An economical frequency synthesizer using interpolation techniques

In this paper, we propose an all-digital frequency synthesizer using interpolation techniques. Unlike the traditional direct digital synthesizer (DDS) which requires a large look-up table to store the phase and its corresponding sine wave, our new design contains only a linear feedback shift register (LFSR) and two banks of memory to store the two reference sequences. This can achieve a cost-efficient design. The novelty of this idea lies in that one can synthesize the desired frequency between these two reference frequencies through a threshold device. The frequency resolution of this synthesizer is determined by the order of the LFSR. The maximal spur around the synthesized frequency may be controlled by the numerical precision of these two reference frequencies. The percentage error of the synthesized frequency is identical to that of the system clock. This design is mainly used in implementing an economical digital modulator for continuous phase frequency shift keying (CPFSK) and other related wireless communication arts.