Pulse Interference Effects in a Phase-Lock Loop

The ability of a phase-lock loop (PLL) demodulator to acquire and remain locked on a desired signal in the presence of receiver noise and pulse interference such as that generated by radar equipments was determined by a measurement program. The measurements were conducted over a wide range of PLL operating conditions for a typical space instrumentation receiver. The PLL and interference signal parameters varied during the test included the desired signal-to-noise ratio, the signal-to-interference ratio, the off-set of the pulse carrier frequency from the desired signal, the pulse width, and pulse repetition rate of the interference signal and receiver tracking bandwidth. The behavior of a phase-lock loop during normal signal reception, signal fade and signal acquisition in the presence of pulse interference signals was examined by repeated measurements of loop performance for the selected test conditions. The input voltage to the voltage controlled oscillator (VCO) was instrumented to record oscillator sweeping, signal or interference acquisition, and loss of lock. The measurements indicated the range of pulse interference characteristics which could cause false lock or unlock problems in the PLL. In general, the effects of the interference were found to be strongly related to the simulated radar duty cycle, and nominally influenced by the amplitude, frequency separation between desired and undesired signals as well as the operating signal-to-noise ratio.