Phase-Locked Loops with Signal Injection for Increased Pull-In Range and Reduced Output Phase Jitter

Second-order phase-locked loops with signal injection (PLLI's) have orders of magnitude larger pull-in ranges than conventional second-order PLL's with identical natural frequencies and damping factors. The PLLI utilizes a little-known characteristic of the Van der Pol oscillator, and for practically no additional hardware the pullin range is increased sufficiently to eliminate in many cases the need for crystal control of the voltage-controlled oscillator (VCO). In nonlinear systems (e.g., a retiming chain of a digital repeater) where the in-phase and in-quadrature noise components are correlated, the PLLI in addition features a significant reduction in output phase jitter. A 44.6MHz PLLI, designed for timing recovery in a fiber optic repeater, has 8.4-kHz conventional minimum jitter bandwidth. To reduce time jitter accumulation in a chain of repeaters, the damping factor was increased to 4 resulting in a
Q
of 1300. The pull-in range of this loop is 9.6 MHz compared to 18 kHz for the same loop without signal injection. The static phase error is
\pm 0.6\deg
for ± 0.3 MHz oscillator drift. By injecting with 18° phase lag, the PLLI can take advantage of the correla tion between amplitude and phase noise components and the total output phase jitter is reduced by 10.5 dB due to destructive interference.