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Methods are presented for determining the modulation limits of phase lock loop (PLL) FM demodulators. These limits establish the maximum deviation that the PLL can support (i.e., remain "inlock") based on the loop design parameters. The modulation limits are derived from experimental data on first and second order loops using sinusoidal and Gaussian noise modulation. The analysis of each PLL is broken into two regions, low frequency and high frequency modulation. It is shown that the deviation limit remains constant with modulation frequency for a first order loop in the low frequency region. In the high frequency region the deviation limit increases with increasing modulation frequency for both first and second order loops. The deviation limit in a second order loop increases with decreasing modulation frequency in the low frequency region and is a function of the loop damping.