General information and parameters of radio signals. The main characteristics of the signals. List of symbols

As a carrier of messages, high-frequency electromagnetic waves (radio waves) of the corresponding range are used, which can propagate over long distances.

The carrier frequency fluctuation emitted by the transmitter is characterized by: amplitude, frequency and initial phase. In general, it is represented as:

i \u003d I m sin (ω 0 t + Ψ 0),

where: i- instantaneous value of the current of the carrier oscillation;

I m - amplitude of the current of the carrier oscillation;

ω 0 - angular frequency of the carrier wave;

Ψ 0 – the initial phase of the carrier wave.

The primary signals (transmitted message converted to electrical form) that control the operation of the transmitter can change one of these parameters.

The process of controlling high-frequency current parameters using a primary signal is called modulation (amplitude, frequency, phase). The term "manipulation" is used for telegraphic transmissions.

In radio communication, for the transmission of information, radio signals are used:

radiotelegraph;

radiotelephone;

phototelegraph;

telecode;

complex types of signals.

Radiotelegraph communication differs: according to the method of telegraphy; by the way of manipulation; on the use of telegraph codes; by the way of using the radio channel.

Depending on the method and rate of transmission, radiotelegraph communications are divided into manual and automatic. In manual transmission, manipulation is carried out with a telegraph key using the MORSE code. The transmission speed (for auditory reception) is 60–100 characters per minute.

In automatic transmission, the manipulation is carried out by electromechanical devices, and the reception is carried out with the help of printing machines. Transfer rate 900-1200 characters per minute.

According to the method of using the radio channel, telegraph transmissions are divided into single-channel and multi-channel.

By the method of manipulation, the most common telegraph signals include signals with amplitude shift keying (АТ - amplitude telegraph - A1), with frequency shift keying (ChT and DCHT - frequency telegraphy and double frequency telegraphy - F1 and F6), with relative phase shift keying (OFT - phase telegraphy - F9).

For the application of telegraph codes, telegraph systems with the MORSE code are used; start-stop systems with 5 and 6-digit codes and others.

Telegraph signals are a sequence of rectangular pulses (messages) of the same or different duration. The smallest message in duration is called elementary.

Basic parameters of telegraph signals: telegraphy speed (V); manipulation frequency (F); spectrum width (2D f).

Telegraphing speed V is equal to the number of chips transmitted per second, measured in baud. At a telegraphing rate of 1 baud, one elementary message is transmitted per second.

Manipulation frequency F numerically equal to half the speed of telegraphy V and is measured in hertz: F \u003d V / 2 .

Amplitude-shift keyed telegraph signal has a spectrum (Fig. 2.2.1.1), which, in addition to the carrier frequency, contains an infinite set of frequency components located on either side of it, at intervals equal to the manipulation frequency F. In practice, for reliable reproduction of a telegraph radio signal, it is sufficient to accept, in addition to the carrier signal, along three spectrum components located on either side of the carrier. Thus, the spectrum width of an amplitude-shift keyed CW RF signal is 6F. The higher the keying frequency, the wider the spectrum of the HF telegraph signal.

Figure: 2.2.1.1. Time and spectral representation of the AT signal

When frequency shift keying the current in the antenna does not change in amplitude, but only the frequency changes in accordance with the change in the manipulating signal. The spectrum of the FT signal (DCF) (Fig. 2.2.1.2) is, as it were, a spectrum of two (four) independent amplitude-manipulated oscillations with their own carrier frequencies. The difference between the frequency of "pressing" and the frequency of "pressing" is called the frequency separation, denoted ∆f and can be in the range of 50 - 2000 Hz (most often 400 - 900 Hz). The width of the FT signal spectrum is 2∆f + 3F.

Fig. 2.2.1.2. Time and spectral representation of the QT signal

To increase the throughput of a radio link, multichannel radiotelegraph systems are used. In them, on one carrier frequency of the radio transmitter, two or more telegraph programs can be transmitted simultaneously. A distinction is made between frequency division multiplexing, time division multiplexing and combined systems.

The simplest two-channel system is the dual frequency telegraphy (DFC) system. Frequency-keyed signals in the DCT system are transmitted by changing the carrier frequency of the transmitter due to the simultaneous effect of the signals of two telegraph sets on it. It uses the fact that the signals of two devices operating simultaneously can have only four combinations of transmitted messages. With this method, at any time, a signal of one frequency is emitted, corresponding to a certain combination of manipulated voltages. The receiving device has a decoder, with the help of which DC telegraph messages are generated through two channels. Frequency multiplexing means that the frequencies of individual channels are located in different parts of the total frequency range and all channels are transmitted simultaneously.

With time division of channels, the radio line is provided to each telegraph apparatus sequentially using distributors (Fig. 2.2.1.3).

Fig. 2.2.1.3. Multi-channel time division system

For the transmission of radiotelephone messages, mainly amplitude-modulated and frequency-modulated high-frequency signals are used. LF modulating signal is a collection of a large number of signals of different frequencies located in a certain band. The bandwidth of a standard LF telephone signal is typically 0.3–3.4 kHz.

Amplitude modulation (AM) is the simplest and most widespread in radio engineering way of putting information into high-frequency oscillations. At AM, the envelope of the amplitudes of the carrier oscillation changes according to the law, which coincides with the law of variation of the transmitted message, while the frequency and the initial phase of the oscillation are kept unchanged. Therefore, for an amplitude-modulated radio signal, the general expression (3.1) can be replaced by the following:

The nature of the envelope A (t) is determined by the type of the transmitted message.

With continuous communication (Fig. 3.1, a), the modulated oscillation takes the form shown in Fig. 3.1, b. The envelope A (t) coincides in form with the modulating function, that is, with the transmitted message s (t). Figure 3.1, b is built on the assumption that the constant component of the function s (t) is zero (in the opposite case, the amplitude of the carrier oscillation during modulation may not coincide with the amplitude of the unmodulated oscillation) The largest change in A (t) “down” cannot be greater. The change "up" can, in principle, be more.

The main parameter of the amplitude-modulated oscillation is the modulation index.

Figure: 3.1. Modulating function (a) and amplitude-modulated oscillation (b)

The definition of this concept is especially clear for tone modulation, when the modulating function is a harmonic oscillation:

In this case, the envelope of the modulated vibration can be represented as

where is the modulation frequency; - the initial phase of the envelope; - coefficient of proportionality; - the amplitude of the envelope change (Fig. 3.2).

Figure: 3.2. Oscillation modulated in amplitude by a harmonic function

Figure: 3.3. Amplitude-modulated oscillation pulse train

Attitude

called the modulation factor.

Thus, the instantaneous value of the modulated oscillation

With undistorted modulation, the amplitude of the oscillation varies from minimum to maximum.

In accordance with the change in the amplitude, the power of the modulated oscillation averaged over the period of high frequency also changes. The envelope peaks correspond to a power that is (1 to 4 times greater than the power of the carrier oscillation. The average power over the modulation period is proportional to the mean square of the amplitude A (t):

This power exceeds the power of the carrier oscillation by only one-fold. Thus, at 100% modulation (M \u003d 1), the peak power is equal to and the average power (denoted by the power of the carrier wave). Hence, it can be seen that the increase in the oscillation power caused by the modulation, which basically determines the conditions for extracting a message during reception, even with the limiting modulation depth does not exceed half the power of the carrier wave.

When transmitting discrete messages, which are an alternation of pulses and pauses (Fig. 3.3, a), the modulated oscillation has the form of a sequence of radio pulses shown in Fig. 3.3, b. This means that the phases of high-frequency filling in each of the pulses are the same as when "cutting" them from one continuous harmonic oscillation.

Only under this condition, shown in Fig. 3.3, b, the sequence of radio pulses can be interpreted as an oscillation modulated only in amplitude. If the phase changes from pulse to pulse, then we should talk about mixed amplitude-angular modulation.

2.1.1. Deterministic and random signals

Deterministic signalIs a signal, the instantaneous value of which at any moment of time can be predicted with a probability equal to one.

An example of a deterministic signal (Fig. 10) can be: sequences of pulses (the shape, amplitude and position in time of which are known), continuous signals with given amplitude-phase relationships.

Methods for specifying the MM signal: analytical expression (formula), oscillogram, spectral representation.

An example of an MM of a deterministic signal.

s (t) \u003d S m Sin (w 0 t + j 0)

Random signal - a signal, the instantaneous value of which at any moment of time is unknown in advance, but can be predicted with a certain probability, less than one.

An example of a random signal (Fig. 11) can be the voltage corresponding to human speech, music; a sequence of radio pulses at the input of the radar receiver; interference, noise.

2.1.2. Signals used in radio electronics

Continuous in magnitude (level) and continuous in time (continuous or analog) signals - take any values \u200b\u200bs (t) and exist at any moment in a given time interval (Fig. 12).

Continuous in magnitude and discrete in time signals are set at discrete time values \u200b\u200b(on a counting set of points), the signal magnitude s (t) at these points takes any value in a certain interval along the ordinate.

The term "discrete" characterizes the way of setting the signal on the time axis (Fig. 13).

Quantized in magnitude and continuous in time signals are set on the entire time axis, but the value s (t) can only take discrete (quantized) values \u200b\u200b(Fig. 14).

Quantized in magnitude and discrete in time (digital) signals- the values \u200b\u200bof the signal levels are transmitted in digital form (Fig. 15).

2.1.3. Pulse signals

Pulse - an oscillation that exists only within a finite period of time. In fig. 16 and 17 show a video pulse and a radio pulse.

For a trapezoidal video pulse, enter the parameters:

A is the amplitude;

t and is the duration of the video pulse;

t f - the duration of the front;

t cf - cut duration.

S p (t) \u003d S in (t) Sin (w 0 t + j 0)

S in (t) -video pulse - the envelope for a radio pulse.

Sin (w 0 t + j 0) -filling the radio pulse.

2.1.4. Special signals

Switch-on function (single function (fig. 18) or Heaviside function) describes the process of transition of some physical object from "zero" to "single" state, and this transition occurs instantly.

Delta function (Dirac function) is a pulse, the duration of which tends to zero, while the pulse height increases indefinitely. It is customary to say that the function is concentrated at this point.

	(2)
	(3)

According to the principle of information exchange, three types of radio communication are distinguished:

simplex radio communication;

duplex radio communication;

half-duplex radio communication.

By the type of equipment used in the radio communication channel, the following types of radio communication are distinguished:

telephone;

telegraph;

data transmission;

facsimile;

television;

radio broadcasting.

By the type of radio communication channels used, the following types of radio communication are distinguished:

surface wave;

tropospheric;

ionospheric;

meteoric;

space;

radio relay.

Types of documented radio communications:

telegraph communication;

data transfer;

facsimile communication.

Telegraph communication - for the transmission of messages in the form of alphanumeric text.

Data transfer for the exchange of formalized information between a person and a computer or between computers.

Facsimile for the transmission of still images by electrical signals.

1 - Telex - for the exchange of written correspondence between organizations and institutions using typewriters with electronic memory;

2 - Tele (video) text - to receive information from a computer to monitors;

3 - Tele (bureau) fax - fax machines are used for receiving (either from users or enterprises).

The following types of radio communication signals are widely used in radio networks:

A1 - AT with continuous vibration manipulation;

A2 - tone-modulated vibration manipulation

ADS - A1 (B1) - ОМ with 50% carrier

AZA - A1 (B1) - OM with 10% carrier

AZU1 - A1 (Bl) - OM without carrier

3. Features of the propagation of radio waves of various bands.

Propagation of radio waves in the myriameter, kilometer and hectometer ranges.

To assess the nature of the propagation of radio waves of a particular range, it is necessary to know the electrical properties of the material media in which the radio wave propagates, i.e. know and ε A of the earth and the atmosphere.

The total current law in differential form states that

those. the change in time of the flux of magnetic induction causes the appearance of a conduction current and a displacement current.

We write this equation taking into account the properties of the material environment:

λ < 4 м - диэлектрик

4 m< λ < 400 м – полупроводник

λ\u003e 400 m - conductor

Sea water:

λ < 3 м - диэлектрик

3 cm< λ < 3 м – полупроводник

λ\u003e 3 m - conductor

For myriameter wave (SVD):

λ \u003d 10 ÷ 100 km f \u003d 3 ÷ 30 kHz

and kilometer (DV):

λ \u003d 10 ÷ 1 km f \u003d 30 ÷ 300 kHz

in terms of its electrical parameters, the earth's surface approaches an ideal conductor, while the ionosphere has the highest conductivity and the lowest dielectric constant, i.e. close to the guide.

RV of VLF and LW bands practically do not penetrate into the ground and ionosphere, reflecting from their surface and can propagate along natural radio paths over considerable distances without significant loss of energy by surface and space waves.

Because Since the wavelength of the VLF range is commensurate with the distance to the lower boundary of the ionosphere, the concept of a simple and surface wave loses its meaning.

The RV propagation process is considered as occurring in a spherical waveguide:

Inner side - earth

Outer side (at night - layer E, during the day - layer D)

The waveguide process is characterized by insignificant energy losses.

Optimal RV - 25 ÷ 30 km

Critical RV (strong attenuation) - 100 km or more.

The following phenomena are inherent: - fading, radio echo.

Fading (fading) as a result of the interference of RVs that have traveled different paths and have different phases at the receiving point.

If in antiphase at the receiving point there is a surface and a spatial wave, then this is fading.

If there are sky waves in antiphase at the receiving point, then this is far fading.

A radio echo is a repetition of a signal as a result of the successive reception of waves reflected from the ionosphere a different number of times (near radio echo) or arriving at the receiving point without and after going round the globe (far radio echo).

The earth's surface has stable properties, and the places where the conditions of ionization of the ionosphere are measured have little effect on the propagation of the VLF RV range, then the value of the radio signal energy changes little during the day, year, and in extreme conditions.

In the range of km waves, both surface and space waves (both day and night) are well pronounced, especially at λ\u003e 3 km.

Surface waves when emitted have an elevation angle of no more than 3-4 degrees, and space waves are emitted at large angles to the earth's surface.

The critical angle of incidence of the RV km range is very small (in the daytime to the D layer, and at night to the E layer). Beams with elevation angles close to 90 ° are reflected from the ionosphere.

Surface waves in the km range, due to their good diffraction ability, can provide communication over distances of up to 1000 km or more. However, these waves are strongly attenuated with distance. (The surface wave is less intense by 1000 km than the spatial one).

Over very long distances, communication is carried out only by a spatial km wave. Near fading is observed in the region of equal intensity of surface and spatial waves. The conditions for the propagation of km of waves practically do not depend on the season, the level of solar activity, weakly depend on the time of day (the signal level is higher at night).

Reception in the km range is rarely degraded by strong atmospheric interference (thunderstorm).

When passing from KM (LW) km to the hectometer range, the conductivity of the earth and the ionosphere decreases. ε of the earth and approaching ε of the atmosphere.

Losses in the ground are increasing. The waves penetrate deeper into the ionosphere. At a distance of several hundred kilometers, space waves begin to dominate, since surface ones are absorbed by the ground and attenuate.

At a distance of about 50-200 km, surface and space waves are equal in intensity and close fading can occur.

Fading is frequent and deep.

With a decrease in λ, the fade depth increases with a decrease in the cutoff duration.

Fading is especially severe at λ\u003e 100 m.

The average duration of fading ranges from several seconds (1 sec) to several tens of seconds.

The conditions of radio communication in the hectometer range (MW) depend on the season and time of day. the D layer disappears, and the E layer is higher, and in the D layer there is a large absorption.

The communication range is longer at night than during the day.

In winter, reception conditions are improved due to a decrease in the electron density of the ionosphere and are weakened in atmospheric fields. In cities, reception is highly dependent on industrial interference.

SpreadRV - decameter range (KV).

In the transition from NE to HF, losses in the ground greatly increase (the earth is an imperfect dielectric), in the atmosphere (ionosphere), it decreases.

Surface waves on natural HF radio paths are of low value (weak diffraction, strong absorption).