Variable Equalizers 



By H. W. BODE 



THE use of equalizing structures to compensate for the variation 

 in the phase and attenuation characteristics of transmission lines 

 and other pieces of apparatus is well known in the communication 

 art. Ordinarily, of course, an equalizer has a definite characteristic 

 fixed by the apparatus with which it is to be associated. It may 

 happen, however, that the characteristics demanded of the equalizer 

 cannot be prescribed in advance, either because the characteristics of 

 the associated apparatus are not known with sufficient precision, or 

 because they vary with time. Examples are found in the equalization 

 of transmission lines the exact lengths of which are unknown, or the 

 characteristics of which may be affected by changes in temperature and 

 humidity. 



In recent years the problem of providing equalizers which will meet 

 such conditions as these has assumed particular importance because 

 of the large variations in line attenuation which may result from 

 temperature changes in new carrier systems. In some of these the 

 maximum change in attenuation is more than 1 db per mile. Evi- 

 dently, if a reasonable standard of quality is to be maintained for 

 systems the overall length of which may be several hundred or several 

 thousand miles, these variations must be compensated for with great 

 accuracy. Moreover, since the total amount of correction must 

 necessarily be divided up into much smaller amounts appearing at 

 many points, and since the daily cycle of temperature changes may be 

 large, it is almost essential that the adjustments made be so simple 

 that they can readily be performed automatically by a suitable 

 auxiliary circuit. 



The variable equalizers described here are attempts to meet this 

 problem. In order to secure the maximum simplicity it has been 

 assumed that the characteristics of the structure are controlled by a 

 single variable element, which in most cases is a variable resistance. 

 It has also been assumed that the temperature coefficient is known as 

 a function of frequency and that it is the same at all temperatures. 

 What is required, then, is a structure by means of which an arbitrary 

 multiple of a given attenuation characteristic can be introduced into 

 a circuit by changes of a single element. 



229 



