Effect of an AGC amplifier on the performance of a generalised PLL in the presence of a large interfering signal

The influence of an interfering signal on the synchronisation capabilities of a generalised PLL receiver is analysed. The presence of an interfering signal has the effect of introducing parasitic-phase error and parasitic-phase modulation in the reconstructed carrier at the VCO output. When the interfering signal becomes large, synchronisation is lost. The limiting interference-to-carrier ratio and the frequency gap between the desired carrier and the undesired interferer, for which the PLL maintains a carrier lock, are specified. These limits are obtained for a generalised PLL configuration with an AGC amplifier preceding the loop. The results obtained are applied to three basic configurations, namely, short-, long- and split-loop receivers. A comparative study of these results, with similar loop configurations, without an AGC preceding the loop is also presented.