Spur reduction in wideband PLLs by random positioning of charge pump current pulses

Charge pump PLL is prone to reference spurs due to non-idealities like feedthrough, charge pump current mismatch and loop filter leakage. To resolve the problem of reference spurs, a randomization technique that converts the spurs to wideband noise is proposed. The added wideband noise has insignificant contribution to the output phase noise inside the PLL bandwidth. Analytical expressions are derived to study the effect of the randomization on the output phase noise. A 1 GHz output frequency PLL with a bandwidth of 1 MHz and a 20 MHz reference frequency is simulated to test the idea. From the simulated results we could see a spur reduction of 20 dB in the nominal case. The settling time of the simulated PLL measured for a 25.2 ppm settling error is 4.5 μs.