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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. The sigma-delta modulator displaces any quantization noise in the ratio control signal away from the frequency of the phase control signal and multiplies thereof. A direct frequency synthesizer may be implemented by means of the same sigma-delta modulator and variable modulus divider, but arranged so that the reference frequency signal is applied to the input of the divider, which divides it directly to provide the desired output frequency.

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