Railway Frequency-shift Signal Detection Based On Fpga Technology Research And Implementation

Railway transportation exerts an important role in the economic construction of China, so that the security and reliability of railway traffic and inspect the parameter of track FSK signal accurately become important questions to discuss. The frequency-shift signal in the railway track reflects the speed and status of the running train, and accurate detection of FSK signal plays a key role in safe running of the train. FSK is the abbreviation of frequency-shift keying. There are two main kinds of standard in China’s railway track circuits:self-developed18information FSK standard and introduced UM-71track circuit system, which are based on the frequency to transfer controlling signal. With the highly development of railway transportation, traditional detection of FSK signal can not satisfy the requirements of increasing speed. Therefore, innovative solutions are needed to explore for detecting FSK signals.Based on some relevant studies and analysis of existing demodulating methods, this paper proposed a comprehensive solution which is based on FPGA and AVR single chip machine. By comparing the characteristic of two traditional detecting methods for FSK signal, including time domain and frequency domain, the measuring width in time domain is given and according to the uninterrupted count and multi-period measurement technology, the solution based on FPGA is raised for frequency measurement. The parameter of frequency to be detected includes:carrier frequency、upper side frequency、down side frequency、the base frequency even the single frequency signal.FSK signal is pre-processed through the programmable band-pass filter of the channel circuit and via the interface to FPGA, which starts to measure by counting when the SCM requires. Gate time is set by the carrier frequency’s cycle and compare the count difference between the upper、down side frequency and the carrier frequency, so that we can distinguish the two side frequency by the different positions of borrow pulse, and recover the base frequency signal. For the purpose of reducing the error and improving the accuracy, synchronous multi-period measurement is applied in a period of the base frequency to count, respectively for the upper、down side frequency, and then the data is uploaded to SCM by SPI, which is as the master to calculate the actual value by the count value.The detection based on time domain, with the feature of simple structure and high measurement accuracy, can satisfy the requirement of measurement under the condition of simulation test in the lab. Finally, the error of measurement for the side frequency and base frequency of FSK signal are within±0.3Hz, and for the single frequency is within±0.2Hz.