An interrogator (1) for an identification and telemetry system comprises: a transmitter (2) containing an antenna (3) for transmitting an interrogation signal incorporating modulated radio frequency energy at an interrogation frequency; means (36, 37, 38, 39) for generating said interrogation signal connected to said antenna (3); an antenna (3) for receiving a complete label reply signal from a signal responding label (4) containing an oscillator (9); means (40) for separating the interrogation and reply signals; means (42) for transmitting the separated reply signal to a receiver (12); and means (13, 48) within the receiver for decoding the reply signal; wherein said radio frequency energy includes a waveform having a period of substantially uniform amplitude; said oscillator (9) is used for forming said reply signal and said period is sufficient to allow coherent operation of the oscillator (9) within the label (4) for a period sufficient for the label (4) to transmit at least one complete label reply.