This paper describes an IR-UWB transmitter with an asymmetric pulse which is featured with a steep rising edge and a slow falling edge. The proposed asymmetric pulse not only achieves a better ranging resolution but also reduces the inter-symbol interference (ISI) for high data rate communication. For low data rate communication, a frequency hopping (FH) method is applied to the transmitter to enhance the link margin. The proposed UWB transmitter is designed in 65nm CMOS. Compared to the conventional triangle pulse, the ranging variation of the proposed asymmetric pulse is reduced from 120ps to 10ps over PVT variations, the transmitter achieves a maximum data rate of 500Mb/s with a pulse energy of 11.5pJ/bit while meeting the UWB spectral mask.