulator (PPM) for aggressive duty-cycling of the Tx. The chip is fabricated with 90nm CMOS technology and consumes 540 μW from 1 V power supply resulting in 45 pJ/bit energy efficiency with -26 dBm of output power.