1442 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



One model of the equalizer was completely equipped with these rheo- 

 stats. By allowing for the equivalent length of cable within the rheostat, 

 an essentially pure resistive termination was obtained. 



IX. OTHER COMPONENTS 



Other components reciuired for the equalizer included stepped rheo- 

 stats, delay line or delay networks, a suitable summing network, and a 

 loss equalizer. 



The stepped-switch rheostats were made from standard switch parts 

 with eleven positions. Deposited carbon resistors, 205D, were used for 

 the steps. The mid-position corresponded to the circuit impedance level, 

 75 ohms. The other steps were arranged to provide equal increments of 

 echo amplitude measured in decibels. With careful control of lead lengths, 

 special shielding and a coaxial cable connector, satisfactory control of 

 the return loss of this rheostat was obtained. 



Resistance pads were added to the switch assemblies associated with 

 each of the echo terms. The loss of each pad was determined so that the 

 corresponding term would have the desired maximum amplitude. In 

 addition, the losses of the pads associated with the leading echo terms 

 were increased to compensate for the midband loss of the delay line be- 

 tween the leading coupler and the corresponding lagging coupler. This 

 insured that the two echos would have equal amplitudes. 



The delay required between taps in the delay line is 0.025 microsecond, 

 corresponding to a change in phase shift of 180° from 60 to 80 mc. In 

 order for the equalizer cosine characteristics to have maxima at the band 

 edges, the total phase shift at 60 and 80 mc must be successive integral 

 multiples of 180°. Since the phase shift of coaxial patch cable is closely 

 linear and proportional to length, it could be used for the delay line. 

 Lumped-element delay networks consisting of two or more all-pass sec- 

 tions are a feasible alternative and would reduce the over-all size and 

 weight. In view of the additional development effort involved to produce 

 these and the experimental nature of this equalizer, it was decided to 

 use coaxial patch cord. The type selected, 728A cable, has a polyethylene 

 dielectric and is tested during production for return loss in the 50- to 95- 

 mc band. The length reciuired for each section is about 15.6 feet. This 

 much cable has a loss of about 0.3 db at 70 mc. 



It was originally proposed to use a series of directional couplers for 

 summing the echo voltages with the main signal. This would provide 

 additional isolation between terms. However, tests on a preliminary 

 model indicated this isolation was not required in this application. In- 



