(A) study on the performance of N-phase digital tanlock loop for suppressed carrier tracking

Abstract

In this thesis work, the performance of the N-phase digital tanlock loop(DTL) in the presence of phase error disturbance is studied. The N-phase DTL is a digital loop for tracking an N-array phase-shifted suppressed carrier. It has linear phase characteristics in the modulo-2π/N sense as a result of using the $\tan^{-1}[ㆍ]$ function in the phase error detector. In analysis of the loop performance, we assume that the output of the phase error detector is disturbed by Gaussian channel noise and modulation distortion due to the finite bandwidths of the band-pass filters (BPF``s) in the loop. In the presence of phase error disturbance, neglecting quantization effects, we study the statistical characteristics of phase error of the phase detector output and investigate the effects of various system parameters on the system performance. First, we calculate the probability density function of phase error of the phase detector output and verify it by computer simulation. Second, we find the optimum bandwidths of the BPF``s in the loop that yield the minimum variance of the phase error. Third, the performance of the N-phase DTL is compared with that of the digital N-phase I-Q loop which has sinusodial phase characteristics. It has been found that the first order N-phase DTL has wider lock range than the first-order N-phase I-Q loop in the absence of noise, and simpler to implement. Finally, the finite word length effect on the performance of the N-phase DTL is investigated by computer simulation. From the simulation result it has been found that at least 10 bits of word length are required to have satisfactory performance when the number of phases of the phase-shifted signal is 8.