Abstract: Signal transmission apparatus for use, for example, in a passive sensor system such as an in-vehicle tyre pressure sensing system, comprises transmitter circuitry and receiver circuitry coupled together by a coupling, preferably a wireless coupling such as two opposed plate-form antennae. The transmitter circuitry includes a resonator having at least one component whose value influences a natural resonant frequency of the resonator and can be changed in use of the circuitry. The effective value is changed, for example, by a physical parameter being sensed or by a control signal to be transmitted. The receiver circuitry includes a driver section for applying to the resonator an excitation signal having a predetermined excitation frequency that is different from the natural resonant frequency. The transmitter circuitry preferably derives its power supply from the excitation signal via a rectifier circuit. The receiver circuitry detects such a change in the effective value via the coupling.