Measuring modulation index of an amplitude modulated wave with an oscilloscope

A oscilloscope can be used to measure the modulation index of an amplitude modulated wave. Modulation index is a value from 0 to `1, but is often expressed as a percentage.

The discussion here assumes symmetric modulation, it does not apply to super modulation schemes or any other schemes that are asymmetric.

Envelope method

If an oscilloscope is used to display the modulation envelope (as it is known), modulation index can be calculated from measured values of the peak voltage at the crest and valley of the envelope waveform.

Modulation index can be calculated as \(m=\frac{b-a}{b+a}\).

Trapezoidal pattern

If an oscilloscope is setup to display the modulating waveform on the horizontal axis and RF voltage on the vertical axis, it results in a trapezoidal pattern that can be used to calculate modulation index.

Modulation index can be calculated as \(m=\frac{b-a}{b+a}\).

Note that the trapezoidal pattern depends on the horizontal axis signal being in synchronism with the modulated wave. This may not be so with modulators that use DSP techniques.

See also RF Power Terms.