HIGH-SPEED FREQUENCY-SWITCHING SYNTHESIZER USING FRACTIONAL N-PHASE-LOCKED LOOP

A fractional N phase-locked loop frequency synthesizer has been develo ped for small-size and low-power applications. It has fast frequency s witching and low-phase noise. The fractional N-frequency synthesizer r ealizes fast switching by the high-frequency reference signal and by w idening the loop bandwidth. However, when the bandwidth was widened, d eterioration of phase noise is observed. Then the loop bandwidth which is necessary for fast frequency switching and which gives low-phase n oise performance is determined. Based on these conditions, loop filter switching was studied which realized fast switching by widening the l oop bandwidth and the low-phase noise performance by making the loop b andwidth narrow after the frequency switching. In the conventional fil ter switching, VCO control voltage fluctuates at the time of switching and the frequency switching time becomes longer. A new method is prop osed 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 noi se of -120 dBc/Hz at 50 kHz offset from carrier and spurious level of -83 dBc were obtained.