Abstract: Interrogator for an electronic identification system having a first oscillator arranged to vary in frequency in accordance with the frequency of a received signal, a second oscillator arranged to vary in accordance with the frequency of the first oscillator after a delay of a number of cycles of the received signal, a phase discriminator for detecting the phase difference between respective output signals of the first and second oscillators and a phase-change detector arranged to determine the frequency at which the phase changes occur. In this way, a modulation frequency of the received signal can be determined.

Abstract: In a bandpass filter, a first cyclical analogue signal V3 at a center frequency divided by integer N in divide 44 and the divided signal supplied to a digital filter 48 as its system clock; a second cyclical analogue signal at a frequency, to be measured, differing by a small amount from the center frequency, is delayed by N cycles by a dynamic store 54; a substractor 56 subtracts the second signal from the delayed second signal and supplies any difference to the digital filter 48; the output of the digital filter is related to the frequency of the second signal, but the center frequency is suppressed. The filter may be used in an interrogation for an electronic detection system in which an interrogation signal is modified in frequency by a small amount, characteristic of a device to be detected.