Abstract: An indirect frequency synthesizer suitable for use in a cellular radio system has finer resolution and reduced spurious frequencies and/or phase noise, which facilitates a large number of channels in a given bandwidth. The synthesizer comprises a phase detector responsive to a reference signal and a phase control signal for generating a control signal that varies in dependence upon the phase difference between the reference signal and the phase control signal. A voltage controlled oscillator responsive to the control signal generates an output signal whose frequency varies in dependence upon the control signal. A variable modulus divider divides the output signal to provide the phase control signal. The variable modulus divider varies its division ratio in dependence upon a ratio control signal derived by a second or higher order sigma-delta modulator in response to a frequency control signal .delta..phi. and to the phase control signal.