Q.What is Angle modulation? Which are the different types of Angle modulation?

Frequency modulation (FM) is the process by which the frequency of the carrier is varied in accordance with the instantaneous amplitude of the modulating signal, while the carrier's amplitude stays constant. When the message is zero the carrier stays at its rest (centre) frequency $f_c$; a rising message raises the carrier frequency and a falling message lowers it, the extremes occurring at the message peaks.

FM and phase modulation both vary the total phase angle $\phi = (\omega_c t + \theta)$ of the carrier, so together they are called angle modulation. In phase modulation (PM) it is the carrier's phase that is varied in proportion to the message, with amplitude and frequency held constant.

Starting from a message $v_m = V_m\cos\omega_m t$ and carrier $v_c = V_c\sin(\omega_c t + \theta)$, the instantaneous angular frequency during FM is $\omega = \omega_c + K V_m\cos\omega_m t$, where $K$ is the modulator's frequency-sensitivity constant. Integrating the phase leads to the standard FM voltage equation

where $m_f$ is the FM modulation index. Recognising this form lets us read the carrier frequency, modulating frequency and index straight from a given FM expression.

FM was developed as an alternative to AM because AM transmits voice and music inefficiently and is vulnerable to noise. FM's strengths are better noise immunity (information is in frequency, not amplitude, so amplitude noise can be stripped off) and high-fidelity reproduction, which is why FM is used at VHF for quality music and speech broadcasts — although it does need a larger transmission bandwidth.