Detector device

A method of operating a detector device for detecting an object in a detecting zone by emitting an electro-magnetic signal using a generated signal into the detecting zone and receiving a first electro-magnetic signal therefrom through a conductor, the detector device comprising at least a field-distorter comprising:

• disposing a semiconductor device adjacent to the conductor, the conductor for carrying the received first electro-magnetic signal;
• the field-distorter being responsive to an input signal to shift the phase of the received first electromagnetic signal to produce a phase shifted signal in the conductor;
• mixing the phase shifted signal with the generated signal to produce a combined signal having a characteristic determined by the input signal, indicative of the correct operation or otherwise of the detector device, wherein the characteristic of the combined signal corresponds to a change in a decomponent of the combined signal;
• the change being responsive to the input signal;
• the method comprising:
• applying a signal to the field distorter to vary the electrical or electro-magnetic characteristics of the field distorter and thereby influence at least one characteristic of the first electro-magnetic signal;
• and
• producing an output signal indicative of the degree of influence exerted on the received first electro-magnetic signal.