Effects of interfering signals in phase locked loops

Advantages concerning the use of phase-locked loops (PLL) are related to the ability of the PLL to separate small signals from large noises with good signal to noise ratios. PLL have, however, the drawback that a periodically interfering signal at the input of the PLL can sometimes fall out of the lock. An analysis is conducted of the stability in first order PLL. It is shown that this analysis can also be used for second order PLL under the assumption that the wanted signal and the interfering signal have a small frequency difference. A stabilization effect related to the interfering signal is considered. If a PLL is unlocked to the wanted signal, it can be locked by an interfering signal. To achieve this effect, particular frequencies and amplitudes of the interfering signals must be selected.