A power transmitter (101) of a wireless power transfer system provides wireless power to a power receiver (105). The power transmitter (101) comprises a variable resonance circuit (201) generating an inductive power transfer signal in response to a drive signal. The resonance circuit comprises a capacitive and inductive impedance (201, 203), at least one of which is variable. The resonance frequency can be varied by at least one of the impedances being variable in response to a control signal. A driver (205) generates the drive signal with a variable drive frequency. A frequency modulator (305) applies frequency modulation to the drive signal by varying the variable drive frequency in response to data values to be transmitted to the power receiver (105). An adapter (309) generates the control signal in response to the data values such that the variable resonance frequency follows the variations in the drive frequency resulting from the frequency modulation of the drive signal.