REAL GENERATORS 



switches. We shall continue to discuss the Tform, but it must be remembered 

 that these remarks apply equally to the pi configuration, also to the H and O 

 forms, which are only trivial modifications of the T and pi. 

 The of a stepped attenuator may be varied by : 



(1) changing the resistances in use for three new ones {Figure 2.33); 



(2) using tapped resistances, which is sometimes more economical (Figure 

 2.34); 



(3) having a varying number of fitted T's in cascade (Figure 2.35). Clearly 

 if a number of attenuator 'sections' are designed to work between a certain 

 generator and load, i.e. have the same characteristic resistance, then they can 

 be connected together to form a chain and their 0's will be multipUcative. 



(4) Having a fijced number of varying T's in cascade (Figure 2.36). Here 

 again, the overall d is the product of the 0's for the several sections. 



The decibel (abbreviation: dB) — The transmission factor, d, was intro- 

 duced because it enables us to compute the resistance values required for an 

 attenuator section. In dealing with a number of sections, however, there is 

 a much more convenient unit — the decibel. 



Suppose we have a collection of attenuator sections, all of the same charac- 

 teristic resistance, having d values of 0-1, 0-2, 0-3 .. . 0-9, and suppose we 

 have to make up an attenuator of overall d = 0-126. It takes a little time to 

 see that the way to do this is to use the 0-2, 0-7 and 0-9 sections, whereas no 

 one has any difficulty in seeing which coins from a handful of loose change 

 go to make, say, 6/4d. It is, of course, easier to add than to find factors. 

 Thus, if for our attenuator sections we can find a unit which expresses the 

 input-output ratio and which is additive when sections are connected in 

 cascade, then the calculations become very easy. 



The Bel is defined as 



logio y 



where P^ and /*2 are two powers. 

 Thus the Bel can be used to compare powers flowing in the same part of a 

 circuit at different times — an important application in connection with 

 filters — or to compare powers in two different circuits at the same time, which 

 is the application relevant to attenuators. 



R(^r) 



Figure 2.37 



The Bel is rather a large unit, and a more popular one which avoids frac- 

 tions and decimal points is the dB, which is 1/lOth of a Bel. One dB is of 

 physiological interest, in that an increase of one dB in the power supplied 

 to a telephone receiver or loudspeaker is about the smallest increment that 

 can be detected by the ear. 



Figure 2.37 shows a generator matched to a load with a single section T 



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