ELECTRICAL MEASUREMENT 



87 



is omitted. Then the two branches of the circuit will be identical, 

 provided the attenuation of the standard attenuator is equal to that 

 of the apparatus under test. Accordingly, the method of measure- 

 ment is to switch the detector first to one and then to the other branch, 



STANDARD 

 ATTENUATOR 



APPARATUS 

 UNDER 

 Z3 TEST 



-fc^=!. 



zaf ^ n 



A 



STANDARD 

 ATTENUATOR 



AMPLIFIER 

 UNDER 

 TEST 





-fcr=i 



DETECTOR 



f— <> 



ziH 



DETECTOR 



Fig. 11 — Circuits for measuring attenuation and gain. A. Arrangement for 

 measuring loss. B. Arrangement for measuring gain 



adjusting the standard attenuator until an equal output is obtained 

 for either switch position. The attenuator then reads directly the 

 loss in the apparatus. The total input of the circuit is independent 

 of the switch position, since the impedance conditions remain un- 

 changed in switching. 



If the apparatus under test has not the same impedance as the 

 standard attenuator, the input impedance Z3 and the matching net- 

 work Z2 are adjusted so that the circuit still reads directly. 



The standard attenuator is a resistance network capable of variation 

 in small steps, each step consisting of a network of the L, T or // type, 

 the resistance values being such as to give the desired attenuation 

 between the output and input terminals. It is usually calibrated in 

 0.1-T.U. steps and may read as high as 100 T.U. corresponding to a 

 ratio of power output to power input of ten billion to one or, if the 

 impedances are the same, which is usually the case, corresponding to 

 a current or voltage ratio of 100,000 to 1. 



