Power and signal isolation are crucial to guarantee system safety and reliability in harsh industry environments. Conventionally, separate transformers or high-voltage capacitors are used for power and signal isolation [1]. To obtain a compact design, previous works used high-frequency power transfer that reduced the size of the transformers, while fulfilling power and data transfer using a single pair of transformers, as shown in Fig. 31.5.1. A primary-side LC-oscillator improved the efficiency due to the charge recycling of parasitic capacitance. Then [2] used amplitude shift keying (ASK) to transmit bidirectional data, i.e., forward data through primary-side supply modulation and backward data through secondary-side capacitance-amplitude (C-Amp.) modulation. However, the small modulation depth of the backward ASK (5% in measurement) makes it susceptible to voltage noise. Conversely, frequency shift key (FSK) is preferable for the applications that demand high-quality data. A straightforward implementation is to drive a power amplifier (PA) with a frequency-modulated signal [3], allowing a large modulation depth. However, it exhibits a high driving loss at the 100-MHz carrier frequency. Meanwhile, [3] only supported forward data transfer, and hence needs another transformer for backward data. To fulfill a high-efficiency FSK, an oscillator with frequency modulation is preferred. In [4], a backward FSK facilitates the frequency splitting effect of the LC oscillator. It selects one of the two splitting resonant frequencies by modulating the secondary capacitance (C-f Mod.) with C1 and S1. However, before each data bit transmission, the damping resistors R1 and R1' (controlled by switches S2 and S2') should quench the existing oscillation. This results in an increased settling time and hence lower data rate (e.g., oscillating frequency up to 30MHz, and data rate 5Mbps in [4]). Furthermore, [4] did not include the forward data transmission.