New fractional phase-locked loop frequency synthesizer using a sigma-delta modulator

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 only integral multiples of the reference signal. 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 the fractional divider system. The new fractional PLL frequency synthesizer with ΣΔ modulator is described. A complete fractional PLL was simulated and partly constructed and measured.