Study On Track Circuit Signal Receiving Based On 32-bit Fixed Point DSP

At present 16-bit fixed point DSP and 32-bit floating point DSP are usually used in receiving railway signal. However, 16-bit fixed point DSP has poor computing ability and low speed, and 32-bit floating point DSP comprises complicated circuits which result in high cost. Based on the ZPW-2000 track circuit, this paper studied how to receive and demodulate the railway signal with 32-bit fixed point DSP TMS320F2812.In this paper, both the software and hardware of the minimum receiving system are first designed on the basis of theoretic analysis and simulation,and then the performance of the system is tested and examined. The main achievements of this paper are as follows:1. An algorithm of zooming spectrum called ZOOM-FFT is analyzed theoretically and simulated. It has been proved that ZOOM-FFT can greatly improve the frequency resolution and reduce the sampling rate without increasing sampling numbers, and at the same time meet the sampling theorem. By frequency spectrum simulation of windowed ZPW-2000 signal, the hamming window and hann window are proved to be able to effectively suppress the interference on spectrum caused by truncated signal. The two main problems in spectral analysis are resolved.2. The actual effect of digital filters and FFT analysis realized respectively by 32-bit floating point, 32-bit fixed point and 16-bit fixed point operation are compared by simulation. Accordingly the comments on the computing ability of 32-bit fixed point DSP are made that the coefficient quantization, rounding error and overflow of 32-bit fixed point operation have little effect on filters and FFT analysis compared to 32-bit floating point operation, and that 32-bit fixed point DSP has a better performance than 16-bit fixed point DSP.3. According to the experimental report of the subjective cab signal used on the railway between Dalian and Qinhuangdao and the calculation of harmonic current of multiple units done by Siemens, the requirement for anti-interference of receiving the railway signal is analyzed, and an in-band SNR of 2:1 is determined to be the anti-interference index of signal receivers.4. According to the minimum short circuit current of rails and the analog input voltage range of the AD converters on TMS320F2812, the dynamic range of the input signal is calculated and the scaling value of 32-bit fixed point operation is determined. 5. A minimum receiving module circuit containing two CPUs for comparing the decoding results is designed. The receiving circuit is simplified by realizing AD, decoding and output control with one single DSP.6. In the decoding experiment by hardware simulation, the decoding of frequency and amplitude indices is tested, the anti-interference ability is verified and the response time is estimated.During the study Matlab is used for the software simulation, while the development board, emulator and Code Composer Studio Version 2.0 are involved in the hardware simulation for testing and verification.It has been proved by the whole study that it is feasible to receive the railway signal with 32-bit fixed point DSP which has a higher operation precision than 16-bit fixed point DSP and a more simplified circuit, a lower cost and a lower power than 32-bit floating point DSP.