A DFT-based approach for precise tuning of a band-pass filter for the extraction of control signals from power lines

The authors discuss some of the theoretical considerations and practical problems associated with the measurement and precise tuning of a bandpass filter and introduce a discrete Fourier transform (DFT) based approach for such measurement. An effective filter tuning algorithm which requires testing at two frequency points is presented. The process is completely PC-based and software oriented and is fast and low-cost compared with the conventional analog instrument approach. In the algorithm, phase difference measurements of input and output are needed. The accuracy of the phase difference measurements under the worst conditions is estimated. Adaptation of the DFT for phase measurement together with other enhancements resulted in a reliable and robust test system. The phase measurement errors of the DFT-based testing method are analyzed in detail. Prototype test results and performance evaluation of the system are reported