Analysis of error propagation and design of filter cutoff frequency in a quantized feedback dc‐restoration system

Abstract

AbstractIn the regenerative repeater in digital transmission, the low‐frequency component of the transmitted pulse is cut off by the coupling circuits at the input and the output. However, by applying the quantized feedback dc‐restoration circuit in the repeater the digital regenerative transmission with low dc‐balance transmission code can be achieved. In the quantized feedback system, the component lost in the low‐frequency cutoff characteristics of the transmission system is restored by feeding back the low‐frequency component of the pulse regenerated in the repeater to the decision circuit input. Because of this mechanism, however, an error propagation occurs where an error component is fed back to produce a distortion and to enlarge the error, when a decision error is produced in the repeater. This paper derives the state‐transition diagram which approximates the error propagation. By the analysis of the state‐transition diagram, the relation between the filter cutoff frequency and the error propagation characteristics is determined. Furthermore, it is shown by a simulation experiment that the result of the analysis is a good approximation to the error propagation. Based on these results, a design method is presented for the cutoff frequencies of the forward and the feedback low‐pass filters in the quantized feedback system.