This paper proposes a frequency-domain OOK (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. Unlike the conventional OOK modulation, the carrier waveform in the time domain can be always present regardless of data pattern, thus offering a packet-level duty cycle operation without affecting overall spectrum occupation. Compared to the BFSK modulation, more efficient bandwidth control can be achieved for the same data rate with a symmetric FM template. The F-OOK signal is generated by a PLL based modulator in which only high-pass modulation is performed, which is different from the conventional two-point modulation, thus significantly relaxing design complexity. A prototype transmitter implemented in 65nm CMOS consumes 1.9mW from a 0.8V supply with a data rate of 750kb/s, achieving an energy efficiency of 2.5nJ/bit.