MEASURING SYSTEM FUR VIDEO 223 



and phase, and indicates their difference on the direct reading scales of the 

 iiKhcator meters. 



If the measuring attenuator is set at 60 db loss, and the range attenuator 

 // at 40 db loss, "6"' and "X" channels are in balance when the apparatus 

 under test is replaced by a zero loss strap. The phase-shifter has, by design, 

 20 db loss; so that under these conditions ".S" and "X" channels are nomi- 

 nally in balance, except for small residual phase and transmission dif- 

 ferentials which may be zeroed-out by initial adjustment of the phase-shifter 

 and of the relative gain between "5" and "X" channel amplifiers within 

 the detector. Null readings on the phase and transmission difference 

 indicating meters tell when exact phase and transmission balance between 

 the two channels has been established. The phase-shifter and attenuator 

 dials are arranged to read zero after this initial balance has been made. 

 To measure apparatus transmission and phase, the strap is replaced by the 

 apparatus under test and the balance restored by adjustment of the phase- 

 shifter and the measuring attenuator. The insertion phase and trans- 

 mission of the apparatus under test are then read directly from the cali- 

 brated dials of the phase shifter and attenuator. 



When measuring loss, attenuation in the measuring attenuator is reduced 

 by the amount of attenuation introduced in the high-frequency portion of 

 "X" channel by the "apparatus under test." In measuring gain, the 

 attenuation through the measuring attenuator must be increased by the 

 amount of apparatus gain. To insure that "5" and "X" channel modulators 

 are not overloaded by excessive input, range attenuator I is set to 40 db 

 loss during gain measurements. This attenuator is common to both chan- 

 nels and therefore introduces no phase differential. Simultaneously and 

 automatically, the range attenuator II immediately following the "5" 

 modulator, is operated, removing 40 db loss from the 31 kc standard channel. 



The measuring attenuator is self-computing and indicates directly in 

 illuminated figures the gain or loss of the apparatus under test. A simple 

 switching arrangement automatically controls the dial-lighting circuit of 

 the measuring attenuator. When measuring gain the dial indications 

 increase in one direction, and when measuring loss the indications increase 

 in the opposite direction (Fig. 3). 



In addition to the null-balance method, a deflection method of measure- 

 ment using direct reading scales of the phase and transmission difference 

 indicating meters is also possible. An automatic volume control circuit 

 assures invariance of the indicator scale factors with either the modulator 

 frequency-transmission characteristic, or input voltage variation at the 

 "5'" modulator caused by reflections from apparatus under test. The auto- 

 matic volume control circuit regulates the output voltage of the slave 

 oscillator to maintain the amplitude of the "5" channel input to the dif- 



