Parameter Optimization for Rise Time of Sub nanosecond Pulser Based on Avalanche Transistors Ming-xiang Gao, Yan-zhao Xie, Ya-han Hu Xi an Jiaotong University 2017/05/08
Contents 1 Introduction 2 Principles and design of pulser 3 The limiting factors of rise time 4 Development of a bipolar pulser 5 Conclusions
1 Introduction Sub nanosecond pulser is widely used: UWB radar EMC test Bioelectromagnetism Technical points of sub nanosecond pulser: Pulse rise time bandwidth of EM radiation Pulse amplitude Power of EM radiation High repetition frequency application requirements
2 Principles and design of pulser The main principles of the sub nanosecond pulser based on avalanche transistors can be summarized as follows: 1. The avalanche effect of the transistor; 2. The Marx circuit; 3. The characteristic impedance matching of pulse wave propagation.
2 Principles and design of pulser 1.The avalanche effect of the transistor The avalanche effect let the current flowing through the collector "avalanche" growth. This principle directly determines the short rise time of pulse. 2.The Marx circuit The capacitors are parallel connected during the charging process. The avalanche breakdown of transistors makes the capacitors connected in series and discharge on the load.
2 Principles and design of pulser 3.The characteristic impedance matching It is necessary to consider the problem of characteristic impedance matching for sub nanosecond pulser. (The wave propagation of pulse forming) The microstrip line and transmission cable should be selected with the characteristic impedance of 50 Ω. The packaging of circuit devices should be surface mount device (SMD).
2 Principles and design of pulser The typical parameters of the developed pulser: Load: 50 Ω Rise time: 180ps Amplitude: 3.5kV (48 stages Marx circuit) Repetition frequency: 50 khz Prototype of pulser
3 The limiting factors of rise time The rise time of the pulse is the key performance of the pulser. The main limiting factors of rise time are summarized. 1. Switching device characteristic frequency f T 2. Parameters of Marx circuit The capacitor in each stage mainly affects the width of the pulse. With the capacitor increasing, the transistor will keep longer discharge duration, which maintain the charge growth with a result of longer rise time.
3 The limiting factors of rise time 3. PCB design A compact circuit arrangement can improve the synchronization of transistors, while reducing the charge loss. The microstrip structures should be designed with the characteristic impedance of 50 Ω. 4. Trigger signal a fast pulse signal or square wave signal with the amplitude of 5 V or more. It has been confirmed that a long rise time of the trigger signal will cause longer rise time and instability of the output pulses.
4 Development of a bipolar pulser Using a bipolar pulse is a feasible method to improve the subnanosecond pulse bandwidth, especially to improve high frequency band. Bipolar pulses could be generated in two ways: Short circuit method is to parallel connect a short circuit cable at the end of the unipolar pulser, thereby an opposite polarity pulse can be superimposed on the original pulse. Power combiner method is to superimpose positive and negative pulses by a RF power combiner with a certain time interval.
4 Development of a bipolar pulser Thesecondmethodisappliedinthispaper. The negative pulse can be easily generated by changing the circuit structure (Exchange the transistor connection point). It is possible to adjust the time interval between positive pulse and negative pulse by extending the transmission cable.
4 Development of a bipolar pulser In order to compare the radiation spectrums generated by unipolar and bipolar pulses, a small impulse radiating antenna (IRA) with a diameter of 40cm is developed as the radiation antenna. Spectrums of the E field under unipolar pulse Spectrums of the E field under bipolar pulse Obviously, the spectral bandwidth of the radiation field under bipolar pulse excitation is significantly wider, especially the spectral components above 1 GHz.
5 Conclusions This paper describes commonly used sub nanosecond pulse source based on avalanche transistors and highlighted the importance of considering characteristic impedance matching during pulse forming and propagation. The main limiting factors of pulse rise time are summarized, including the characteristics of avalanche transistor, the parameters of Marx circuit device, the design and layout of circuit board structure and the condition of trigger signal. Finally, the developed bipolar pulser is introduced, which can effectively improve the bandwidth of the radiation signal, especially the high frequency component.
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