A circuit block configured to be connected to a resonant circuit, the circuit block having a pair of connections for connection of the circuit block into the resonant circuit as a capacitor, said connections configured to enable the circuit block to draw power from an external radio frequency signal to power up the circuit block, the circuit block comprising: a first non-linear capacitor (C2, C3), comprising a pair of series coupled capacitors with a transistor coupled across one of the capacitors of the pair, said first non-linear capacitor has one of two different values of capacitance dependent on an instantaneous value of a varying amplitude of voltage of a resonant signal on the non-linear capacitor, the capacitance of said first non-linear capacitor configured to switch between one of said two different values and the other of said two different values during a cycle of the resonant signal by varying a gate voltage on the transistor, to thereby provide an effective average capacitance value based on a duty cycle of said capacitance switching; a power supply circuit coupled to said non-linear capacitor to provide a power supply for the circuit block; a plurality of second capacitors (C5 - C14) each coupled to a respective second capacitor switch to enable a said second capacitor to be switched in or out of parallel connection with said first non-linear capacitor; and a tuning control circuit powered by said power supply, coupled to said second capacitor switches, and having an input to sense an amplitude of said resonant signal; and wherein said tuning control circuit is configured to, once the circuit block is powered up, control said second capacitor switches to successively switch each said second capacitor in or out of said parallel connection with said first non-linear capacitor dependent on said amplitude of said resonant signal, each said successive switch after a delay for the non-linear capacitor to adjust to recover amplitude of the resonant signal, said control until the effective average capacitance value of said first non-linear capacitor is substantially a single one of said two different values such that in said resonant circuit said circuit block then appears to behave substantially as a fixed value capacitor. a first non-linear capacitor (C2, C3), comprising a pair of series coupled capacitors with a transistor coupled across one of the capacitors of the pair, said first non-linear capacitor has one of two different values of capacitance dependent on an instantaneous value of a varying amplitude of voltage of a resonant signal on the non-linear capacitor, the capacitance of said first non-linear capacitor configured to switch between one of said two different values and the other of said two different values during a cycle of the resonant signal by varying a gate voltage on the transistor, to thereby provide an effective average capacitance value based on a duty cycle of said capacitance switching; a power supply circuit coupled to said non-linear capacitor to provide a power supply for the circuit block; a plurality of second capacitors (C5 - C14) each coupled to a respective second capacitor switch to enable a said second capacitor to be switched in or out of parallel connection with said first non-linear capacitor; and a tuning control circuit powered by said power supply, coupled to said second capacitor switches, and having an input to sense an amplitude of said resonant signal; and wherein said tuning control circuit is configured to, once the circuit block is powered up, control said second capacitor switches to successively switch each said second capacitor in or out of said parallel connection with said first non-linear capacitor dependent on said amplitude of said resonant signal, each said successive switch after a delay for the non-linear capacitor to adjust to recover amplitude of the resonant signal, said control until the effective average capacitance value of said first non-linear capacitor is substantially a single one of said two different values such that in said resonant circuit said circuit block then appears to behave substantially as a fixed value capacitor.