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Abstract

A fractional-N frequency synthesizer for use in a transmitter for transmitting TDMA signals, comprises a fourth order sigma-delta modulator(16) having an input for digitized signals, a FIR filter(18) having 2 taps coupled to an output of the sigma-delta modulator, the FIR filter (18) serving to increase the number of states on its output by one over the number of states on its input which is connected to an output of the sigma-delta modulator, and a phase locked loop(PLL) including a frequency divider(20) with an incremental ratio of 0.5 and having a control input coupled to an output of the FIR filter. The frequency divider (20) by having an incremental ratio of 0.5 enables the PLL to have a reference oscillator(24) operating at half the sampling frequency of the sigma-delta modulator(16). The frequency synthesizer as a consequence has been found to give a 12 dB improvement in noise level and also continuous tuning can be achieved because the successive groups of three half ratios adjacent tuning ranges made possible by using overlap.

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