Decremeter

an instrument for measuring the attenuation of electrical signals on wire or cable lines, in wave guides, and in such equipment as attenuators and valves. The unit of measurement of attenuation A is the neper

or the decibel A_dB = 10 log(P_out/P_in), where P is the power of the signal at the input or output of the line or equipment.

For measurements at low frequencies the substitution method is generally used. A voltage from an oscillator is fed to the input of the line, and a vacuum-tube voltmeter is connected to the output. Then a variable attenuator is connected between the oscillator and the voltmeter and is adjusted to obtain the previous voltmeter reading. The amount of attenuation is then read from the scale of the variable attenuator. Attenuation is measured on high- and superhigh-frequency coaxial and wave guidelines in a similar manner. For the measurement of high attenuations (of 60–120 dB) at superhigh frequencies, a complicated decremeter is used that includes oscillators for the input of the line under test and standard superheterodyne receivers for its output.