Phase Modulation

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• published: 30 Nov 2010
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Phase ModulationFrequency and phase modulation have become widely spread in the VHF range.One of the advantages of phase modulation is that it's easy to use.For example, VHF direct transformation radio stations which use phase modulation are among the simplest ones, but still remain as effective as the radio stations that use single-band modulation.In addition, phase modulation has other advantages.Thus, the power of a transmitter does not change during the modulation. It remains constant and equals the peak value.The transmitter's power amplifier operates at a constant signal amplitude, so there are no special requirements to its linearity.The transmitter does not require high-power audio signal, so its scheme and design turn out to be simpler than a single-band transmitter.During phase modulation the phase of high-frequency oscillations varies in accordance with the sound signal.The maximum phase deviation, expressed in radians, is called a modulation index.When you change the phase of the signal, the frequency also changes, as the frequency is actually the speed of the phase change over time.In general, the phase and frequency modulations are very similar.They differ only in the spectral composition of the modulating signal applied to the modulator.During the pure sound modulation the phase and the frequency modulations are indistinguishable at all.Phase-modulated signal at the modulator's output has a constant modulation index at any modulation frequency.The difference of the phase modulation from the frequency one is that the latter requires a frequency detector to receive a frequency-modulated signal.In the case of phase modulation, you can receive a signal using a regular receiver with amplitude modulation.A small detuning of the receiver weakens one of the sidebands, so the AM detector outputs a signal.One can also receive a modulated signal using single-band modulation receivers.A phase-modulated signal may be received even with FM receivers.In this case, treble frequencies of the sound spectrum would be accentuated.