Abstract: A mixer receives a frequency shift keyed (FSK) radio signal having two transmission frequencies and mixes the radio signal with a first or a second local oscillator signal from a local oscillator to provide a mixed signal. The mixed signal lies within a high or low band of frequencies depending upon the frequency of the FSK signal and upon the local oscillator signal received by the mixer. A detector receives the mixed signal and detects in which band of frequencies the mixed signal lies, and generates a switching signal in dependence upon the band of frequencies in which the mixed signal lies. If the FSK signal changes to a different transmission frequency and this causes the mixed signal to lie in the low band of frequencies, then a switching signal is generated which switches the local oscillator such that the oscillator generates a different local oscillator signal which causes the mixed signal to shift to the high band of frequencies.

Abstract: The divider is formed by a dividing counter 7 to which an input pulse train is fed via an OR gate 3. The counter 7 has a fixed division ratio and the OR gate is controlled by a logic circuit which receives control pulses and controls the OR gate to remove one pulse from the input pulse train for each control pulse in order to change the overall division ratio of the divider. The invention is particularly suitable as an adjustable ratio divider for use in frequency synthesizers.