Phase modulation as a method of data transmission

As is known, the 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). It follows that there are three independent parameters in the signal of the carrier frequency, affecting 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} + Δφ ∙ 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 in 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) into 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 phase angle θ (t) changing in time is equal to Θ (t) = ω _n t + φ ₀ + msin Ωt, so that ω = d Θ (t) / dt = ω _n + mΩ cosΩt, where mΩ = Δφ _max Ω = Δω _n = kEΩ is the maximum frequency deviation from ωH at FM, directly proportional to the amplitude and frequency of the modulating oscillation.

Thus, with 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 mean value (deviation) varies directly in proportion to the amplitude and frequency of the modulating voltage.

Depending on the application conditions, phase modulation has several varieties. One of them, in particular, is relative phase shift keying.

In this form, depending on the modulating signal, only the phase of the signal changes, and the frequency and amplitude remain unchanged. With an OMP, the information value is not the absolute phase change, but its change relative to the previous value.

An electronic circuit that causes a change in the phase angle of the modulated oscillation (with respect to the unmodulated carrier) in accordance with the modulating signal is called a phase modulator.

Many types of such images have been developed. The scheme of a simple modulator contains a varicap-diode, capable of changing the junction capacitance under the influence of a control voltage. In this scheme, the modulating voltage changes the capacitance of the varicap. The phase shift depends on the relative magnitude of the capacitance of this diode and the load resistance R.

Thus, this shift depends on the modulating voltage. This also determines the phase modulation of the radio signal. However, such a shift is nonlinearly related to the modulating voltage, the varicap capacitance is nonlinearly related to the modulating voltage, which creates additional problems in the construction of phase modulators.

In its pure form, phase modulation has not been widely used due to its inherent serious drawback - low noise immunity.