TRACKING PERFORMANCE OF A PHASE LOCKED LOOP WITH A LINEAR PHASE DETECTOR

Abstract

In this paper the performance of a phase locked loop with a linear phase detector is analyzed in terms of its inherent capability to establish a coherent phase reference of phase modulated signals. After introducing the general analysis for arbitrary modulating waveforms, the case of bi-phase modulation of the carrier is investigated in detail. A comparison performed with a "standard" phase locked loop with a product type of phase detector shows that the "linear" phase locked loop may realise up to 5-dB improvement in tracking performance of bi-phase modulated carriers when the modulation index is ≤ 1.25 radian. A comparison with data-aided and squaring loops (in case of suppressed carrier modulation) shows that the latter types are superior with respect to their tracking performance. However it is considered that the capability, inherent to the "linear" phase locked loop, to establish a coherent phase reference signed for a wide class of phase modulated signals, such as multi-level PM, combined AM and PM and multiplexed bi-phase channels, is an attractive feature, which can be utilized for coherent demodulation of the same class of modulated signals.