Frequency is one of the characteristics of the amplitude fluctuations of a certain physical quantity relative to its average value. In physics, there are many types of oscillations of various nature, for example, oscillations of the electromagnetic field. In addition to spontaneous electromagnetic oscillations of a natural nature, there are oscillations, the occurrence of which is set by the will of man to solve certain problems arising from the needs of human activity. Most often, using vibrational processes, the issues of energy and / or information transfer are solved, i.e. communication, broadcasting, television, etc.
The process of information transfer includes the preparation of the corresponding energy field and its modulation with a useful signal and consists in the generation by the transmitter of a carrier frequency with certain parameter values - amplitude and frequency. The first parameter is proportional to the field energy and determines the transmission distance, and the second - to the recipient. At this stage, we only have a carrier frequency signal that acts as a “carrier” of information.
The information content of the energy field is introduced into the signal by a corresponding change in some of its parameters. In practice, various options for changing field parameters are used - this process is called carrier frequency modulation. In this case, a change in the amplitude of the carrier or a deviation of the frequency of the carrier is applied. At the stage of formation of radio communications, amplitude modulation was most often used when information is contained in the amplitude of the carrier frequency signal. In this case, the changes in the carrier amplitude are exactly
repeat the information analog signal. The consumer receives a signal by
the carrier frequency, and then selects the demodulator contained in it
information. At low frequencies, up to the shortwave range, only amplitude modulation is used. Its main drawback is the change in the amplitude of the carrier frequency, which is why the signal-to-noise ratio, a very important indicator of the quality of the communication channel, is reduced.
In addition to amplitude modulation, frequency information is also used to transmit information, at which frequency deviation is applied. Frequency advantage
modulations - higher noise immunity, therefore in professional
Communication systems use only frequency modulated signals. An example of the use of such signals is VHF broadcasting, television, special communications.
Deviation is the maximum change in the carrier frequency relative to its average value. In this case, the spectrum of frequency-modulated carrier oscillations depends on the amplitude of the useful signal, and its amplitude does not change, due to which the stability of communication is initially much higher.
Devices for modulating the carrier frequency with a signal are called frequency modulators. Their role in the process of transmitting a radio signal is to control the transmitter carrier frequency generators. Frequency deviation determines
bandwidth requirements of both the transmitter and receiver.
Since the main requirement for the communication channel is the more accurate transmission of the original signal, the passband of the input and output circuits of the receiver and transmitter should be at least double the frequency band of the original signal. However, here you have to look for a "middle ground", because the causes of deviation on the side of the transmitter can also be associated with spurious oscillations caused by both interference from foreign emissions and the creation of targeted interference, and one of the ways to deal with such signal distortions is to reduce the bandwidth. They usually talk about the ratio of basic
deviation characteristics - swing bands and the width of the spectrum of the transmitted signal.
If the modulation indices are large, then both parameters by which
characterized by frequency deviation, have approximately the same values. Deviation is measured in units of frequency Hz.