Investigation of non-linear operation of PLL FM demodulators

A standard PLL FM demodulator has been subjected to a range of input signals intended to induce non-linear behaviour. The non-linear thresholds and circuit responses in terms of phase error histories, Poincare sections, phase-plane portraits, VCO spectra and bifurcation diagrams have been recorded and compared with predicted results. These comparisons have proven to be most convincing and help to establish the nature of the circuit response to a particular modulating waveform. An optimum waveform has been derived which is capable of causing a non-linear response (and associated chaotic windows) with minimum peak deviation. One of the implications for the design of PLL FM demodulators is that a linear design of a stable PLL FM demodulator will not guarantee that all waveforms within the receiver IF bandwidth will be successfully demodulated. A short trial involving an INMARSAT terminal has revealed very similar responses to those observed in the test circuit and so the many results obtained in this work should be applicable to commercial equipment based on similar PLL FM demodulators