Some Notes on the Practical Measurement of the Degree of Amplitude Modulation

Some questions met with when measuring the degree of amplitude modulation (AM) are discussed. The degree of modulation is defined as the ratio of certain amplitude quantities, a clear distinction being made between what are defined as "up" and "down" values. The difference between the unmodulated carrier amplitude and the "mean amplitude during modulation" is stressed. It is shown that for an arbitrary amplitude modulation the effective value of the modulated current is dependent only on the amplitude of the harmonics, whereas the degree of modulation depends largely on the phase as well. From this it follows that any "effective" method of modulation degree measurement must be rejected as being not in accordance with the definition of the quantity to be measured. The effects of nonlinearity in a class B amplifier and of overmodulation in a class C amplifier are discussed, noting the rise of "mean high-frequency amplitude", the formation of harmonics, and the resulting degrees of modulation. After a short critical review of some existing methods, general requirements are proposed for a measuring device, and a description of the principle and performance of a direct reading modulation meter is given. A "ripple meter" constructed to measure exceedingly small degrees of modulation, and developed along lines similar to the modulation meter, is also briefly described with attention to its principle and its capabilities.