A method of operation of a pet door system with a controllable lock and circuitry including a resonant circuit having an antenna and an element to vary the resonant frequency of the circuit according to a setting parameter, a drive circuit for driving the resonant circuit with a drive signal, and a demodulator, the method including a setting-up operation comprising varying the setting parameter while driving the resonant circuit at a first driving frequency, detecting a first value of the setting parameter at which the resonant circuit is substantially in tune while the resonant circuit is driven at the first driving frequency, and storing the first value of the setting parameter. The method further includes a reading operation comprising reading the first value, driving the resonant circuit at the first driving frequency with the setting parameter set to the first value, and attempting to derive data from the demodulator.