Design of Filter Circuits for Impedance Matching of Wideband Transducers

Design of filter circuits for impedance matching of wideband transducers

ABSTRACT

Wide band piezo-composite transducer is a next generation transducer for under water application. The impedance matching on the interface between electro-acoustical transducer and electrical transmitter has been the most important subject to confirm the high transmitting efficiency. Because the impedance of a wide band transducer depends on signal frequency, it is difficult to design a matching network between power amplifier and the transducer for maximum power transfer. A novel method is proposed to attain the goal in which, impedance matching is achieved by designing a lumped network. The values of elements in the lumped network are optimized through number of experiments. This approach really develops an effective method for impedance matching design and reduced the time for trial and error. Moreover this method makes the implementation procedure less complex.

Keywords: Wide-band transducer, impedance matching, tuning, equivalent circuit

1. Introduction

Piezoelectric transducers are used for under water applications. As we know higher frequencies are more easily attenuated in water, so low frequency sound waves are used for under water mapping. Basically a transducer converts the electrical signal from power amplifier to acoustic signal, which propagates under water. A transducer is always operated at its resonance, where it has maximum amplitude of vibration. To have maximum conversion efficiency, both power amplifier and transducer should be electrically matched. In general, a tuning coil designed as an autotransformer is used for tuning as the matching circuit, at resonance frequency of the transducer. This type of matching can be done only for a single frequency and a different method is needed for wide band impedance matching.

2. Wide-band impedance matching

To transfer maximum power from the power amplifier to the transducer a matching network operated in a wideband should have the property as shown in Fig.1. That means while looking from the output of the power amplifier the network should have impedance of 270Ñ--, the output impedance of the power amplifier and looking from the transducer the network should have the conjugate impedance of the transducer.

3. Theory behind the network

Consider the T network shown in Fig.2. The mesh equations for this network is given by

V = I1(Z1/2+Z2) - I2Z2

0 = -I2Z2+I2 (Z1/2+Z2+Z0)

i.e., I1/I2 = (Z1/2+Z2+Z0)/Z2

Now introduce an important parameter for symmetrical T-network under Z0 termination such that

I1/I2 = eг

where г is called the propagation constant and is a complex number. Thus