High-speed frequency-switching synthesizer using fractional N phase-locked loop

A fractional N phase-locked loop frequency synthesizer has been developed for small-size and low-power applications. It has fast frequency switching and low-phase noise. The fractional N-frequency synthesizer realizes fast switching by the high-frequency reference signal and by widening the loop bandwidth. However, when the bandwidth was widened, deterioration of phase noise is observed. Then the loop bandwidth which is necessary for fast frequency switching and which gives low-phase noise performance is determined. Based on these conditions, loop filter switching was studied which realized fast switching by widening the loop bandwidth and the low-phase noise performance by making the loop bandwidth narrow after the frequency switching. In the conventional filter switching, VCO control voltage fluctuates at the time of switching and the frequency switching time becomes longer. A new method is proposed which gives small switching fluctuation and which can shorten the frequency switching time. Finally, the synthesizer circuit in the 900-MHz band was fabricated and the lock-up time of 1.3 ms, the phase noise of -120 dBc/Hz at 50 kHz offset from carrier and spurious level of -83 dBc were obtained.