Attenuators

At the end of this chapter you should be able to:

• understand the function of an attenuator • understand characteristic impedance and calculate for given values

• appreciate and calculate logarithmic ratios • design symmetrical T and symmetrical π attenuators given required attenuation and characteristic impedance

• appreciate and calculate insertion loss • determine iterative and image impedances for asymmetrical T and π networks

• appreciate and design the L-section attenuator • calculate attenuation for two-port networks in cascade

• understand and applyABCDparameters for networks

41.1 Introduction

An attenuator is a device for introducing a specified loss between a signal source and amatched loadwithout upsetting the impedance relationship necessary for matching. The loss introduced is constant irrespective of frequency; since reactive elements (L or C) vary with frequency, it follows that ideal attenuators are networks containing pure resistances. A fixed attenuator section is usually known as a ‘pad’. Attenuation is a reduction in the magnitude of a volt-

age or current due to its transmission over a line or through an attenuator. Any degree of attenuation may be achieved with an attenuator by suitable choice of resistance values but the input and output impedances of the pad must be such that the impedance conditions existing in the circuit into which it is connected are not disturbed. Thus an attenuator must provide the correct input and output impedances as well as providing the required attenuation. Attenuation sections are made up of resistances

connected as T or π arrangements (as introduced in Chapter 34). Two-port networks

Networks in which electrical energy is fed in at one pair of terminals and taken out at a second pair of terminals are called two-port networks. Thus an attenuator is a twoport network, as are transmission lines, transformers and electronic amplifiers. The network between the input port and the output port is a transmission network for which a known relationship exists between the input and output currents and voltages. If a network contains only passive circuit elements, such as in an attenuator, the network is said to be passive; if a network contains a source of e.m.f., such as in an electronic amplifier, the network is said to

Figure 41.1(a) shows a T-network, which is termed symmetrical if ZA =ZB and Figure 41.1(b) shows a π-network which is symmetrical if ZE =ZF . If ZA =ZB in Figure 41.1(a) and ZE =ZF in Figure 41.1(b), the sections are termed asymmetrical. Both networks shown have one common terminal, which may be earthed, and are therefore said to be unbalanced. The balanced form of the T-network is shown in Figure 41.2(a) and the balanced form of the π-network is shown in Figure 41.2(b).