264 



MEASUREMENTS 



equals the pass band of the highly selective tuned amplifier. The high 

 selectivity is usually obtained by means of a quartz filter. The upper side 

 band is passed through the selective tuned amplifier, detected and then 

 measured on a meter. 



The balance bridge for measuring the total distortion is shown sche- 

 matically in Fig. 11.125. A part of the output of the oscillator is fed to the 

 apparatus to be tested and another part to the analyzer. The amplitude 

 and phase relations of the fundamentals from the oscillator and apparatus 

 to be tested are adjusted by means of suitable networks so that none of 



Fig. 11.13. Schematic arrangement of the apparatus employing a cathode ray tube for 

 indicating the non-Hnear distortion of a loud speaker. 



the fundamental remains. The remainder is the total harmonic generated 

 by the system under test. This is measured by means of a root-mean- 

 square meter. 



The above procedure for obtaining the harmonic content requires con- 

 siderable time and sometimes a qualitative indication requiring only a 

 few minutes has been found to be very useful. 



Figure 11.13 shows the circuits used in connection with a modern 

 cathode ray oscillograph, having a stabilizing sweep circuit, to indicate 

 qualitatively the extent to which harmonic distortion is introduced by a 

 loud speaker. When the switch is thrown to the right and the sine wave 

 generated in the oscillator is sent into the amplifying system through the 

 attenuator, the cathode ray oscillograph should show a pure sine wave 



