This paper discusses how Differential Pulse Position Modulation (DPPM) can increase the energy efficiency of an UWB impulse radio transmitter when compared to On-Off Keying (OOK). In DPPM, data words are coded in time intervals between consecutive pulses and it saves energy as less pulses are transmitted per bit. A method is shown for minimizing the energy con sumption per bit of a DPPM transmitter by choosing an optimal data word length. Measurement results show an energy efficiency improvement of 45.2%. The jitter of a low-power, low-performance reference clock used in the modulation could cause an unsatisfactory bit error ratio (BER). However, the measurements show that a 1.4 μW, 10 MHz ring oscillator (RO) can enable a BER better than 10−6. The measured transmitter has been implemented in a 180 nm CMOS technology.