Types of analog modulation. Types of modulation

I’ll warn you right away: it simply won’t work. Modulation is too complicated a thing.

To understand what modulation is, you need to know what frequency is, so let's start with that.
For example, let's take a swing: the swing frequency of a swing is the number of complete oscillations, swings per second.
Full, this means that one oscillation is the movement of the swing from the extreme left position, down, through the center to the maximum level on the right and then again through the center to the same level on the left.
An ordinary yard swing has a frequency of about 0.5 hertz, which means that it completes a full swing in 2 seconds.
The speaker of the sound column swings much faster, reproducing the note “A” of the first octave (440 hertz), it makes 440 vibrations per second.
IN electrical circuits oscillations are a voltage swing, from a maximum positive value, down, through zero voltage to a maximum negative value, up, through zero again to a maximum positive value. Or from maximum voltage, through some average to the minimum, then again through the average, again to the maximum.
On a graph (or oscilloscope screen) it looks like this:

The frequency of voltage fluctuations at the output of a radio station emitting a carrier on channel 18 of grid C in Europe will be 27,175,000 oscillations per second or 27 megahertz and 175 kilohertz (mega - million; kilo - thousand).

To make the modulation visual, let’s invent two certain signals, one with a frequency of 1000 Hz, the second with a frequency of 3000 Hz, graphically they look like this:

Let's notice how these signals are displayed on the graphs on the left. These are frequency and level graphs. The higher the frequency of the signal, the more to the right the signal will be shown on such a graph; the higher its level (power), the higher the line of this signal on the graph.

Now imagine that we have added both of these signals, that is, in finished form, our fictitious test signal is the sum of two signals. How did you put it together? It’s very simple - we put a microphone and sat two people in front of it: a man who screamed at a frequency of 1000 Hz and a woman who squealed at 3000 Hz, at the microphone output we received our test signal, which looks like this:

And it is precisely this test signal that we will “feed” to the microphone input of our fictitious transmitter, studying what is produced at the output (at the antenna) and how all this affects the intelligibility and range of communication.

About modulation in general

The modulated carrier signal at the output of any transmitter in any case (with any modulation) is obtained by adding or multiplying the carrier signal by the signal that needs to be transmitted, for example, the signal from the output of a microphone. The only difference between modulations is what is multiplied, what is added to, and in what part of the transmitter circuit this occurs.
In terms of reception, it all comes down to isolating from the received signal what the signal was modulated with, amplifying it and making it understandable (audible, visible).

Amplitude modulation - AM (AM, amplitude modulation)

As can be seen, when amplitude modulation The voltage level of high-frequency (carrier) oscillations directly depends on the magnitude of the voltage coming from the microphone.
The voltage at the microphone output increases, and the carrier voltage at the transmitter output also increases, that is, more power at the output, less voltage from the microphone, less voltage at the output. When the voltage at the microphone output is at a certain central position, the transmitter emits a certain central power (with AM modulation at 100% and silence in front of the microphone at 50% power).
AM modulation depth is the level of influence of the signal from the microphone on the output power level of the transmitter. If the wobble is 30%, then the strongest negative voltage pulse from the microphone will reduce the output carrier level by 30% of the maximum power.
And this is what the spectrum of a signal with AM modulation looks like (distribution of its components by frequency):

In the center, at a frequency of 27175000 Hz, we have the carrier, and lower and higher in frequency are the “sidebands,” that is, the sum of the carrier signal and the audio frequencies of our test signal:
27175000+1000Hz and 27175000-1000Hz
27175000+3000Hz and 27175000-3000Hz
The carrier-minus-audio signals are the lower sideband, and the carrier-plus-audio signals are the upper sideband.
It is not difficult to notice that only one sideband is enough to transmit information; the second one only repeats the same information, but only with the opposite sign, wasting the transmitter power on radiating this duplicate information into the air.
If you remove the carrier, which useful information does not contain one of the sidebands at all, then the result is SSB modulation (in Russian: OBP) - modulation with one sideband and a missing carrier (single sideband modulation).

SSB modulation (SSB, single sideband modulation)

This is what SSB looks like at the transmitter output:

It can be seen that this signal is not much different from AM modulation. It’s understandable, SSB is a continuation of AM, that is, SSB is created from AM modulation, from the signal of which the unnecessary sideband and carrier are removed.
If you look at the signal spectrum, the difference is obvious:

There is neither a carrier nor a duplicate sideband (this graph shows USB, i.e. single-sideband modulation, where the upper sideband is left, there is also LSB, this is when the lower sideband is left).
There is no carrier, no backup side - all the transmitter power is spent only on transmitting useful information.
It’s just impossible to receive such modulation on a regular AM receiver. To receive, you need to restore the “starting point” - the carrier. This is easy to do - the frequency at which the transmitter operates is known, which means you just need to add a carrier of the same frequency and the starting point will appear. The curious reader has probably already noticed that if the frequency of the transmitter is not known, then the starting point will be incorrect, we will add the wrong carrier, what will we hear? And at the same time we will hear the voice of either a “bull” or a “gnome”. This will happen because the receiver in this type of modulation does not know what frequencies we had initially, whether it was 1000Hz and 3000Hz, or 2000Hz and 4000Hz, or 500Hz and 2500Hz - the “distances” between the frequencies are correct, but began to shift, resulting in either a “pee-pee-pee” or a “boo-boo-boo.”

CW modulation (telegraph)

With the telegraph everything is simple - it is a 100% AM modulation signal, only sharp: either there is a signal at the output of the transmitter or there is no signal. The telegraph key is pressed - there is a signal, released - there is nothing.
The telegraph looks like this on the charts:

Accordingly, the spectrum of the telegraph signal:

That is, the carrier frequency is 100% modulated by pressing the telegraph key.
Why are there 2 rods on the spectrum, slightly departing from the “central frequency” signal, and not just one single one - the carrier?
Everything is simple here: be that as it may, a telegraph is AM, and AM is the sum of carrier and modulation signals, since a telegraph (Morse code) is a series of key presses, these are also oscillations with a certain frequency, albeit low compared to sound. It is at the frequency of pressing the key that the side bands of the telegraph signal recede from the carrier.
How to transmit such signals?
In the simplest case - by pressing the transmit button during silence in front of the microphone.
How to receive such signals?
To receive, you need to turn the carrier that appears on the air in time with the key presses into sound. There are many methods, the simplest is to connect a circuit to the output of the AM receiver detector that beeps every time voltage appears on the detector (i.e., a carrier is supplied to the detector). A more complex and reasonable way is to mix the signal coming from the air with the signal of the generator (local oscillator) built into the receiver, and feed the difference in the signals to an audio amplifier. So if the frequency of the signal on the air is 27175000Hz, the frequency of the receiver generator is 27174000, then at the input of the amplifier audio frequency a signal will arrive 27175000+27174000=54349000Hz and 27175000-27174000=1000Hz, naturally the first of them is not an audio signal but a radio signal, the sound amplifier will not amplify it, but the second one, 1000Hz, is an already audible sound and it will amplify it and we will hear “piiii”, while there is a carrier on the air and silence (air noise) when there is not.
By the way, when two people start transmitting at the same time, I think many people have noticed the “piiiiii” effect that arises from the addition and subtraction of carriers in the receiver. What is heard is the difference between the carrier signals occurring in our receiver.

FM modulation (FM, frequency modulation)

The actual essence of frequency modulation is simple: the carrier frequency changes slightly in time with the voltage at the microphone output. When the voltage at the microphone increases, the frequency also increases; when the voltage at the microphone output decreases, the carrier frequency also decreases.
The decrease and increase in carrier frequency occurs within small limits, for example, for CB radio stations it is plus/minus 3000 Hz with a carrier frequency of about 27,000,000 Hz, for FM broadcast stations it is plus/minus 100,000 Hz.
FM modulation parameter - modulation index. Sound ratio maximum frequency which the transmitter microphone amplifier will pass to the maximum change in carrier frequency at the loudest sound. It is not difficult to notice that for CB it is 1 (or 3000/3000), and for FM broadcast stations it is approximately 6 ... 7 (100000/15000).
With FM modulation, the carrier level (transmitter signal power) is always constant; it does not change depending on the volume of sounds in front of the microphone.
In graphical form, at the output of the FM transmitter, the modulation looks like this:

With FM modulation, as with AM, there is both a carrier and two sidebands at the output of the transmitter, since the carrier frequency dangles in time with the modulating signal, moving away from the center:

DSB, DChT, phase and other types of modulation

To be fair, it should be noted that there are other types of carrier modulation:
DSB - two sidebands and no carrier. DSB, essentially AM modulation in which the carrier has been removed (cut out, suppressed).
DCT - dual-frequency telegraph, in fact, is nothing more than frequency modulation, but by pressing a telegraph key. For example, a dot corresponds to a carrier shift of 1000 Hz, and a dash corresponds to 1500 Hz.
Phase modulation- carrier phase modulation. Frequency modulation at small indices 1-2, there is essentially phase modulation.

In some systems (television, FM stereo broadcasting), the modulation of the carrier is carried out by another modulated carrier, and it already carries useful information.
For example, to put it simply, an FM stereo broadcast signal is a carrier modulated by frequency modulation, the signal itself being a carrier modulated by DSB modulations, where one sideband is the left channel signal, and the other sideband is the right audio channel signal.

Important aspects of receiving and transmitting AM, FM and SSB signals

Since AM and SSB are modulations in which the transmitter output signal is proportional to the voltage coming from the microphone, it is important that it is linearly amplified on both the receiving and transmitting sides. That is, if the amplifier amplifies 10 times, then with a voltage at its input of 1 volt, the output should be 10 volts, and with 17 volts at the input, the output should be exactly 170 volts. If the amplifier is not linear, that is, at an input voltage of 1 volt, the gain is 10 and at the output 10 volts, and at 17 volts at the input the gain is only 5 and the output is 85 volts, then distortion will appear - wheezing and grunting with loud sounds in front of microphone. If the gain, on the contrary, is less for small input signals, then there will be wheezing when quiet sounds and unpleasant overtones even when loud (because at the beginning of its vibration, any sound passes through a zone close to zero).
The linearity of amplifiers for SSB modulation is especially important.

To equalize signal levels in AM and SSB receivers, special circuit components are used - automatic gain controllers (AGC circuits). The task of the AGC is to select such a gain of the receiver nodes that the strong signal (from a close correspondent) and the weak one (from a distant one) ultimately turn out to be approximately the same. If AGC is not used, then weak signals will be heard quietly, and strong ones will tear the receiver's sound emitter to shreds, like a drop of nicotine tears a hamster. If the AGC reacts too quickly to a change in level, then it will begin not only to equalize the levels of signals from close and distant correspondents, but also to “strangle” the modulation within the signal - reducing the gain when the voltage increases and increasing it when the voltage decreases, reducing all modulation to an unmodulated signal .

FM modulation does not require special linearity of amplifiers; with FM modulation, the information is carried by a change in frequency and no distortion or limiting of the signal level can change the frequency of the signal. Actually, in an FM receiver, a signal level limiter is generally installed, since the level is not important, the frequency is important, and changing the level will only interfere with highlighting frequency changes and turning the FM carrier into the sound of the signal with which it is modulated.
By the way, precisely because in the FM receiver all signals are limited, that is, weak noises have almost the same level as a strong useful signal, in the absence of an FM signal the detector (demodulator) makes so much noise - it tries to highlight the change the frequency of the noise at the input of the receiver and the noise of the receiver itself, and in the noise the change in frequency is very large and random, so random ones are heard strong sounds: Loud noise.
In an AM and SSB receiver, there is less noise in the absence of a signal, since the receiver noise itself is still low in level and the noise at the input is small in level compared to the useful signal, and for AM and SSB it is the level that is important.

For a telegraph, linearity is also not very important; there, information is carried by the very presence or absence of a carrier, and its level is only a secondary parameter.

FM, AM and SSB by ear

In AM and SSB signals, impulse noise is much more noticeable, such as the crackling sound of faulty car ignitions, the clicks of lightning discharges, or the rumble from pulse voltage converters.
How weaker signal, the lower its power, the quieter the sound at the receiver output, and the stronger, the louder. Although AGC does its job by leveling signal levels, its capabilities are not endless.
For SSB modulation, it is almost impossible to use a noise suppressor and generally understand when the other correspondent has released the transmission, since when there is silence in front of the microphone in SSB, the transmitter does not radiate anything into the air - there is no carrier, and if there is silence in front of the microphone, then there are no sidebands.

FM signals are less susceptible to impulse noise, but due to loud noise An FM detector in the absence of a signal is simply unbearable to sit without a noise suppressor. Each time the correspondent’s transmission is turned off in the receiver, it is accompanied by a characteristic “poof” - the detector has already begun to convert noise into sound, but the noise suppressor has not yet closed.

If you listen to an AM receiver on an FM receiver or vice versa, you will hear grunting, but you can still make out what they are talking about. If you listen to SSB on an FM or AM receiver, you will only get a wild audio mess of “oink-zhu-zhu-bzhu” and absolutely no intelligibility.
On an SSB receiver you can perfectly listen to CW (telegraph), AM, and, with some distortion, FM with low modulation indices.

If two or more AM or FM radio stations are turned on simultaneously on the same frequency, then the result is a mess of carriers, a kind of squeaking and screeching among which nothing can be heard.
If two or more SSB transmitters turn on at the same frequency, then everyone who spoke will be heard in the receiver, since SSB has no carrier and there is nothing to beat (mix until it whistles). You can hear everyone, as if everyone were sitting in the same room and started talking at once.

If in AM or FM the receiver frequency does not exactly match the transmitter frequency, then distortion and “wheezing” appear on loud sounds.
If the frequency of an SSB transmitter changes in time with the signal level (for example, the equipment does not have enough power), then gurgling can be heard in the voice. If the frequency of the receiver or transmitter floats, then the sound floats in frequency, then “mumbles”, then “chirps”.

Efficiency of modulation types - AM, FM and SSB

Theoretically, I emphasize - theoretically, with equal transmitter power, the communication range will depend on the type of modulation as follows:
AM = Distance * 1
World Cup = Distance * 1
SSB = Distance * 2
In that same theory, energetically, SSB outperforms AM by 4 times in power, or 2 times in voltage. The gain appears due to the fact that the transmitter power is not wasted on emitting a useless carrier and wastingly duplicating the information of the second sideband.
In practice, the gain is less, since the human brain is not used to hearing the noise of the air in the pauses between loud sounds and legibility suffers somewhat.
FM is also modulation “with a surprise” - some smart books say that AM and FM are no better than each other, and even FM is worse, others claim that with low modulation indices (and these are CB and amateur radio stations) FM outperforms AM 1.5 times. In fact, according to the author’s subjective opinion, FM is about 1.5 times more “punchy” than AM, primarily because FM is less susceptible to impulse noise and signal level fluctuations.

AM, FM and SSB equipment in terms of complexity and conversion of one into another

The most complex equipment is SSB.
In fact, an SSB device can easily work in AM or FM after negligible modifications.
It is almost impossible to convert an AM or FM transceiver to SSB (you will need to introduce many, many additional components into the circuit and completely remake the transmitter unit).
From the author: personally, converting an AM or FM device into SSB seems completely insane to me.
I assembled the SSB device from scratch, but not to convert AM or FM into SSB.

The second most difficult is the FM apparatus.
In fact, the FM device already contains in the receiver everything that is needed to detect AM signals, since it also has an AGC ( automatic adjustment amplification) and therefore a detector of the level of the received carrier, that is, essentially a full-fledged AM receiver, only working somewhere inside (the threshold noise suppressor also works from this part of the circuit).
It will be more difficult with the transmitter, since almost all of its stages operate in a non-linear mode.
From the author: it is possible to redo it, but there was never a need for it.

AM equipment is the simplest.
To convert an AM receiver to FM, you will need to introduce new components - a limiter and an FM detector. In fact, the limiter and FM detector are 1 microcircuit and a few parts.
Converting an AM transmitter to FM is much simpler, since you only need to introduce a chain that will “chatter” the carrier frequency in time with the voltage coming from the microphone.
From the author: I converted the AM transceiver to AM/FM a couple of times, in particular the CB radio stations “Cobra 23 plus” and “Cobra 19 plus”.

Test questions for lecture 6

6-1. How are data transmission systems classified based on 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 causes external additive interference?

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

6-9. What causes galvanic interference?

6-10. What causes capacitive interference?

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 interference occurs in a single-wire unbalanced line general view?

6-17. What components does the interference contain? normal looking?

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

6-19. Why does installing a second signal wire significantly weaken magnetic interference?

6-20. Under what condition does installing a second signal wire weaken galvanic interference?

6-21. How can you ensure symmetrical conditions for transmitting signals along both wires of a two-wire line?

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

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

6-24. Describe the design coaxial cable.

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

6-26- What provides the high bandwidth of coaxial cables?

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

6-29. How does the twisting pitch of twisted pair wires affect interference reduction?

6-30. List the main elements of a linear fiber-optic link.

6-31. What is a light guide?

6-32. What causes directed energy transfer in a light guide?

6-33. What determines the nature of the passage of optical radiation through a light guide?

6-34. What optical phenomena accompany the propagation of light along a light guide?

6-35. What is used as light sources and receivers in fiber-optic communication lines?

6-36- What are the main advantages of SPD using fiber-optic communication lines?

6-37. What are they? radio relay lines line of sight?

6-38. How do tropospheric RRLs differ from line-of-sight RRLs?

6-39. How do satellite RRLs differ from tropospheric RRLs?

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

Lecture 7. Continuous modulation and manipulation methods

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

Mathematical model the carrier may serve as a 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 act as 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 general, modulation is a mapping of a set of possible values input signal to a set of values of the informative parameter of the carrier. The device that performs modulation is called a modulator. One modulator input is affected by the implementation of the input signal x(t), on the other – the carrier signal l(t,A). The modulator generates an output signal l(t,A), the informative parameter of which changes over time in accordance with the transmitted signal. In a narrower sense, modulation refers to the effect on the carrier, expressed in the multiplication of information, i.e. modulated parameter by multiplier , Where h(t)- modulating function corresponding to the implementation x(t) input signal, determined so that ½h(t)½£1, A M– modulation coefficient.

The main purpose of modulation is to transfer the spectrum of a signal to a given frequency region to ensure the possibility of transmitting it over a channel and increasing the noise immunity of transmission.

Depending on the type of carrier used in modulation, continuous and impulse types modulation. With continuous modulation, a harmonic oscillation is used as a carrier wave. Pulse modulation uses a periodic sequence of rectangular pulses as a carrier.

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

Modulation is a process transformation of one or more characteristics of a modulating high-frequency oscillation under the influence of 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 for the purpose of transmitting information through. The transmitted data is contained in the control signal. And the carrier function is performed by a high-frequency oscillation, called a carrier. Oscillations of various shapes can be used as carrier vibrations: sawtooth, rectangular, etc., but usually harmonic sinusoidal ones are used. Based on what specific characteristic sinusoidal oscillation changes, there are several types of modulation:

Amplitude modulation

The modulating and reference signals are transmitted to the input of the modulating device, resulting in a modulated signal at the output. The condition for correct conversion is considered to be double the value of the carrier frequency in comparison with maximum value baseband bands. This type of modulation is quite simple to implement, but is characterized by low noise immunity.

Noise instability occurs due to the narrow bandwidth of the modulated signal. It is used mainly in the mid- 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 rather than the power. Therefore, if the signal magnitude increases, then the frequency increases accordingly. Due to the fact that the bandwidth of the received signal is much wider than the original signal value.

This modulation is characterized by high noise immunity, but for its application it is necessary to use the high-frequency range.

Phase modulation

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

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

Undamped functions, noise, a sequence of pulses, etc. can be used as a carrier signal. Thus, with pulse modulation, a sequence of narrow pulses is used as a carrier signal, and a discrete or analog signal. Since the pulse sequence is characterized by 4 characteristics, there are 4 types of modulation:

— frequency-pulse;

— pulse width;

— amplitude-pulse;

- phase-pulse.

To carry out effective signal transmission in any medium, it is necessary to transfer the spectrum of these signals from the low-frequency region to the region of sufficiently high frequencies. This procedure is called modulation in radio engineering.

The essence of modulation is as follows. A certain oscillation (most often harmonic), called a carrier oscillation or simply a carrier, is formed, and any of the parameters of this oscillation changes over time in proportion to the original signal. The original signal is called modulating, and the resulting oscillation with time-varying parameters is called a modulated signal. The reverse process - separating the modulating signal from the modulated oscillation - is called demodulation.

Classification of modulation types:

1) by type of information signal (modulating signal);

Continuous modulation (analog signal);

Discrete modulation (discrete signal);

2) by type of carrier (or carrier frequency)

Harmonic (sinusoidal signal);

Pulse (rectangular periodic pulse).

3) by the type of carrier frequency parameters that undergo changes under the influence of the information signal.

Amplitude modulation;

Frequency modulation;

Phase modulation;

Width modulation;

Pulse width modulation (Figure 1.1).

Figure 1.1 – Types of modulation

General harmonic signal:

S (t) = A cos(ω 0 t+ φ 0).

This signal has three parameters: amplitude A, frequency ω 0 and initial phase φ 0. Each of them can be associated with a modulating signal, thus obtaining three main types of modulation: amplitude, frequency and phase. Frequency modulation and phase modulation are very closely related, since they both affect the argument of the cos function. Therefore, these two types of modulation have common name- corner

modulation.

Currently, an increasing part of the information transmitted through various communication channels exists in digital form. This means that it is not a continuous (analog) modulating signal that is to be transmitted, but a sequence of integers n 0 , p 1, n 2 , ..., which can take values from some fixed finite set. These numbers, called symbols, come from a source of information with a period T, and the frequency corresponding to this period is called the symbol rate: f T = 1/T.

A variant often used in practice is binary sequence of characters when each of the numbers n i can take one of two values - 0 or 1.

The sequence of transmitted symbols is obviously a discrete signal. Since the symbols take values from a finite set, this signal is actually quantized, that is, it can be called digital signal.

A typical approach to transmitting a discrete sequence of characters is as follows. Each of the possible symbol values is associated with a certain set of carrier vibration parameters. These parameters are maintained constant during the interval T, that is, until the next symbol arrives. This actually means converting a sequence of numbers { n k } to step signal S n (t) using piecewise constant interpolation:

s n (t)=f(n k ), kT

Here f is some transformation function. Received signal S n (t) is then used as a modulating signal in the usual way.

This method of modulation, when the parameters of the carrier oscillation change abruptly, is called manipulation. Depending on which parameters are changed, they distinguish between amplitude (AM), phase (PM), and frequency (FM). In addition, when transmitting digital

information, a carrier wave of different shape can be used

from harmonic. Thus, when using a sequence of rectangular pulses as a carrier oscillation, pulse amplitude (APM), pulse width (PWM) and pulse time (PMT) modulation are possible. PAM - pulse amplitude modulation is that the amplitude of the pulse carrier changes according to the law of changes in the instantaneous values of the primary signal.

PFM – pulse frequency modulation. According to the law of changes in the instantaneous values of the primary signal, the repetition rate of the carrier pulses changes.

VIM is time-pulse modulation, in which the information parameter is the time interval between the synchronizing pulse and the information pulse.

PWM – pulse width modulation. The point is that, according to the law of changes in the instantaneous values of the modulating signal, the duration of the carrier pulses changes.

PPM – pulse phase modulation, differs from VIM by the method of synchronization. The phase shift of the carrier pulse does not change relative to the synchronizing pulse, but relative to some conventional phase.

PCM – pulse-code modulation. It cannot be considered as a separate type of modulation, since the value of the modulating voltage is represented in the form of code words.

SIM – counting pulse modulation. It is a special case of PCM, in which the information parameter is the number of pulses in the code group.

At amplitude keying a single symbol is transmitted by HF padding, and a zero symbol by the absence of a signal. The amplitude-manipulated signal is described by the expression:

where the amplitude term can take M discrete values, and the phase term φ is an arbitrary constant. The AM signal shown in Figure 1.2 (c) can correspond to a radio transmission using two signals, the amplitudes of which are 0 and .

Amplitude manipulation is the simplest, but at the same time the least noise-resistant and is currently practically not used.

At frequency discrete modulation(FM, FSK–Frequency Shift Keying) values 0 and 1 of the information bit correspond to their own frequencies of the physical signal with its amplitude unchanged. The general analytical expression for a frequency-shift keyed signal is as follows:

Here the frequency ω i can take M discrete values, and the phase φ is an arbitrary constant. A schematic representation of the FM signal is shown in Figure 1.2 b, where you can observe a typical change in frequency at the moments of transitions between symbols.

Frequency modulation is very noise-resistant, since it is mainly the signal amplitude, not the frequency, that is distorted by interference. In this case, the reliability of demodulation, and therefore the noise immunity, is higher, the more signal periods fall into the baud interval. But increasing the baud interval, for obvious reasons, reduces the speed of information transfer. On the other hand, the signal spectrum width required for this type of modulation can be significantly narrower than the entire channel bandwidth. This leads to the area of application of FM - low-speed, but highly reliable standards that allow communication on channels with large distortions of the amplitude-frequency response, or even with a truncated bandwidth.

At phase shift keying 1 and 0 differ in the phase of the high-frequency oscillation. The phase-keyed signal has the following form:

Here the phase component φ i (t) can accept M discrete values, usually defined as follows:

where E is the energy of the symbol;

T – symbol transmission time.

Figure 1.2a shows an example of binary (M=2) phase shift keying, where characteristic sharp phase changes are clearly visible during the transition between symbols.

In practice, phase shift keying is used when the number of possible initial phase values is small - typically 2.4 or 8. In addition, it is difficult to measure when receiving a signal absolute initial phase value; much easier to determine relative phase shift between two adjacent symbols. Therefore, phase difference or relative phase shift keying is usually used.

At phase difference modulation(DOPSK, TOPSK, DPSK – Differential Phase Shift Keying) the parameter that changes depending on the value of the information element is the phase of the signal with constant amplitude and frequency. In this case, each information element is associated not with the absolute value of the phase, but with its change relative to the previous value.

According to CCITT recommendations, at a speed of 2400 bps, the data stream to be transmitted is divided into pairs of consecutive bits (dibits), which are encoded into a phase change with respect to the phase of the previous signal element. One signal element carries 2 bits of information. If the information element is a dibit, then depending on its value (00, 01, 10 or 11), the phase of the signal can change by 90, 180, 270 degrees or not change at all.

With triple relative phase modulation or eightfold

In phase difference modulation, the data stream to be transmitted is divided into triplets of consecutive bits (tribits), which are encoded into a change in phase with respect to the phase of the previous signal element. One signal element carries 3 bits of information.

Phase modulation is the most informative, however, increasing the number of coded bits above three (8 phase rotation positions) leads to a sharp decrease in noise immunity. Therefore, at high speeds, combined amplitude-phase modulation methods are used.

Amplitude-phase manipulation. Amplitude phase keying (APK) is a combination of ASK and PSK schemes. The ARC modulated signal is shown in Fig. 1.2 G and is expressed as

with indexing of amplitude and phase terms. In Fig. 1. 2 G one can see characteristic simultaneous (at the moments of transition between symbols) changes in the phase and amplitude of the ARC-modulated signal. In the given example M=8, which corresponds to 8 signals (octal transmission). A possible set of eight signal vectors is plotted in phase-amplitude coordinates. Four of the vectors shown have one amplitude, four more have another. The vectors are oriented so that the angle between the two closest vectors is 45°.

Figure 1.2 – Types of digital modulations

If in the two-dimensional space of signals between M dialing signals at a straight angle, the scheme is called quadrature amplitude modulation (QAM).

Quadrature amplitude modulation

It should be noted that another type of linear modulation is quadrature amplitude modulation (QAM), the essence of which is the transmission of two different signals using AM or FM methods on the same carrier frequency. The spectra of these two signals completely overlap and their separation using filters is impossible. To maintain the possibility of signal separation at the receiving side, the oscillation carriers are supplied to the modulators with a phase shift of 90° (in quadrature).

Figure 1.3 shows the QAM signal generation diagram.

Figure 1.3 – Quadrature AM

The advantage of QAM compared to conventional AM or BM is twice the number of signals that can be independently transmitted in the same frequency band.

Angle (frequency and phase) modulation

Angle modulation is usually used when it is necessary to ensure high fidelity of reception of the transmitted message. This is explained by the fact that systems with angular modulation have increased resistance to noise and other types of interference compared to AM. It is known, for example, that FM systems have properties to suppress additive noise interference. This means that when FM is detected, the signal-to-noise ratio is significantly improved. However, this advantage is achieved at the cost of deterioration of other signal parameters, in particular at the cost of increasing the occupied frequency band. Frequency modulation is perhaps the most common example that illustrates methods for increasing the noise immunity of communication systems based on spreading the signal spectrum.

Figure 1.4 shows the timing diagram of the signal with single-tone angle modulation.

Figure 1.4 Angle modulation: a - modulating low-frequency signal; b - single-tone signal with angular modulation

The angular modulation (AM) signal with a harmonic carrier can be written as follows:

u UM (t)= U 0 cos[(t)]=U 0 cos[ω 0 t+φ(t)],

where (t)=ω 0 t+φ(t) – total phase of the signal;

φ(t) – phase, which carries information about the primary signal.

There are two types of PA: phase (PM) and frequency (FM). With PM, phase changes are directly proportional to the primary signal

Where φ 0 is the initial phase.

In FM, the instantaneous frequency of the signal is directly proportional to the primary signal

, where is the conversion coefficient of the control signal into a change in the frequency of the signal at the output of the frequency modulator.

The shapes of PM and FM signals do not differ from each other if the time derivative of the primary signal has the same form as the primary signal itself. This occurs with a sinusoidal primary signal, for example

b(t)=Usint .

The PA signal in this case can be written as follows:

u UM (t)=U 0 cos(ω 0 t+Msint),

where M is the modulation index.

The FM index is determined as

M FM ==K FM U  ( – phase deviation).

The World Cup index is

M FM ==K FM U  /,

where the frequency deviation is K FM U  . therefore, the World Cup index

M FM =/=f / F.

Let's find the signal spectrum for PA with one tone. Let us represent the signal with PA in one tone using the following expression:

(Re is the real part).

Because during the World Cup

M FM =/=f /F,

then we find that for large modulation indices

f mind 2f ,

i.e., the frequency bandwidth at FM is equal to twice the frequency deviation and does not depend on the modulation frequency F.

Figures 1.5 and 1.6 show schemes for obtaining angle modulation signals

where b(t) is the primary signal;

–carrier generator U0cosω0t ;

block -/2 rotates the phase by angle -/2;

We continue the series of general education articles under the general title “Theory of Radio Waves.”
In previous articles we got acquainted with radio waves and antennas: Let's take a closer look at radio signal modulation.

Within the framework of this article, analog modulation of the following types will be considered:
Amplitude modulation

Amplitude modulation with one sideband

Frequency modulation

Linear frequency modulation

Phase modulation

Differential phase modulation

Amplitude modulation
With amplitude modulation, the envelope of the amplitudes of the carrier vibration changes according to a law that coincides with the law of the transmitted message. The frequency and phase of the carrier oscillation does not change.
One of the main parameters of AM is the modulation coefficient (M).
The modulation coefficient is the ratio of the difference between the maximum and minimum values of the amplitudes of the modulated signal to the sum of these values (%).
Simply put, this coefficient shows how much the amplitude of the carrier vibration at a given moment deviates from the average value.
When the modulation factor is greater than 1, an overmodulation effect occurs, resulting in signal distortion.
AM spectrum
This spectrum is characteristic of a modulating oscillation of a constant frequency.
On the graph, the X axis represents the frequency, and the Y axis represents the amplitude.
For AM, in addition to the amplitude of the fundamental frequency located in the center, amplitude values to the right and left of the carrier frequency are also presented. These are the so-called left and right side stripes. They are separated from the carrier frequency by a distance equal to the modulation frequency.
The distance from the left to the right side strip is called spectrum width.
In the normal case, with a modulation coefficient<=1, амплитуды боковых полос меньше или равны половине амплитуды несущей.
Useful information is contained only in the upper or lower side bands of the spectrum. The main spectral component, the carrier, does not carry useful information. The transmitter power during amplitude modulation is mostly spent on “heating the air”, due to the lack of information content of the most basic element of the spectrum.
Single sideband amplitude modulation
Due to the ineffectiveness of classical amplitude modulation, single sideband amplitude modulation was invented.
Its essence is to remove the carrier and one of the sidebands from the spectrum, while all the necessary information is transmitted over the remaining sideband.
But in its pure form, this type did not take root in household radio broadcasting, because In the receiver, the carrier must be synthesized with very high accuracy. Used in compaction equipment and amateur radio.
In radio broadcasting, AM with one sideband and a partially suppressed carrier is more often used:
With this modulation the quality/efficiency ratio is best achieved.
Frequency modulation
A type of analog modulation in which the carrier frequency changes according to the law of the modulating low-frequency signal. The amplitude remains constant.
a) - carrier frequency, b) modulating signal, c) modulation result
The largest frequency deviation from the average value is called deviation.
Ideally, the deviation should be directly proportional to the amplitude of the modulating oscillation.
The frequency modulation spectrum looks like this:
It consists of a carrier and sideband harmonics symmetrically lagging behind it to the right and left, at a frequency that is a multiple of the frequency of the modulating oscillation.
This spectrum represents a harmonic vibration. In the case of real modulation, the spectrum has more complex shapes.
There are broadband and narrowband FM modulation.
In broadband, the frequency spectrum significantly exceeds the frequency of the modulating signal. Used in FM radio broadcasting.
Radio stations mainly use narrowband FM modulation, which requires more precise tuning of the receiver and, accordingly, is more protected from interference.
Broadband and narrowband FM spectra are presented below
The spectrum of narrowband FM resembles amplitude modulation, but when you consider the phase of the sidebands, these waves appear to have constant amplitude and variable frequency, rather than constant frequency and variable amplitude (AM). With wideband FM, the carrier amplitude can be very small, which results in high FM efficiency; this means that most of the transmitted energy is contained in the side frequencies that carry information.
The main advantages of FM over AM are energy efficiency and noise immunity.
Linear frequency modulation is a type of FM.
Its essence lies in the fact that the frequency of the carrier signal changes according to a linear law.
The practical significance of linear frequency modulated (chirp) signals lies in the possibility of significant compression of the signal during reception with an increase in its amplitude above the noise level.
Chirps are used in radar.
Phase modulation
In reality, the term phase manipulation is more commonly used, because They mainly modulate discrete signals.
The meaning of PM is that the phase of the carrier changes abruptly with the arrival of the next discrete signal, different from the previous one.
From the spectrum you can see the almost complete absence of a carrier, which indicates high energy efficiency.
The disadvantage of this modulation is that an error in one symbol can lead to incorrect reception of all subsequent ones.
Differential phase shift keying
In the case of this modulation, the phase does not change with each change in the value of the modulating pulse, but with a change in the difference. In this example, when each “1” arrives.
The advantage of this type of modulation is that if a random error occurs in one symbol, this does not entail a further chain of errors.
It is worth noting that there are also phase manipulations such as quadrature, which uses a phase change within 90 degrees and higher-order PM, but their consideration is beyond the scope of this article.
PS: I want to note once again that the purpose of the articles is not to replace a textbook, but to tell you “at a glance” about the basics of radio.
Only the main types of modulations are considered to create the reader’s idea of the topic.