Indirect modulation of fractional-N synthesizers is an energy-efficient architecture capable of moderate data rates, and is well-suited for use in sensor networks or WLAN. Although the architecture is used primarily at low RF frequencies, the capability for fractional- N synthesizers at Ku-band and above currently exist in available silicon technology. Recent demonstrations at 10- 25GHz show promising results, although power consumption at this higher frequency remains high for small batterypowered devices. This work implements a fully-integrated fractional-N synthesizer optimized for power efficient modulation at 15.8 to 18.9GHz with an 80MHz reference. Binary and 4-ary FSK modulation of up to 8Mbps is achieved while consuming 16mW in IBM 0.18um SiGe BiCMOS.