The principle of transmitting information over radio waves
In this article, we will try to understand the principles of transmitting information over a radio channel.
1) What are radio waves and where do they come from?
Radio waves are electromagnetic oscillations that carry through space the energy emitted by the generator of electromagnetic oscillations, electromagnetic oscillations, in turn, arise when the polarity of the electric field changes, for example, when an alternating electric current passes through the conductor. The speed of propagation of radio waves in vacuum is equal to the speed of light 299,792,458 m/s or 300,000 km/s or 1080 million kilometers per hour. To roughly imagine this speed, let's give some comparisons, so a radio wave can overcome the length of the Earth's equator in 134 ms, a radio wave will get from the Earth to the Moon in 1.225 seconds, and from the Earth to the Sun in 8.3 minutes.
In order to understand how radio waves arise in the generator of electromagnetic oscillations, let's consider a diagram of the simplest ideal oscillatory circuit.
The figure below shows the simplest closed oscillatory circuit consisting of a charged capacitor and an inductor (1), the charged capacitor begins to discharge through the inductance, in which electromagnetic induction occurs and energy accumulates (2), at this moment the capacitor plates are completely discharged, then the current flows through the inductance and flows to the reverse plates of the capacitor, fully charging them with energy (3), when charged, the capacitor again starts to discharge back through the inductance (4) and so on in reverse order (5) each time charging and recharging with a certain oscillation frequency.
In order to get an open oscillatory circuit, it is necessary to move the capacitor plates apart, in this case we get an open circuit radiating electromagnetic waves into space (A), by expanding the capacitor plates in different directions (B) we get an open oscillatory circuit in which electromagnetic - radio waves are radiated into space. If we replace the inductance with a conventional electrical signal generator (C), we get an antenna constantly emitting radio waves into space with the frequency of the generator. Figure (D) shows a schematic representation of the antenna.
The main characteristic of radio waves is the frequency, which shows how often the direction of the electric current changes in the generator of electromagnetic oscillations, which means the frequency of the emitted radio waves.
If we imagine the process of changing the electromagnetic field in the form of a change graph, we get the picture shown in the figure below, we can see the change in the field over time - a constant recharge of the capacitor plates with transients.
The main parameters of radio waves are amplitude and wavelength, the wavelength in turn is related to frequency.
Amplitude - corresponds to the magnitude of the electric and magnetic fields.
Wavelength- corresponds to the distance between two wave crests, two points of the wave that are in the same phase, is related to the rate of change in the strength of the electromagnetic field.
Frequency - the number of waves in a certain period of time, measured in hertz [Hz]. One hertz, equal to one oscillation of an electrical signal, for 1 second of time [frequency calculation formula f \u003d c / λ f - frequency in hertz, c - speed of light equal to 300,000,000 m / s, λ - wavelength in meters]
2) After we figured out what radio waves are, let's figure out how you can transmit information over radio waves, imagine that we are faced with the task of transmitting some bit sequence 010101, a logical unit can be marked with an excellent amplitude level or excellent frequency or phase shift. Therefore, the main of some methods of information presentation is amplitude modulation, frequency modulation, phase modulation.
Amplitude change - called amplitude modulation, AM modulation
The basic principle is a change in the level of electromagnetic field strength by the transmitting side.
To denote zero, we take the amplitude level at the base value, and to denote unity, we will increase the amplitude by a small value. The graph shows how the amplitude of the radio wave changes depending on the bit sequence, zero corresponds to the base amplitude, and units to a higher value. Amplitude modulation is less common due to the technical complexity of implementation and low resistance to interference, for example, a source of electromagnetic radiation that is not connected to the receiving and transmitting side can interfere with the transmission, for example, a lightning strike will briefly increase the amplitude and a false signal will appear at the output in the form of noise.
Frequency change - called frequency modulation, FM modulation
The basic principle is a change in the frequency of radio emission.
To denote zero, we take the base value of the frequency, and to denote one, we will change the frequency value upwards. The graph shows how the frequency changes depending on the bit sequence, zero corresponds to the base frequency value, and one corresponds to a higher frequency different from the base one. Frequency modulation has become more widespread due to the ease of implementation - it is only necessary to increase the frequency by changing the frequency characteristics of the oscillatory circuit. Also, this modulation is more noise-resistant, external noise can increase the amplitude of the signal, but the frequency will remain the same, after passing through a series of filters, we will get the original sequence.
Phase change - called phase modulation Phase-shift keying (PSK)
The basic principle is an abrupt change in the phase shift of the carrier wave.
To denote zero, we take the absence of a phase shift, and to denote a logical unit, in the original digital sequence, change the phase of the harmonic message by 180 °. The graph shows how the phase shift occurs when a logical unit is transmitted. Phase modulation has also become widespread due to its good noise immunity and ease of implementation. The radiated power of a transmitter with phase modulation is always at the same level, unlike amplitude and frequency modulation, which reduces the basic requirements for microelectronic components.
3) After we figured out what radio waves are and how information can be transmitted through them, let's look at the hardware implementation of transmitting and receiving information. As an example, let's take voice transmission from a transmitter to a receiver, we will transmit information using frequency modulation.
Transmitter - transmitter - transmitter
Consists of an oscillator, also called an oscillator, and a modulator that changes the base frequency of the radio wave. It works as follows:
- To generate the base frequency of the radio wave, we use an LC oscillatory circuit consisting of a capacitor C2 and an inductance L1, with this we create a base frequency at the output of the antenna.
- The voice exerts pressure on the microphone and creates slight electrical oscillations in it, which, entering the Base of the transistor, slightly open the Collector-Emitter junction in it. The greater the volume, the greater the level of electrical oscillation created at the output of the microphone and the more the transistor junction opens.
- The open transition of the transistor changes the frequency characteristics of the oscillating circuit, and therefore the frequency at the output of the antenna will change depending on the incoming signal, in this case, the frequency modulation of the voice signal occurs
Receiver - receiver - receiver
Consists of a receiving device and a demodulator. To receive a voice signal transmitted by our transmitter, we need to demodulate the signal, it works as follows:
- With a tuning capacitor C2, we set up an oscillatory circuit consisting of a capacitor C2 and an inductance L1, so that an oscillation frequency equal to the base frequency of the transmitting signal arises in it.
- The frequency change received by the antenna, which is different from the base one, causes a resonance in the circuit, which slightly increases the voltage at the base of the transistor, slightly opening the Collector-Emitter junction, the higher the resonance level, the more the transistor is open, the open transistor, in turn, changes the characteristics of the receiving circuit after the transistor and at the output microphone, the sound transmitted by our transmitter appears
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