Amplitude modulation (AM) is the simplest and most common method in radio engineering of putting information into high-frequency oscillations. With AM, the envelope of the amplitudes of the carrier oscillation changes according to a law that coincides with the law of change in the transmitted message, while the frequency and initial phase of the oscillation are maintained 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 on the form shown in Fig. 3.1b. The envelope A(t) coincides in form with the modulating function, i.e. 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 equal to zero (otherwise, the amplitude of the carrier oscillation during modulation may not coincide with the amplitude of the unmodulated oscillation). The largest change A(t) "down" cannot be greater than . The change "up" can, in principle, be greater.

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

Rice. 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 oscillation 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).

Rice. 3.2. An oscillation modulated in amplitude by a harmonic function

Rice. 3.3. Oscillation modulated by the amplitude of the pulse train

Attitude

is called the modulation factor.

Thus, the instantaneous value of the modulated oscillation

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

In accordance with the change in amplitude, the average for the period also changes. high frequency modulated oscillation power. The peaks of the envelope correspond to a power that is 14 times greater than the power of the carrier wave. 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 wave by only a factor of 1. Thus, at 100% modulation (M = 1), the peak power is equal to a average power(through denotes the power of the carrier oscillation). This shows that the increase in the power of the oscillation due to modulation, which basically determines the conditions for isolating the message upon reception, even at the limiting depth of modulation does not exceed half the power of the carrier oscillation.

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. 3.3b. This means that the phases of high-frequency filling in each of the pulses are the same as when they are "cut" 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 speak of mixed amplitude-angle modulation.

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

simplex radio communication;

duplex radio communication;

half duplex radio.

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

telephone;

telegraph;

data transmission;

facsimile;

television;

broadcasting.

According to 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.

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 a computer.

Fax communication 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 at enterprises).

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

A1 - AT with CW keying;

A2 - manipulation of tone-modulated oscillations

ADS - A1 (B1) - OM 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 ranges.

Propagation of radio waves of 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 atmosphere.

The total current law in differential form says that

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

Let us write this equation taking into account the properties of the material medium:

λ < 4 м - диэлектрик

4 m< λ < 400 м – полупроводник

λ > 400 m - conductor

Sea water:

λ < 3 м - диэлектрик

3 cm< λ < 3 м – полупроводник

λ > 3 m - conductor

For myriameter wave (SVD):

λ = 10 ÷ 100 km f = 3 ÷ 30 kHz

and kilometer (DV):

λ = 10 ÷ 1 km f = 30 ÷ 300 kHz

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

The RV bands of the LLW and LW practically do not penetrate into the earth and the ionosphere, being reflected from their surface, and can propagate along natural radio paths over considerable distances without significant loss of energy by surface and spatial waves.

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

The RV propagation process is considered to take place in a spherical waveguide:

Inner side - ground

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 the surface and spatial waves are in antiphase at the receiving point, then this is fading.

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

A radio echo is a repetition of a signal as a result of 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 rounding the globe (far radio echo).

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

In the range of km waves, both surface and spatial waves are well expressed (both during the day and at night), especially at waves λ> 3 km.

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

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

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

Over very long distances, communication is carried out only by a spatial km wave. In the region of equal intensity of the surface and spatial waves, near fading is observed. The conditions for the propagation of km waves are practically independent of the season, the level of solar activity, and weakly depend on the time of day (the signal level is higher at night).

Reception in the km range rarely deteriorates due to strong atmospheric interference (thunderstorm).

In the transition from KM (LW) km to the hectometric range, the conductivity of the earth and the ionosphere decreases. ε of the earth and approaches ε of the atmosphere.

Ground losses are on the rise. Waves penetrate deeper into the ionosphere. At a distance of several hundred kilometers, sky waves begin to predominate, because surface ones are absorbed by the earth and die out.

At a distance of approximately 50–200 km, surface and sky waves are equal in intensity and near fading may occur.

The fading is frequent and deep.

As λ decreases, the fading depth increases as the blocking duration decreases.

Particularly strong fading at λ greater than 100 m.

The average duration of fading varies from a few seconds (1 sec) to several tens of seconds.

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

Communication range at night is greater than during the day.

In winter, reception conditions improve 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.

SpreadingR.V.- decameter range (HF).

When moving from SW to SW, the losses in the earth increase greatly (the earth is an imperfect dielectric), and in the atmosphere (ionosphere) it decreases.

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

Radio signals are called electromagnetic waves or electrical high frequency vibrations that encapsulate the transmitted message. To form a signal, the parameters of high-frequency oscillations are changed (modulated) using control signals, which are voltages that change according to a given law. Harmonic high-frequency oscillations are usually used as modulated ones:

where w 0 \u003d 2π f 0 – high carrier frequency;

U 0 is the amplitude of high-frequency oscillations.

The simplest and most commonly used control signals are harmonic oscillation

where Ω is a low frequency, much less than w 0 ; ψ is the initial phase; U m - amplitude, as well as rectangular pulse signals, which are characterized by the fact that the voltage value U ex ( t)=U during the time intervals τ and, called the duration of the pulses, and is equal to zero during the interval between pulses (Fig. 1.13). Value T and is called the pulse repetition period; F and =1/ T and is the frequency of their repetition. Pulse Period Ratio T and to the duration τ and is called the duty cycle Q impulse process: Q=T and /τ and.

Fig.1.13. Rectangular pulse train

Depending on which parameter of the high-frequency oscillation is changed (modulated) with the help of a control signal, amplitude, frequency and phase modulation is distinguished.

With amplitude modulation (AM) of high-frequency oscillations by a low-frequency sinusoidal voltage with a frequency of Ω mod, a signal is formed, the amplitude of which changes with time (Fig. 1.14):

Parameter m=U m / U 0 is called the amplitude modulation factor. Its values ​​are in the range from one to zero: 1≥m≥0. Modulation factor expressed as a percentage (i.e. m×100%) is called the amplitude modulation depth.

Rice. 1.14. Amplitude modulated radio signal

With phase modulation (PM) of a high-frequency oscillation by a sinusoidal voltage, the signal amplitude remains constant, and its phase receives an additional increment Δy under the influence of the modulating voltage: Δy= k FM U m sinW mod t, Where k FM - coefficient of proportionality. A high-frequency signal with phase modulation according to a sinusoidal law has the form

At frequency modulation(FM) control signal changes the frequency of high-frequency oscillations. If the modulating voltage changes according to a sinusoidal law, then the instantaneous value of the frequency of the modulated oscillations w \u003d w 0 + k World Cup U m sinW mod t, Where k FM - coefficient of proportionality. The greatest change in frequency w with respect to its average value w 0 equal to Δw М = k World Cup U m, is called the frequency deviation. The frequency modulated signal can be written as follows:

The value equal to the ratio of the frequency deviation to the modulation frequency (Δw m / W mod = m FM) is called the frequency modulation ratio.

Figure 1.14 shows high-frequency signals for AM, PM and FM. In all three cases, the same modulating voltage is used. U mod, changing according to the symmetrical sawtooth law U mod( t)= k Maud t, Where k mod >0 on time interval 0 t 1 and k Maud<0 на отрезке t 1 t 2 (Fig. 1.15, a).

With AM, the signal frequency remains constant (w 0), and the amplitude changes according to the law of the modulating voltage U AM ( t) = U 0 k Maud t(Fig. 1.15, b).

The frequency modulated signal (Fig. 1.15, c) is characterized by a constant amplitude and a smooth change in frequency: w( t) = w0 + k World Cup t. In the time span from t=0 to t 1 the oscillation frequency increases from the value w 0 to the value w 0 + k World Cup t 1 , and on the segment from t 1 to t 2 the frequency decreases again to the value w 0 .

The phase-modulated signal (Fig. 1.15, d) has a constant amplitude and frequency hopping. Let's explain this analytically. With FM under the influence of modulating voltage

Fig.1.15. Comparative view of modulated oscillations with AM, FM and FM:
a - modulating voltage; b – amplitude modulated signal;
c – frequency-modulated signal; d - phase modulated signal

signal phase receives an additional increment Δy= k FM t, therefore, a high-frequency signal with phase modulation according to the sawtooth law has the form

Thus, on the segment 0 t 1 the frequency is w 1 >w 0 , and on the segment t 1 t 2 it is equal to w 2

When transmitting a sequence of pulses, for example, a binary digital code (Fig. 1.16, a), AM, FM and FM can also be used. This type of modulation is called keying or telegraphy (AT, CT and FT).

Fig.1.16. Comparative view of manipulated oscillations in AT, PT and FT

With amplitude telegraphy, a sequence of high-frequency radio pulses is formed, the amplitude of which is constant during the duration of the modulating pulses τ and, and is equal to zero the rest of the time (Fig. 1.16, b).

With frequency telegraphy, a high-frequency signal is formed with a constant amplitude and a frequency that takes two possible values ​​(Fig. 1.16, c).

With phase telegraphy, a high-frequency signal is formed with a constant amplitude and frequency, the phase of which changes by 180 ° according to the law of the modulating signal (Fig. 1.16, d).

Lecture #5

T theme #2: Transmission of DISCRETE messages

Lecture topic: DIGITAL RADIO SIGNALS AND THEIR

Features Introduction

For data transmission systems, the requirement for the reliability of the transmitted information is most important. This requires logical control of the processes of transmission and reception of information. This becomes possible when digital signals are used to transmit information in a formalized form. Such signals make it possible to unify the element base and use correction codes that provide a significant increase in noise immunity.

2.1. Understanding Discrete Messaging

Currently, for the transmission of discrete messages (data), as a rule, the so-called digital communication channels are used.

Message carriers in digital communication channels are digital signals or radio signals if radio communication lines are used. The information parameters in such signals are amplitude, frequency and phase. Among the accompanying parameters, the phase of the harmonic oscillation occupies a special place. If the phase of the harmonic oscillation on the receiving side is precisely known and this is used when receiving, then such a communication channel is considered coherent. IN incoherent In the communication channel, the phase of the harmonic oscillation on the receiving side is not known and it is assumed that it is distributed according to a uniform law in the range from 0 to 2  .

The process of converting discrete messages into digital signals during transmission and digital signals into discrete messages during reception is illustrated in Fig. 2.1.

Fig.2.1. The process of converting discrete messages during their transmission

Here it is taken into account that the main operations for converting a discrete message into a digital radio signal and vice versa correspond to the generalized block diagram of the discrete message transmission system discussed in the last lecture (shown in Fig. 3). Consider the main types of digital radio signals.

2.2. Characteristics of digital radio signals

2.2.1. Amplitude-shift keyed radio signals (aMn)

Amplitude shift keying (AMn). Analytical expression of the AMn signal for any moment of time t looks like:

s AMn (t, )= A 0  (t) cos(  t ) , (2.1)

Where A 0 ,   And  - amplitude, cyclic carrier frequency and initial phase of the AMn radio signal,  (t) – primary digital signal (discrete information parameter).

Another form of writing is often used:

s 1 (t) = 0 at  = 0,

s 2 (t) = A 0 cos(  t ) at  = 1, 0  t T ,(2.2)

which is used in the analysis of AMn signals in a time interval equal to one clock interval T. Because s(t) = 0 at  = 0, then the AMn signal is often referred to as a signal with a passive pause. The implementation of the AMn radio signal is shown in Fig. 2.2.

Fig.2.2. Implementation of the AM radio signal

The spectral density of the AMn signal has both a continuous and a discrete component at the carrier frequency   . The continuous component is the spectral density of the transmitted digital signal  (t) transferred to the carrier frequency region. It should be noted that the discrete component of the spectral density takes place only at a constant initial phase of the signal  . In practice, as a rule, this condition is not met, since, as a result of various destabilizing factors, the initial phase of the signal randomly changes in time, i.e. is a random process  (t) and is uniformly distributed in the interval [-  ;  ]. The presence of such phase fluctuations leads to “blurring” of the discrete component. This feature is also characteristic of other types of manipulation. Figure 2.3 shows the spectral density of the AMn radio signal.

Fig.2.3. Spectral density of the AMn radio signal with a random, uniform

distributed in the interval [-  ;  ] initial phase 

The average power of the AM radio signal is equal to
. This power is equally distributed between the continuous and discrete components of the spectral density. Consequently, in the AMn radio signal, the share of the continuous component due to the transmission of useful information accounts for only half of the power emitted by the transmitter.

To form the AMn radio signal, a device is usually used that provides a change in the amplitude level of the radio signal according to the law of the transmitted primary digital signal  (t) (for example, an amplitude modulator).