| Ultrasound is getting more and more attention for it’s unique characteristics. It has played an important role in many fields such as communication, energy and medical treatment.Generally speaking, ultrasound system has several components including transmitter system, transducer or transducer array, data acquisition system and signal processing system. As an important part of transmitter system and data acquisition system, impedance matching network is a bridge connecting the transducer and electrical modules. In transmitter system, the impedance matching network is designed for improving effective power sent to transducer from the power souce and avoiding the damage of components of source caused by reflected pulse.Most traditional ultrasound devices are narrowband which can only work within a narrow range near resonant frequency. The topology of the impedance matching networks of these devices is simple. As the development of ultrasound technology, wideband ultrasound or multiband ultrasound is needed in some applications. A simple matching network cannot meet these needs anymore. Complicated calculating process is always needed before a satisfactory result is brought to and the final structure of matching networks becomes more and more complicated.Through the history of the development of the broadband matching network designing method, it is mainly used for designing broadband impedance matching networks of microwave and radio systems. Although the working frequency is low and the bandwidth is narrow for most of ultrasound systems, it is still not a simple mission to design impedance matching networks for them. The impedance and application environment of a broadband ultrasound transducer are usually different from those of a microwave system, and the performance of the matching networks will not be satisfactory without considering the difference.The main goal of this dissertation is to study the impedance matching network that will be used in the broadband ultrasound systems. Theory and experiment are all included. In theory part, beginning from basic knowledge of broadband impedance matching, methods that are widely applied to the engineering practice and the results of different methods are discussed. Based on the analysis of characteristics and shortcomings of broadly used methods A new method based on genetic algorithm is developed, which combines the advantage of real frequency method and direct optimization method. Then, some deficiencies of current methods are improved by the genetic algorithm. Finally, a impedance matching network for a multi-transducers system is designed and results of different methods are compared.In chapter one, the reasons for selecting the subject and some background knowledge are introduced, including the architecture of typical ultrasound systems, principle of piezoelectric ultrasound transducer and impedance matching network, development and current situation of designing method of broadband impedance matching network.In chapter two, some mathematical concepts that will be used in this dissertation are discussed.In chapter three, two classes of methods of designing broadband impedance matching network are discussed. First, three popular analytic theory are introduced. Second, three computer aided designing methods,including real frequency method, Smith chart and direct optimization method, are introduced. The characteristic the CAD methods have in common is that the topology of the load is not necessary. Finally, global optimization algorithms are introduced in direct optimization method. As examples, simulated annealing algorithm and particle swarm optimizer are used for designing and the results of different methods are compared.In chapter four, a new method based on genetic algorithm is introduced. The basic feature of this method is that both the topology and parameters if components can be optimized.In chapter five, a real impedance matching network for a multi-transducer ultrasound logging system is designed by different methods and results of simulation are compared. To prove the effectiveness of the network, actual circuits are made and tested.At last, in chapter six, a summary and prospect are made. |
