INTERFERENCE ASSESSMENT: COMPARISON OF REED TRACK CIRCUIT COMPUTER MODEL AND DFT BASED ANALYSIS

Abstract

Compatibility of electric traction vehicles with existing signaling systems is usually assessed by comparing a frequency domain representation of the line current of the vehicle with frequency domain limits. This frequency domain representation of the current is normally reached by applying a discrete Fourier transform (DFT) or a fast Fourier transform algorithm to a time interval of the current. The length of the time interval (window length) and the way in which the values in this time interval are weighted (window function) influence the frequency domain representation considerably and must therefore be selected in accordance with the signaling system to be protected from interference. This paper describes a model of a reed track circuit receiver based on a linear filter and its validation based on laboratory tests. It then compares the frequency domain representation of this filter to frequency domain representations of window functions used with DFT. From this comparison window length and window function, as well as a postprocessing algorithm for the DFT, results are selected. The aim of this selection is to obtain an assessment method based on DFT that comes close to the behavior of the filter and hence, to the behavior of the track circuit receiver. Transients are important when assessing compatibility. They can best be treated using time domain methods. Time domain comparisons of the output of a sliding DFT algorithm to the output of the linear filter demonstrate the suitability of the proposed algorithm to the assessment of transients.