This paper describes a balanced frequency shift keying (FSK) modulation, namely quasi-balanced FSK (QB-FSK), for energy-efficient high-data-rate communication. Not suffering from data-pattern dependency, the proposed modulation method enables frequency modulation (FM) over 100 Mb/s by utilizing a wideband phase-locked loop (PLL). The QB-FSK signal is generated with the same baseband clock frequency as the non-return zero (NRZ)-coded binary FSK (BFSK) signal, resulting in improved bandwidth efficiency compared to other balanced FSK signals. For demodulation, the receiver employs a sideband energy detection (SB-ED) method. A FM discriminator converts the QB-FSK signal to an on-off keying (OOK) signal. After that, a band-pass filter (BPF) is used to filter out 1/f noise and dc offset. Based on the proposed QB-FSK modulation and the SB-ED method, a prototype FSK transceiver is implemented in 65-nm CMOS for high-data-rate sub-6-GHz proprietary wireless connectivity. The receiver successfully demodulates data at a rate of 200 Mb/s with a sensitivity of -67 dBm, while the transmitter can achieve a maximum data rate of 400 Mb/s. The 200-Mb/s transceiver achieves an energy efficiency of 0.14 nJ/b.