860 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



been removed the process is complete and the equalization residue must 

 be composed solely of those terms not provided by the equalizer. 



The harmonic analyzer method requires tuning or switching and 

 thus to simplify the method still further the actual field equipment uses 

 a power indicator in place of the analyzer as noted on Fig. 14. Given 

 a spectrum of signals of differing frequencies, removing any one reduces 

 the total power. Therefore the entire spectrum may be applied to a 

 power indicator and the reading reduced by adjusting the various equa- 

 lizer controls. In practice this process is assisted by filtering out the 

 high harmonics which cannot be equalized. This reduces the total power 

 and increases the ease of reading the meter. While the method has been 

 demonstrated using an ordinary 60 cps wattmeter, an electronic watt- 

 meter is used in the field equipment. 



TIME 



Fig. 15 — Method of scanning the system gain characteristic to convert cosines 

 of frequency into cosines of time. 



While the receiver unit can be quite simple, the sweep oscillator is 

 complicated by such things as circuits to control the warping and to 

 hold the sweep limits accurately. In practice the sweep is between 0.3 

 and 8.5 mc and is at a 37 cps rate. Further there are six pilots on the 

 system and although the dynamic regulators at the adjusting point are 

 paralyzed during the cosine adjustment the pilots to intermediate regu- 

 lators must not be disturbed. Thus the sweep frequency is shifted very 

 rapidly through the pilots. When the sweep frequency gets within about 

 25 kc of a pilot it is shifted suddenly to the other side of the pilot fre- 

 quency. This materially reduces the interference to the pilot without 

 producing transients in the receiver. 



While other methods of cosine equalizer adjustment were tested and 

 found to work satisfactorily, the above method was found to be superior. 

 In addition, removal of the filtering permits the power meter to read 

 the rms equalization error and thus the equalization operator can deter- 

 mine the quaUty of the job and observe whether the state of the line is 



