Modulation Index
Modulation index, a dimensionless number, is a parameter that quantifies the extent to which a carrier wave is modified by the modulating signal in a communication system. Its definition depends on the type of modulation used, such as amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM). The modulation index is used because it affects the bandwidth, signal power, and potential for distortion in communication systems.
Amplitude Modulation (AM) - In AM, the modulation index (\(m\)) indicates how much the amplitude of the carrier wave is varied by the information signal.
It is given by the formula \(\; m = A_{mod} \;/\; A_{carrier} \;\) where \(\;A_{mod}\;\) is the amplitude of the modulating signal, and \(\;A_{carrier}\;\) is the amplitude of the carrier signal.
When \(\;m = 1\), the modulation is said to be 100% modulated. If \(m > 1\), overmodulation occurs, leading to distortion.
Frequency Modulation (FM) - In FM, the modulation index (\(\beta\)) is the ratio of the frequency deviation (\(\Delta f\)) to the frequency of the modulating signal (\(f_m\)). \(\;\;\beta = \Delta f \;/\; f_m\;\;\)
The modulation index here determines the bandwidth of the transmitted signal and the amount of frequency deviation caused by the modulating signal.
Phase Modulation (PM) - In PM, the modulation index is defined as the peak phase deviation (\(\Delta \theta\)) caused by the modulating signal.
Total Modulation Index formula |
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| \( m_t \;=\; \sqrt{ m_1^2 + m_2^2 + m_3^2 \;... } \) | ||
| Symbol | English | Metric |
| \( m_t \) = Total Modulation Index | \(dimensionless\) | \(dimensionless\) |
| \( m_1, m_2, m_3 \) = Modulation Index of Signal | \(dimensionless\) | \(dimensionless\) |
Tags: Communication System