Design Of The UWB Pulse Generator In The System Of Dangerous Goods Detection

With the increasing urban population, public transport system develops gradually. However, a series of incidents including explosions and hacking of buses occurred, bringing huge losses to people's lives and property. Therefore, it is of great necessity to detect inflammable and explosive materials, metal knives and other dangerous goods of passengers and luggage on buses, which is significant to avoid potential safety problems. In recent years, Ultra-wideband radar technology has made rapid development. With its low power consumption, penetration, high detection precision, interference, and the relatively simple structure, etc., Ultra-wideband technology can well meet the requirements of the bus system to detect dangerous goods. In all applications of Ultra-wideband technology, one of the most main parts is the narrow pulse signal generator, on which this paper will make a thorough analysis.Firstly, this paper explains the background and significance of dangerous goods detection on buses and makes a distinction of the operating principle and the advantages and disadvantages among the current mainstream of X-ray imaging, metal detectors, gas analysis and other testing methods. It describes the superiority of ultra-wideband technology applied in dangerous goods testing on buses.Secondly, this paper introduces four different stages of ultra-wideband technology and the development process of narrow pulse generated technology. It also introduces the definition, time-frequency characteristics, technological advantages and applications of UWB. It shows three ways to achieve Ultra-wideband technology, illustrating the applicability of IR-UWB in the dangerous goods detection system. It lays stress on the requirements of FCC(Federal Communications Commission) on emission mask for various types of Ultra-wideband devices, providing guidelines for the narrow pulse waveform design subsequently. Besides, the present study shows the overall structure of Ultra-wideband system, including the expected performance indicators of narrow pulse signal source system.Thirdly, this paper makes a detailed study and design on the Ultra-wideband narrow pulse waveform. Based on the thought from time-domain to frequency-domain, the study focuses on the time-domain and frequency-domain characteristics of the Gaussian pulse and its n-order derivative, sinusoidal modulation Gaussian narrow pulse, sinusoidal modulation triangular narrow pulse, and Morlet wavelet pulse. Using Matlab, the present study introduces the effect of pulse shape factor on the pulse's time-domain waveform and frequency-domain, as well as the match situation of the above pulse's power spectral density on the FCC spectral mask requirements. Based on the thought of frequency-domain to time-domain, the study applies direct frequency domain method to investigate waveform characteristics of the raised cosine pulse, and finally select the first derivative of Gaussian pulse as the transmit pulse in the dangerous goods detection system.This paper also stresses on the narrow pulse generation techniques, including the tunnel diode, the step recovery diodes, the avalanche transistors and the logic gate's competitive-adventure phenomenon. Combining basic generating circuit, this study introduces the working principle of the above types of technology in details. Narrow pulse circuit based on a single avalanche transistor BJT-NPNBFS17 is designed by Multisim. The circuit is optimized by connecting the boost inductor, as a result, the pulse amplitude is raised by 16.2 V to 24.5 V. The present study makes a distinction between the pros and cons of RC differentiator and LC oscillation mode. At last, we obtain the expected pulse with the width of 1.7 ns and the magnitude of 22.5 V through LC oscillator mode.The present study solves the problem of mutual restraint between the width and amplitude of UWB pulse. To further reduce the width and increase the amplitude, we draws the method of Marx generator "parallel charging, discharging tandem". A narrow pulse generating circuit is designed, based on four avalanche transistor in the present study. The present study simulated and tested the impact on the narrow pulse when different values are taken based on bias voltage, storage capacitors and collector resistance and other component parameters. Finally, the optimum parameters are obtained, and a bipolar first differential Gaussian pulse with PRF of 10 MHz, width of 643 ps and the amplitude of 40.7 V is gained. The pulse performance is expected, and the circuit is relatively simple, which is suitable in UWB dangerous goods detection system. This paper makes a research on UWB pulse waveform and designs the narrow pulse generator, which also has theoretical and practical significance on other UWB devices.