Modulation Index
Understanding Modulation Index
The modulation index controls the trade-off between bandwidth, power efficiency, and signal quality for analog modulation schemes. It is the single most important parameter in AM and FM system design.
AM Modulation Index
- m = 0: No modulation, only carrier. Maximum wasted power.
- m = 0.5: 50% modulation. Moderate sideband power.
- m = 1: 100% modulation. Maximum sideband power without distortion.
- m > 1: Overmodulation. Envelope crosses zero, causing distortion.
FM Modulation Index
- beta < 1: Narrowband FM. Bandwidth approximately 2 x f_message.
- beta > 1: Wideband FM. Bandwidth approximately 2 x (delta_f + f_message) (Carson's rule).
- FM broadcast: delta_f = 75 kHz, f_message = 15 kHz, beta = 5. BW = 180 kHz.
Frequently Asked Questions
What is modulation index?
Modulation index measures the degree of modulation. For AM: ratio of modulating signal amplitude to carrier amplitude (0-1). For FM: ratio of peak frequency deviation to modulating frequency. It determines bandwidth, efficiency, and signal quality.
What happens with AM overmodulation?
When AM modulation index exceeds 1, the envelope crosses zero, creating distortion and unwanted sideband splatter. The demodulated signal is clipped. Broadcast AM must stay below 100% modulation (m = 1).
Why does FM sound better than AM?
FM broadcast uses a high modulation index (beta = 5), trading bandwidth for noise immunity. The FM discriminator rejects amplitude noise, and the deviation ratio provides ~25 dB SNR improvement over AM at the same carrier power.