Electrical Communication is a free textbook on the basics of communication technology. See the editorial for more information.... 
Home Electric Networks Pads and Attenuators  


Pads and AttenuatorsIt is often desired in communication circuits to reduce the amplitude of a signal wave. For example, the program signal delivered over a telephone line may be transmitted at a power level sufficient to "override" line noise (Chapter 14). A pad, defined^{1} as "a nonadjustable transducer for reducing the amplitude
Notes: Values of the reflection loss for any two impedances Z_{x} and Z_{y} are here given as a function of the ratio
where A is the ratio of the magnitudes and θ is the difference between the angles of the two impedances. The ratio is always to be taken by dividing the larger impedance by the smaller, i.e., so that A will not be less than unity. It is immaterial whether θ is positive or negative. Negative values of reflection losses are reflection gains. Examples: (1) Find reflection loss for the impedances 200/60° and 700/30°
Reflection loss = 1.4 db. (2) Find reflection loss for the impedances 400/35° and 680/45°
Reflection loss = 1.8 db, (a gain) of a wave without introducing appreciable distortion," may be necessary at the receiving end of the line to reduce the signal volume to the desired amount. An attenuator, defined^{1} as an "adjustable transducer for reducing the amplitude of a wave without introducing appreciable distortion," is used in radio speechinput equipment to vary the volume of the program signal. Pads are made of fixed resistors and will function over a wide frequency range, determined by the highfrequency characteristics of the resistors. Pads are commonly made in the form shown in Fig. 22, particularly if they are to be used with balanced circuits. In the design of pads, the facts known are usually the impedances of the devices with which the pad is to be associated and the loss in decibels that the pad is to have. In accordance with the theory on page 156, the iterative impedance of the pad of Fig. 22 is
the letter R_{K} being used because the iterative impedance wall be resistance. The power loss in decibels introduced by the pad can be found from power, voltage, or current ratios. If a voltage E_{12} is impressed at the sending end, the input current will be I_{12} = E_{12}/R_{K}, because the pad is symmetrical and is terminated in R_{K}. The voltage across the junctions xy will be
The current through R_{K} will be E_{xy}/(0.5R_{1} + R_{K}), and this current multiplied by R_{K} will give the output voltage E_{34} across R_{K}. Thus, and
As an illustration of the use of this equation, suppose that a pad such as Fig. 22 is to introduce a loss of 10 decibels and must have an iterative impedance of 600 ohms. The voltage ratio E_{12}/E_{34} = 10^{0.05x10} = 3.162, and, from equation 56a, the value of R_{1} will be 622.5 ohms. With R_{K} and R_{1} both known, from equation 55 R_{2} = 422 ohms. Thus the pad of Fig. 22 should be composed of four 156ohm resistors, and one 422ohm resistor. Sometimes unsymmetrical taper pads^{6} having different image impedances are used between circuits of different iterative impedances. Attenuators are made of variable resistors, usually arranged so that they are varied by a common knob. An attenuator of the configuration shown in Fig. 22 is a symmetrical, balanced device. The various resistances that an attenuator must have can be calculated as just explained for each different attenuator setting in decibels. Or calculations can be made at several settings, and curves can be plotted from which the values at other settings can be obtained. An unbalanced pad commonly used is shown in Fig. 23a. This pad also can be designed as just explained. The Lsection attenuator of Fig. 23(b) is widely used in unbalanced circuits where the input and the output impedances offered by the attenuator need not remain fixed. This is an unsymmetrical device, but it is satisfactory for many purposes. Equations for the design of this attenuator can be derived in the same way as for the symmetrical pad.


Home Electric Networks Pads and Attenuators 