Fast frequency switching synthesizer

Claim 1

A frequency synthesizer comprising: a phase locked loop including a voltage-controlled oscillator, a high frequency divider, a phase frequency comparator and a low-pass filter; a divisor controller circuit for supplying a divisor to the high frequency divider; and means for controlling the phase-locked loop so that a loop bandwidth of the phase locked loop is widened when an output frequency of the voltage-controlled oscillator is switched, and is narrowed when the switched output frequency is substantially stabilized after passing of a predetermined period, wherein the divisor controller circuit includes an accumulator having a phase adder and a phase register, wherein when the output frequency of the voltage-controlled oscillator is switched, the phase register is initialized, the phase adder adds its input data and the output of the phase register to make sum data that is given back to the phase register, the phase register inputs the data at the same time as changing of an output signal of the high frequency divider, the divisor controller circuit enlarges the divisor cyclically by incrementing the divisor only when the accumulator overflows, the high frequency divider divides the output signal of the voltage-controlled oscillator in accordance with the divisor, the phase frequency comparator generates a phase difference between an output signal of the high frequency divider and a reference signal, the phase difference signal is filtered by the low-pass filter and given to the voltage-controlled oscillator, which switches the output frequency to be equal to a product of the reference frequency and an average divisor of the high frequency divider, and wherein when the switched output frequency is substantially stabilized after passing of a predetermined period, the loop bandwidth is narrowed at the timing that the phase register becomes a predetermined value.