Fractional phase-locked loop frequency synthesizer

The fractional frequency synthesizer is similar to the divide-by-N phase-locked loop (PLL) (divider in feedback path). However, the output frequency of the voltage-controlled oscillator (VCO) is not restricted to integral multiples of the reference signal only. Rather, it can also be locked to the fractional multiples, with the result of a substantial reduction of the frequency tuning step in the useful bandwidth of the PLL without reduction of its pass band. Consequently, these frequency synthesizers have both high frequency resolution and short settling time, two essential requirements for modern applications, for example, in modern radio sets. The major difficulties are spurious signals generated in fractional divider system. In this paper, the new fractional PLL frequency synthesizer with Σ-Δ modulator is described. A complete fractional PLL was simulated, constructed and measured.