28.4 A High-Q Resonant Inductive Link Transmit Modulator/Driver for Enhanced Power and FSK/PSK Data Transfer Using Adaptive-Predictive Phase-Continuous Switching Fractional-Capacitance Tuning

As well as transferring power, inductively coupled systems such as RFID and wireless charging commonly require a downlink channel to transfer data to the receiving function, for simplicity usually using the same carrier frequency used for the power transfer. A high-Q resonant transmitter coil is highly desirable to create the strong magnetic field required fora practical operating range. However, this not only raises major problems with sensitivity to tolerances and environmental factors, but also seriously restricts the available bandwidth and hence downlink data-rate. Amplitude Shift or On-Off Keying (ASK/OOK) are commonly used to allow simple demodulation, but in addition to the 0 factor restricting the data-rate, the average power transfer will be reduced by around 50%. Frequency Shift Keying (FSK) or Phase Shift Keying (PSK) are attractive inasmuch as the nominally constant envelope provides a potentially higher power throughput, but the data-rate issue with a high-Q transmitter still remains. This is obvious for FSK, where by definition operation cannot be maintained away from the transmitter antenna's resonance frequency. Less obviously, for PSK applied to a nominally constant frequency carrier, the stored energy in the transmit tuned circuit will slow the phase transitions making demodulation more difficult; for binary PSK the amplitude will also drop significantly at each symbol transition. Note that the receiver Q factor is usually lower to avoid the need for active tuning in a micropower circuit.