This paper introduces the extended ΔΦmethod for measuring cycle-to-cycle period jitter in PLL outputs. The theoretical basis for this method is derived from the limited condition for the average period and analytic signal theory. Sinusoidal jitter measurements verify the relationship between cycle-to-cycle period jitter and timing jitter. To validate the method, experimental data from jitter measurements on a PowerPCTM microprocessor is analyzed in the frequency domain. Comparisons of phase quantization errors are made between the extended ΔΦmethod and the conventional zero-crossing method.