The process is called modulation. conversion of one or more characteristics of the modulating high-frequency oscillation when exposed to a control low-frequency signal. As a result, the spectrum of the control signal moves to the high frequency region, where the transmission of high frequencies is more efficient.

Modulation is performed in order to transmit information through. The transmitted data is contained in the control signal. And the function of the carrier is carried out by a high-frequency oscillation, called the carrier. In the role of the carrier oscillations, oscillations of various shapes can be used: sawtooth, rectangular, etc., but harmonic sinusoidal ones are usually used. Based on which particular characteristic of the sinusoidal oscillation changes, several types of modulation are distinguished:

Amplitude modulation

The modulating and reference signals are transmitted to the input of the modulating device, as a result, we have a modulated signal at the output. The condition for correct conversion is twice the value of the carrier frequency in comparison with the maximum value of the bandwidth of the modulating signal. This type of modulation is quite simple in execution, but has a low noise immunity.

Noise immunity arises due to the narrow bandwidth of the modulated signal. It is used mainly in the medium and low frequency ranges of the electromagnetic spectrum.

Frequency modulation

As a result of this type of modulation, the signal modulates the frequency of the reference signal, not the power. Therefore, if the magnitude of the signal increases, then, accordingly, the frequency increases. Due to the fact that the bandwidth of the received signal is much wider than the original signal value.

Such modulation is characterized by high noise immunity, however, for its application, the high-frequency range should be used.

Phase modulation

During this type of modulation, the modulating signal uses the phase of the reference signal. With this type of modulation, the resulting signal has a fairly wide spectrum, because the phase turns 180 degrees.

Phase modulation is actively used to form noise-immune communications in the microwave range.

Continuous functions, noise, a sequence of pulses, etc. can be used as a carrier signal. So, in pulse modulation, a sequence of narrow pulses is used as a carrier signal, and a discrete or analog signal acts as a modulating signal. Since the pulse train is characterized by 4 characteristics, there are 4 types of modulation:

- frequency-pulse;

- pulse-width;

- amplitude-pulse;

- phase-pulse.

Continuous modulation methods.

Signal conversion methods.

Electrical signals to be transmitted in telemechanics systems, in most cases, lie in the low-frequency part of the spectrum (in the range from zero to several tens of hertz). The direct transmission of these signals between the PU and the CP is sometimes used in the so-called intensity systems, but the range of such systems is limited and rarely exceeds several tens of meters, since low-frequency signals are most susceptible to interference when they are transmitted over long distances. Since the bandwidth of overhead communication lines usually starts from 0.5 kHz, in order to match the low-frequency signal with the high-frequency communication line, the spectrum of the transmitted signal is transferred to the high-frequency region.

To do this, the low-frequency signal is brought into one-to-one correspondence with one of the parameters of the high-frequency oscillation, called bearing. This transformation of the spectrum is called modulation, and the modulation device - modulator.There are continuous, pulse and digital modulation methods.

Continuous modulation methods.

In continuous modulation methods, a continuous harmonic oscillation generated by a high-frequency generator is used as a carrier. Depending on which particular parameter of the carrier oscillation changes in accordance with the change in the low-frequency signal, amplitude (AM), frequency (FM) and phase (PM) modulations are distinguished.

Consider amplitude modulation (Fig. 14.1). Let there be a modulating input signal (see Fig. 14.1, a) and a carrier harmonic oscillation (see Fig. 14.1, a), moreover, the carrier frequency is much greater than the frequency of the input signal, and the initial phases and will be taken equal to zero. As a result of modulation, the amplitude of the carrier wave becomes related to the modulating signal as follows:

where is the amplitude of the carrier signal; X- amplitude of the input signal; - modulation factor.

Then the expression for the modulated signal will look like

Opening the brackets, by the cosine product theorem we get

those. the modulated signal consists of three components with frequencies , and and , respectively, with amplitudes and . Therefore, the bandwidth of the communication line must be at least 2 for such a signal.

Rice. 14.1. Amplitude modulation: a– input signal; b– modulated signal; v– detective-

broken signal; G– block diagram of signal conversion.

If the input signal is periodic with a frequency , but has a complex shape, then according to the Fourier transform it can be represented as a sum of harmonic components with frequencies, etc. Accordingly, components with frequencies will appear in the spectrum of the modulated signal etc. With pulsed and non-periodic input signals, this series turns out to be infinite, but the power of the higher harmonic components is very small, and in practice the spectrum of the modulated signal can be considered limited.

Thus, regardless of the signal shape, as a result of modulation, its spectrum is transferred from the low-frequency region to the high-frequency region: from frequency to frequency. The frequency of the high-frequency oscillation is selected depending on the type and bandwidth of the communication line. By itself, the modulated oscillation does not carry information, therefore, when received, it is inversely converted, highlighting the original low-frequency signal. Such a transformation is called demodulation, and the corresponding device demodulator.

To demodulate AM oscillations, the signal is passed through an amplitude detector, which is a one- or two-half-wave rectifier. As a result, a demodulated signal is obtained, the shape of which (for a full-wave rectifier) ​​is shown in Fig. 14.1, v. This signal contains the original component with a frequency , for the selection of which a low-pass filter (LPF) with the corresponding frequency response is used.

A significant disadvantage of the amplitude modulation method is its low noise immunity. This is because an interference signal with a frequency , which is always present in the communication line, when added to the useful signal , changes, first of all, its amplitude. And since the amplitude of the AM oscillation is an informative parameter, after demodulation, the selected signal (see Fig. 14.1, G) differs markedly from the transmitted signal.

Test questions for lecture 6

6-1. How are data transmission systems classified according to the signal propagation medium they use?

6-2. What is used as a continuous transmission medium?

6-3. What is used as an open transmission medium?

6-4. List the types of wired communication lines?

6-5. What causes multiplicative noise?

6-6- What causes internal additive noise?

6-7. What is the cause of external additive noise?

6-8. List the main types of external additive noise?

6-9. What is the cause of galvanic interference?

6-10. What causes capacitive pickups?

6-11. What causes magnetic interference?

6-12. What causes electromagnetic interference?

6-13. What is used as the second wire in a single wire unbalanced line?

6-14. Why is a single wire line called unbalanced?

6-15. Draw the equivalent circuit of a single-wire unbalanced line?

6-16- Why does a single-wire unbalanced line cause common noise?

6-17. What are the components of a normal type interference?

6-18. What is the second signal wire used for in the simplest case?

6-19. Why does installing a second signal wire significantly reduce magnetic pickup?

6-20. Under what condition does the installation of a second signal wire attenuate galvanic pickup?

6-21. How can symmetrical signal transmission conditions be ensured on both wires of a two-wire line?

6-22. Why does twisting wires virtually eliminate the magnetic component of interference?

6-23. What tool is used to reduce capacitive interference?

6-24. Describe the construction of a coaxial cable.

6-25. What are the advantages of coaxial cable over balanced cable?

6-26- What makes coaxial cables high bandwidth?

6-27. How is the operating current distributed in the outer and inner wires of the coaxial cable depending on the frequency of the operating current?

6-28. How is the influencing current distributed in the outer and inner wires of the coaxial cable depending on the frequency of the influencing current?

6-29. How does the pitch of twisted-pair wires affect noise attenuation?

6-30. List the main elements of the FOCL linear path.

6-31. What is a light guide?

6-32. What is the directional transfer of energy in the fiber?

6-33. What determines the nature of the passage of optical radiation through the fiber?

6-34. What optical phenomena accompany the propagation of light through a fiber?

6-35. What is used as sources and receivers of light in FOCL?

6-36- What are the main advantages of SPD using FOCL?

6-37. What are line-of-sight radio relays?

6-38. How are tropospheric RRLs different from line-of-sight RRLs?

6-39. How are satellite RRLs different from tropospheric RRLs?

6-40. How does a satellite repeater differ from repeaters used on conventional RRL?

Lecture 7. Continuous methods of modulation and manipulation

When transmitting information over a continuous channel, a certain physical process is used, called a carrier or carrier.

The mathematical model of the carrier can be the function of time l(t,A,B,…), which also depends on the parameters A, B,….

Some function parameters are fixed under given transmission conditions, and then they can play the role of identifying parameters, i.e. they can be used to determine whether a given signal belongs to a certain class of signals.

Other parameters are affected by the transmitter. This effect on them is called modulation, and these parameters play the role of informative parameters.

In the general case, modulation is a mapping of the set of possible values ​​of the input signal to the set of values ​​of the informative parameter of the carrier. The device that modulates is called a modulator. One input of the modulator is affected by the implementation of the input signal x(t), on the other - a signal-carrier l(t,A). The modulator generates an output signal l(t,A), the informative parameter of which changes in time in accordance with the transmitted signal. In a narrower sense, modulation refers to the effect on the carrier, expressed in the multiplication of the informative, i.e. modulated parameter per multiplier , where h(t)- modulating function corresponding to the implementation x(t) input signal, defined so that ½h(t)½£1, a M is the modulation factor.

The main purpose of modulation is to transfer the signal spectrum to a given frequency region to enable its transmission over the channel and increase the noise immunity of the transmission.

Depending on the type of carrier used in the modulation, continuous and pulse types of modulation are distinguished. Continuous modulation uses a harmonic wave as the carrier. Pulse modulation uses a periodic train of rectangular pulses as the carrier.

Consider the basic principles of continuous modulation methods, when harmonic voltage is used as a carrier or carrier or modulated voltage, where is the voltage amplitude, is the carrier frequency, is the initial phase (Fig. 2.7).

LikBez > Radio communication

The first experience of transmitting speech and music by radio using the amplitude modulation method was made in 1906 by the American engineer R. Fessenden. The carrier frequency of 50 kHz of the radio transmitter was generated by a machine generator (alternator); for its modulation, a carbon microphone was switched on between the generator and the antenna, which changed the attenuation of the signal in the circuit. Since 1920, vacuum tube generators have been used instead of alternators. In the second half of the 1930s, with the development of ultrashort waves, amplitude modulation gradually began to be forced out of broadcasting and radio communications on VHF by frequency modulation. Since the middle of the 20th century, single-sideband modulation (SSB) has been introduced in service and amateur radio communications at all frequencies, which has a number of important advantages over AM.

The issue of transferring to the UBP and broadcasting was raised, but this would require the replacement of all broadcasting receivers with more complex and expensive ones, therefore it was not implemented. At the end of the 20th century, the transition to digital broadcasting began using amplitude-shifted signals. The oscillation parameter (amplitude, frequency, phase) that changes during the modulation process determines the name of the modulation. Accordingly, amplitude, frequency, phase. Mixed modulation is also possible, for example, amplitude-phase. The modulated signal is the result of superimposing the oscillations of the modulating signal on the oscillations of the carrier frequency.

In many cases, the modulating signal is in the form of a pulse, and the resulting burst of high frequency pulses. In multichannel communication systems, a sequence of radio pulses is used as an information carrier. Such a sequence is determined by four parameters amplitude, frequency, duration (width) and phase. Accordingly, several variants of pulse modulation are possible. Namely: amplitude-pulse, phase-pulse, frequency-pulse, pulse-width, code-pulse modulation. Pulse types of modulation are distinguished by increased noise immunity compared to the modulation of a continuous harmonic signal.

In terms of range, AM modulation loses to FM, as can be seen from the figure, the signal amplitude at some points in time with AM is less than with FM, hence the shorter range. To transmit the carrier frequency of a conventional radio signal with AM, a part of the power of the transmitting equipment (about 50%) is used. The way to increase the communication range on AM is to switch to single sideband modulation, which makes it possible to use the entire power of the transmitting equipment to transmit only the useful signal. There are other types of modulations, but they are less common or have applied value.

Signal modulation is the process of changing one signal to match the shape of another signal.
Modulation is carried out to transmit data using electromagnetic radiation. Usually, a sinusoidal signal (carrier) is subjected to modification. Distinguish:
- amplitude modulation;
- frequency modulation;

Modulation is a process in which a high frequency wave is used to carry a low frequency wave.

Amplitude modulation
In amplitude modulated (AM) systems, the modulating wave changes the amplitude of the high frequency carrier wave. An analysis of the output frequencies shows the presence of not only the input frequencies Fc and Fm, but also their sum and difference: Fc + Fm and Fc - Fm. If the modulating waveform is complex, such as a speech signal that consists of many frequencies, then the sums and differences of the various frequencies will occupy two bands, one below and one above the carrier frequency. They are called upper and lower lateral. The upper band is a copy of the original spoken signal, only shifted by the frequency Fc. The lower band is an inverted copy of the original signal, i.e. the upper frequencies in the original are the lower frequencies in the lower side. The lower sideband is a mirror image of the upper sideband with respect to the carrier frequency Fc. An AM system that transmits both side and carrier is known as a double sidebaud (DSB) system. The carrier carries no useful information and can be dropped, but with or without carrier, the bandwidth of the DSB signal is twice the bandwidth of the original signal. To narrow the band, it is possible to replace not only the carrier, but also one of the side ones, since they carry the same information. This type of operation is known as Single SideBand Suppressed Carrier (SSB-SC).
Demodulation of the AM signal is achieved by mixing the modulated signal with a carrier of the same frequency as on the modulator. The original signal is then obtained as a single frequency (or frequency band) and can be filtered from other signals. When using SSB-SC, the demodulation carrier is generated locally and may not match in any way the carrier frequency at the modulator. The small difference between the two frequencies is the cause of the frequency mismatch that is inherent in telephone circuits.

Amplitude modulation using digital signals
A special case of amplitude modulation is when the lower of the two amplitude levels is brought to zero, then the modulation process consists of turning the carrier on and off. However, surges in transmitted energy make this technique unsuitable for data transmission over communication networks.

Types of modulation: FM, AM, SSB
What is allowed, how the type of modulation affects the communication range.
Features of working with SSB.
In Russia, in the CB band, it is allowed to use frequency (FM), amplitude (AM) and single-sideband (SSB) modulation. What modulation is better to choose for communication?

First of all, your modulation must match that of your correspondent. The vast majority of Russian CB users use FM. It provides the highest quality sound if the signal of the correspondent is strong enough. Using FM allows you to suppress most types of interference that are amplitude in nature. The disadvantage of FM is the high noise level of the detector in the absence of a signal, which requires an accurate setting of the noise suppressor threshold.

AM is used for medium to long distance communications when the correspondent's signal is too weak to take advantage of FM. The maximum communication range when using AM and FM is almost the same.

Radio communication using a single sideband has such great advantages over AM and FM that it has completely replaced them in professional and amateur radio communications. SSB appeared on amateur radio bands in the fifties. In 195b there were only a few dozen amateur SSB radio stations in the world, in 1961 their number already exceeded 20,000. The first Soviet shortwave operator who made money on SSB was Georgy Rumyantsev (UA1DZ), one of the oldest Russian radio amateurs L. Labutin (UA3CR), who started working on SSB in 1958, did a lot to popularize work on SSB.

SSB modulation came to CBS much later: abroad - in the 90s, in Russia - only in the very last years.

The main reason for the low use of SSB in the CB band is the higher price of SSB transceivers, which is 3-5 times higher than the prices of AM / FM stations, the second reason is the peculiarities of working on SSB, which require a higher operator qualification.

When receiving a station with SSB modulation, you need to use the fine tuning knob to achieve the best intelligibility and naturalness of the correspondent's voice. This is what prevented the widespread use of SSB in car radios, for manual tuning of which the driver should not be distracted while driving. However, quite decent SSB car stations have recently appeared on the market, but the price is only 1.5-2 times more expensive than AM, FM stations, which have a frequency stability that is quite sufficient to work on SSB when the car is moving.

It must be borne in mind that even with fine tuning, the sound of the correspondent's voice when working on SSB still remains unnatural, with a specific "synthesized" timbre, which, however, does not interfere with the reception of information.

The main advantage of SSB compared to AM and FM is the gain in the power of the useful radiated signal, which is 9 dB, or 8 times. According to the rules adopted in Russia, the carrier power of a CB radio station with AM and FM modulation types and the peak power with SSB modulation should not exceed 10 watts. Where does the winning come from?

With SSB modulation, the carrier and one of the sidebands are not emitted, allowing all permitted power to be emitted as a single sideband. The power carrying useful speech information with AM and FM is at best 1.25 W, and with SSB it is all 10 W. Thus, when receiving an SSB signal from a transmitter with a peak power of 10 W, the audibility will be the same as when receiving an AM transmitter with a power of 80 W!

However, the benefits of SSB are not limited to this. AM and FM stations radiate carrier power all the time, whether you speak or remain silent in front of the microphone. SSB stations do not radiate any power between words. In addition to saving energy and facilitating the mode of the transmitter output stage, this provides additional benefits when working in a channel overloaded with stations. When using AM or FM modulations, the inclusion of a more powerful station completely "presses" the weaker one, making reception impossible, when using SSB in the pauses between the words of a powerful station, the weak station continues to be heard. It is possible not only to follow the station, but also to capture the meaning of the message. In practice, in such cases, it is possible to agree on a transition to another frequency. If the signal strength of the interfering stations is not much higher than the received one, and the frequencies of all stations are exactly the same, you will understand most of the information of the desired station, similar to how you understand the interlocutor when talking in an environment of talking people. In practice, the frequencies of interfering stations always differ from those received, therefore, due to a violation of the relationship between the frequency components of the spectrum, the speech of correspondents of interfering stations becomes illegible and it is much easier to focus all attention on the intelligible speech of your correspondent. This is only true, of course, in the case of interference from other SSB stations. If the interfering station operates with amplitude or frequency modulation, SSB does not provide advantages.

It is for this reason that CBS users in the range, in which there is no frequency separation for working with different types of modulation, agree among themselves on which channels only SSB can be used. So CBS users in Europe have agreed on the preferential use of the D band for working with SSB, leaving the C band for AM and FM.

All these advantages of SSB modulation make it possible, ceteris paribus, to obtain a communication range of 50-75% more than with AM or FM.

6. Types of modulation. Introduction to the specialty

6. Types of modulation

Telecommunication signaling principles

The transfer of a signal from one point in space to another is carried out by the telecommunication system. An electrical signal is, in fact, a form of representation of a message for transmission by a telecommunication system.

The source of the message (Fig. 6.1) generates a message a(t), which is converted into an electrical signal s(t) with the help of special devices. When transmitting speech, such a transformation is performed by a microphone, when transmitting an image, by a cathode-ray tube, and when transmitting a telegram, by the transmitting part of the telegraph apparatus.

To transmit a signal in a telecommunication system, you need to use some kind of carrier. As a carrier, it is natural to use those material objects that tend to move in space, for example, an electromagnetic field in wires (wire communication), in open space (radio communication), a light beam (optical communication). On fig. 6.2 shows the use of the frequency scale and waves of various types for various types of communication.

Thus, at the transmission point (Fig. 6.1), the primary signal s(t) must be converted into a signal v(t), convenient for its transmission over the appropriate propagation medium. The inverse transformation is performed at the receiving point. In some cases (for example, when the propagation medium is a pair of physical wires, as in a city telephone), this signal conversion may be absent.

The signal delivered to the receiving point must be again converted into a message (for example, using a telephone or loudspeaker when transmitting speech, a cathode ray tube when transmitting an image, the receiving part of a telegraph apparatus when transmitting a telegram) and then transmitted to the recipient.

The transmission of information is always accompanied by the inevitable action of interference and distortion. This leads to the fact that the signal at the output of the telecommunication system and the received message may differ to some extent from the signal at the input s(t) and the transmitted message a(t). The degree of compliance of the received message with the transmitted one is called the fidelity of information transmission.

For different messages, the quality of their transmission is evaluated differently. The received telephone message must be legible enough, the subscriber must be recognizable. For a television message, there is a standard (a table well known to all viewers on the TV screen), which evaluates the quality of the received image.

A quantitative assessment of the fidelity of the transmission of discrete messages is the ratio of the number of erroneously received message elements to the number of transmitted elements - the error rate (or error rate).

Amplitude modulation

Usually, a high-frequency harmonic oscillation - a carrier oscillation - is used as a carrier. The process of converting the primary signal consists in changing one or more parameters of the carrier wave according to the law of change of the primary signal (i.e., endowing the carrier wave with the characteristics of the primary signal) and is called modulation.

We write the harmonic oscillation chosen as the carrier in the following form:

This oscillation is completely characterized by three parameters: amplitude V, frequency w and initial phase j. Modulation can be done by changing any of the three parameters according to the law of the transmitted signal.

The change in the amplitude of the carrier wave over time is proportional to the primary signal s(t), i.e. V(t) = V + kAM s(t), where kAM is the proportionality factor, is called amplitude modulation (AM).

The carrier oscillation with the amplitude modulated according to the law of the primary signal is: v(t) = V(t)cos(wt + j). If the same harmonic oscillation (but with a lower frequency W) s(t) = ScosWt is used as the primary signal, then the modulated oscillation will be written in the form (j = 0 is taken for simplicity): v(t) = (V + kAMScosWt) coswt.

We take V out of the brackets and denote DV = kAMS and MAM = DV/V. Then

The parameter MAM = DV/V is called the amplitude modulation depth. For MAM = 0 there is no modulation and v(t) = v0(t), i.e. we obtain an unmodulated carrier wave (2.1). Typically, the carrier amplitude is chosen to be greater than the amplitude of the primary signal, so that MAM is 1.

On fig. 6.3 shows the form of the transmitted signal (a), the carrier waveform before modulation (b) and the amplitude-modulated carrier waveform (c).

Having multiplied in (6.2), we obtain that the amplitude-modulated oscillation

consists of the sum of three harmonic components with frequencies w, w + W and w – W and amplitudes V, MAMV/2 and MAMV/2, respectively. Thus, the spectrum of an amplitude-modulated oscillation (or AM oscillation) consists of the frequency of the carrier oscillation and two side frequencies, symmetrical with respect to the carrier, with the same amplitudes (Fig. 6.4, b). The spectrum of the primary signal s(t) is shown in fig. 6.4, a.

If the primary signal is complex and its spectrum is limited by frequencies and (Fig. 6.4, c), then the spectrum of the AM oscillation will consist of a carrier wave and two side bands symmetrical with respect to the carrier (Fig. 6.4, d).

An analysis of the energy relations shows that the main power of the AM oscillation lies in the carrier oscillation, which does not contain useful information. The lower and upper sidebands carry the same information and have lower power.

Angle modulation

It is possible to change in time in proportion to the primary signal s(t) not the amplitude, but the frequency of the carrier oscillation:

where is the coefficient of proportionality; value - is called the frequency deviation (in fact, this is the maximum deviation of the frequency of the modulated signal from the frequency of the carrier oscillation).

This type of modulation is called frequency modulation. On fig. 6.5 shows the change in the frequency of the carrier wave during frequency modulation.

When the phase of the carrier oscillation changes, we obtain phase modulation

where is the coefficient of proportionality; is the phase modulation index.

There is a close relationship between frequency and phase modulation. We represent the carrier oscillation in the form

where j is the initial phase of the oscillation, and Y(t) is its full phase. There is a connection between the phase Y(t) and the frequency w:

. (6.6)

Let us substitute expression (6.3) for w(t) in (6.6) with frequency modulation:

Value is called the frequency modulation index.

The frequency-modulated oscillation will be written as:

The phase-modulated oscillation, taking into account (6.4) for j(t), is the following:

From the comparison of (6.7) and (6.8) it follows that by the appearance of the signal v(t) it is difficult to distinguish which modulation is applied - frequency or phase. Often both of these types of modulation are called angular modulation, and MFM and MPM are called angular modulation indices.

The carrier wave subjected to angular modulation (6.7) or (6.8) can be represented as a sum of harmonic oscillations:

Here M is the index of angular modulation, which takes the value of MFM at FM and MFM at PM. The amplitudes of the harmonics in this expression are determined by some coefficients, the values ​​of which for various arguments are given in special reference tables. The larger M, the wider the spectrum of the modulated oscillation.

Thus, the spectrum of a modulated carrier with angular modulation, even with a harmonic primary signal s(t), consists of an infinite number of discrete components that form the lower and upper sidebands of the spectrum, symmetrical with respect to the carrier frequency and having the same amplitudes (Fig. 6.6).

If the primary signal s(t) has a shape other than sinusoidal and occupies a frequency band from to , then the spectrum of the modulated oscillation with angular modulation will have an even more complex form.

Sometimes, the modulation of a harmonic carrier wave in amplitude, frequency, or phase by discrete primary signals s(t), for example, telegraph or data transmission, is considered separately. On fig. 6.7 shows a discrete primary signal (a), a carrier wave modulated in amplitude (b), frequency (c), and phase (d).

The modulation of a harmonic carrier wave by the primary signal s(t) is called continuous, since a continuous periodic signal is chosen as the carrier.

Comparison of various types of continuous modulation makes it possible to reveal their features. With amplitude modulation, the width of the spectrum of the modulated signal is, as a rule, much smaller than with angular modulation (frequency and phase). Thus, there is a saving in the frequency spectrum: for amplitude-modulated signals, a narrower frequency band can be allocated during transmission. As will be shown below, this is especially important when building multichannel transmission systems.

Pulse modulation

Often, a periodic sequence of relatively narrow pulses is used as a carrier. A sequence of rectangular pulses of the same sign is characterized by the following parameters (Fig. 6.8): pulse amplitude V; duration (width) of pulses; repetition rate (or clock frequency), where T is the pulse repetition period (); the position (phase) of the pulses relative to the clock (reference) points. The ratio is called the pulse duty cycle.

According to the law of the transmitted primary signal, it is possible to change (modulate) any of the listed parameters of the pulse sequence. In this case, the modulation is called pulsed.

Depending on which parameter is modulated by the primary signal s (t), there are: amplitude-pulse modulation (AIM), when, according to the law of the transmitted signal (Fig. 6.8, a), the amplitude of the pulses changes (see Fig. 6.8, b); pulse-width modulation (PWM), when the pulse width changes (Fig. 6.8, c); pulse-frequency modulation (PFM) - the pulse repetition rate changes (see Fig. 6.8, d); pulse-phase modulation (PPM) - the phase of the pulses changes, i.e. temporal position relative to clock points (see Fig. 6.8, e).

The PIM and PFM modulation are combined into pulse-time modulation (PIM). Between them there is a relationship similar to the relationship between phase and frequency modulation of a sinusoidal oscillation.

Rice. 6.10. AIM signal spectrum

As an example, in fig. 6.10 shows the spectrum of the AIM signal when the pulse sequence is modulated by a complex primary signal s(t) with a frequency band from 0 to W. It contains the spectrum of the original signal s(t), all harmonics of the clock frequency (i.e. frequencies etc.) and sidebands around clock harmonics.

The spectra of PWM, PFM and PIM signals are even more complex.

The pulse sequences shown in fig. 6.8 are called sequences of video pulses. If the propagation medium allows, then video pulses are transmitted without additional conversions (for example, by cable). However, video pulses cannot be transmitted over radio links. Then the signal is subjected to the second stage of transformation (modulation).

By modulating a harmonic carrier oscillation of a sufficiently high frequency with the help of video pulses, radio pulses are obtained that can propagate through the air. The signals obtained as a result of combining the first and second stages of modulation can be called AIM-AM, PIM-AM, PIM-FM, etc.

Comparison of pulse modulation types shows that AIM has a smaller spectrum width compared to PWM and PWM. However, the latter are more resistant to interference. To justify the choice of modulation method in the transmission system, it is necessary to compare these methods according to various criteria: energy costs for signal transmission, noise immunity (the ability of modulated signals to withstand the harmful effects of interference), equipment complexity, etc.

Control questions

1. What is the structure of the messaging device?

2. What is the principle of amplitude (frequency, phase) modulation?

3. What is the difference between continuous modulation and pulse modulation?

4. How is the restoration of the original signal from the modulated one?

Bibliography

1. Telecommunication systems: Textbook for universities; Ed. V.P. Shuvalov. - M.: Radio and communication, 1987. - 512 p.

2. Baskakov S.I. Radio circuits and signals: Textbook. - 3rd ed., revised. and additional - M .: Higher. school, 2000. - 462 p.