This paper presents the results of an investigation concerning the performance degradation of a phase-locked loop (PLL) due to continuous wave (CW) interference and additive white Gaussian noise. The performance measures evaluated include the probability density function (pdf) of phase error and the cycleslipping rate. The measures are characterized in terms of the ratio of interference to the desired signal power and the loop signal-to-noise ratio. Since the interference term in the loop phase error signal is periodic, the conditional pdf of the phase error process is cyclostationary. A Fourier series expansion is applied as an approach to obtain a solution to the cyclostationary process. Linear performance results are derived and compared to the analytic results.