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.

The occurrence of cycle slipping in a Phaselock Loop (PLL) FM Demodulator is discussed. Single spikes occur in a first order loop as the modulation limit is exceeded with sinusoidal modulation. A second order loop, however, experiences complete break-up, commonly referred to as bursting, of the demodulated output when the modulation limit is exceeded. An explanation is provided for the phenomenon and the result is generalized to ann-th Order PLL.