THE L3 SYSTEM — EQUALIZATION AND REGULATION 



859 



phase-frequency relationship. The sweep oscillator therefore scans at a 

 linear rate in cosine degrees vs time. Upon reaching 8.5 mc the sweep 

 reverses and returns to zero. If the cosine shapes were linear cosines of 

 frequency the sweep would be a triangular wave. 



Assume that the line is perfectly equalized except that the first har- 

 monic term is misadjusted. As the sweep goes from to 8.5 mc the voltage 

 at x-x follows the first harmonic curve of Fig. 16 from to ^i . When the 

 oscillator scans back to the voltage at x-x follows the first harmonic 

 curve from ^i to 2 'i . Then the cycle repeats. The voltage at x-x thus 

 becomes a pure cosine of time oscillation of frequency, }4ti . Although 

 the equalizer shapes are actually cosine on a decibel rather than an 

 amplitude basis this has little practical effect because for small deviations 

 the two are nearly identical. 



If instead of a first harmonic error the second or third cosine harmonic 



Fig. 14 — Block diagram showing the method used to adjust cosine or other 

 orthogonal equalizers. 



is misadjusted, the voltage at x-x will follow the appropriate curve of 

 Fig. 16. The dc component measures the zero harmonic but in practice 

 only the ac components are measured and the flat gain is set as a final 

 step to make the pilot levels correct at the line output. If it takes 0.01 

 seconds for the oscillator to scan up and back, the first harmonic pro- 

 duces 100 cps output from the detector. The second harmonic equaUzer 

 produces 200 cps, the third 300 cps, etc. Therefore at the output of the 

 detector there exist a set of audio frequency harmonics whose amplitudes 

 are a measure of the equalization error of the setting of the cosine con- 

 trols of corresponding periodicity. 



These harmonics can be separated by convention filtering techniques, 

 for example, by an audio tuned detector or harmonic analyzer as noted 

 on Fig. 14. The analyzer can be tuned to 100 cps and the first harmonic 

 control rotated to remove the 100 cps component. Then tuning to 200 

 cps the second control is operated, etc. This null method is similar to 

 bridge balancing and may be instrumented to similar high precision. 

 After all of the harmonics corresponding to equalizer controls have 



