NEW TRANSMISSION MEASURING SYSTEMS 3 



of 20 db can be measured without an amplifier and without using 

 abnormal testing power. Vacuum tubes formerly used to rectify the 

 received alternating current testing power have been largely super- 

 seded by the copper-oxide rectifier which is small, inert and requires 

 no added power for its operation. The combination of this rectifier 

 with sensitive meters greatly simplifies and reduces the cost of the 

 measuring apparatus. Figure 1 shows the simplicity of the circuit 

 of a transmission measuring set which will measure losses up to 20 db. 

 While indicating meters have been used in transmission measuring 

 for many years, it is only recently that they have been calibrated 

 directly in db. This is the preferable way of measuring and its adop- 

 tion has been delayed only because of the limitations of available 

 apparatus and circuits which prevented a stable device from being 

 developed. The earlier copper-oxide rectifiers were too unstable for 



CALIBRATING 

 RESISTANCE 



AA^/ DECIBEL 



METER 



HOLDING 

 COIL 



Fig. 1 — Simplified circuit of transmission measuring set having a range of 20 db 



without an amplifier. 



measuring and until the development of the negative feedback ampli- 

 fier, all vacuum tube amplifiers varied in amplification with changes 

 in filament and plate voltage and the aging of tubes. The voltage of 

 available power plants, while sufficiently stable for commercial use, 

 caused the pointer of the meter in a transmission measuring set to 

 fluctuate and over intervals of a few hours the change might be as 

 much as one db. Smaller changes occurred frequently. These 

 changes were slow enough so that measurements could be made, but 

 they required frequent adjustment of amplifier gain to compensate. 

 With such instability, the most rapid and accurate measurements can 

 be made by using the comparison method of testing where a known 

 "standard" power is attenuated by calibrated potentiometers or net- 

 works until it equals the unknown received testing power. A meter 

 in this case serves merely as a means for telling when the two are 

 equal and the result is read from calibrated dials on the attenuator. 

 A majority of the measuring sets now in the plant operate on this 



