This paper describes a frequency-domain on-off keying (F-OOK) modulation method which utilizes power detection of both a carrier and a sideband by modulating the carrier frequency with a pre-selected modulation template. Compared to on-off keying and binary frequency-shift keying modulations, more efficient bandwidth control is achieved for the same data rate with a symmetric FM template. Since the proposed modulation maintains the average center frequency of the carrier regardless of data pattern, the F-OOK signal can be generated by using only the high-pass modulation path of the conventional phase-locked loop-based two-point modulator, thus relaxing design complexity. A prototype transmitter implemented in 65-nm CMOS consumes 1.9 mW from a 0.8-V supply with a data rate of 750 kb/s, achieving an energy efficiency of 2.5 nJ/b.