This paper discusses the capture bandwidth of the phase-locked loop using the phase plane method. The phase-locked loop consists of sinusoidal phase comparator

nonlinear ratio-plus-integral filter

and voltage-controlled oscillator （VCO）. In the discussion

the practical saturating nonlinearity of the VCO was considered. From this work was derived the general relation between the capture bandwidth and the coefficient of non-linearity of the filter

and that of time constant of the filter and the coefficient of non-linearity of the VCO. Its several special cases were discussed

and experimental results given. These results accord with the theoretical analysis fundamentally.