As is known, a radio frequency signal consists of a carrier, which is based on radio emission in the form of a simple harmonic oscillation u (t) = U cos (ωt + φ). From this it follows that in the carrier frequency signal there are three independent parameters, acting on which you can capture changes in the control signal.
This implies the possibility of three types: amplitude (AM), frequency (FM) and phase modulation (FM).
Phase modulation is a method of transmitting analog or digital information by changing the initial angle (phase) φ 0 of the carrier frequency of the transmitted signal.
With it, the phase φ (t) depends on the amplitude of the control (modulating) signal, i.e. φ (t) = ω 0 t + Δ φ ∙ sinΩt + φ 0 = = φ 0 + ke (t), where k is the proportionality coefficient.
The phase-modulated signal is generally described by the expression u (t) = U n sin [ωt + φ (t)].
When modulating with one tone [e (t) = E sin Ωt], we have: φ (t) = φ 0 + kE sin Ωt = φ 0 + Δφ max sin Ωt.
After substituting the value of φ (t) in the equation of the phase-modulated signal, we obtain u (t) = U n sin (ω n t + φ 0 + Δφ max sin Ωt), where Δφ max is the maximum phase change proportional to the amplitude of the control voltage. Δφ max is otherwise called the angular modulation index and is denoted by m.
As can be seen, with FM m = Δφ max = kE. The instantaneous value of the time-varying phase angle Θ (t) is Θ (t) = ω t + φ 0 + msin Ωt, so that ω = d Θ (t) / dt = ω + mΩ cosΩt, where mΩ = Δφ max Ω = Δ ω n = kEΩ is the maximum frequency deviation from ω n for FM, which is directly proportional to the amplitude and frequency of the modulating oscillation.
Thus, in the case of FM, the modulation index characterizing the maximum phase change is proportional to the amplitude of the control signal and does not depend on the modulation frequency. The change in frequency relative to the average value (deviation) changes in direct proportion to the amplitude and frequency of the modulating voltage.
Depending on the application, phase modulation has several varieties. One of them, in particular, is relative phase manipulation.
In this form, depending on the modulating signal, only the phase of the signal changes, and the frequency and amplitude remain unchanged. When OFM informative value is not the absolute change in phase, but its change relative to the previous value.
An electronic circuit that causes a change in the phase angle of a modulated wave (relative to an unmodulated carrier) in accordance with a modulating signal is called a phase modulator.
Many types of such images have been developed. A simple modulator circuit contains a varicap - diode, capable of changing the transition capacitance under the action of a control voltage. In this circuit, the modulating voltage changes the capacitance of the varicap. The phase shift depends on the relative capacitance of this diode and the load resistance R.
Thus, this shift depends on the modulating voltage. This determines the phase modulation of the radio signal. Nevertheless, such a shift is nonlinearly related to the modulating voltage, the capacitance of the varicap is nonlinearly related to the modulating voltage, which creates additional problems in the design of phase modulators.
In its pure form, phase modulation has not found wide application due to its inherent serious drawback - low noise immunity.