When faced with new concepts in everyday life, many try to find answers to their questions. This is why it is necessary to describe any phenomena. One of them is such a concept as modulation. This will be discussed further.

general description

Modulation is the process of changing one or a whole set of high-frequency vibration parameters in accordance with the law of low-frequency information messages. The result of this is a transfer of the spectrum of the control signal to the high frequency region, since effective broadcasting into space requires that all transceiver devices operate at different frequencies without interrupting each other. Thanks to this process, information vibrations are placed on a carrier that is known a priori. The control signal contains the transmitted information. High-frequency vibration takes on the role of a carrier of information, due to which it acquires the status of a carrier. The control signal contains the transmitted data. There are different types of modulation, which depend on what form of oscillation is used: rectangular, triangular or some other. With a discrete signal, it is customary to talk about manipulation. So, modulation is a process that involves oscillations, so it can be frequency, amplitude, phase, etc.

Varieties

Now we can consider what types of this phenomenon exist. Basically, modulation is a process in which a low-frequency wave is transferred to a high-frequency one. The most commonly used types are frequency, amplitude and phase. When the frequency changes, when the amplitude changes, the amplitude changes, and when the phase changes, the phase changes. There are also mixed types. Pulse modulation and modification are separate types. In this case, the parameters of the high-frequency oscillation change discretely.

Amplitude modulation

In systems with this type of change, the amplitude of the carrier wave changes at a high frequency using a modulating wave. At the output, not only the input frequencies are detected, but also their sum and difference. In this case, if the modulation is a complex wave, such as speech signals consisting of many frequencies, then the sum and difference of frequencies will require two bands, one below the carrier and the other above. They are called lateral: top and bottom. The first is a copy of the original one, shifted to a certain frequency. The lower band is a copy of the original signal that has been inverted, that is, the original high frequencies are the lower frequencies in the lower sideband.

The lower sidewall is a mirror image of the upper sidewall relative to the carrier frequency. A system that uses amplitude modulation, transmitting the carrier and both sidebands, is called two-way. The carrier contains no useful information, so it can be removed, but in any case the signal bandwidth will be twice as large as the original one. Band narrowing is achieved by displacing not only the carrier, but also one of the sidebands, since they contain the same information. This type is known as single sideband suppressed carrier modulation.

Demodulation

This process requires mixing the modulated signal with a carrier of the same frequency as that emitted by the modulator. After this, the original signal is obtained as a separate frequency or frequency band, and then filtered from other signals. Sometimes the carrier for demodulation is generated locally, but it does not always coincide with the carrier frequency on the modulator itself. Due to the small difference between frequencies, mismatches appear, which is typical for telephone circuits.

This uses a digital baseband signal, meaning it allows more than one bit per baud to be encoded by encoding a binary data signal into a signal with multiple levels. Bits of binary signals are sometimes split into pairs. For a pair of bits, four combinations can be used, with each pair being represented by one of four amplitude levels. This encoded signal is characterized in that the modulation baud rate is half that of the original data signal, so it can be used for amplitude modulation in the usual way. It found its application in radio communications.

Frequency modulation

Systems with such modulation assume that the carrier frequency will change according to the shape of the modulating signal. This type is superior to amplitude in terms of resistance to certain influences present on the telephone network, so it should be used at low speeds where there is no need to involve a large frequency band.

Phase-amplitude modulation

To increase the number of bits per baud, you can combine phase and amplitude modulation.

One of the modern methods of amplitude-phase modulation is one that is based on the transmission of several carriers. For example, an application uses 48 carriers separated by a 45 Hz bandwidth. By combining amplitude and phase modulation, each carrier is allocated up to 32 discrete states per baud period, allowing 5 bits per baud to be carried. It turns out that this whole set allows you to transfer 240 bits per baud. When operating at 9600 bps, the modulation rate requires only 40 baud. Such a low figure is quite tolerant of amplitude and phase jumps inherent in the telephone network.

Pulse code modulation

This type is usually considered as a system for broadcasting, for example, voice in digital form. This modulation technique is not used in modems. This involves gating the analog signal at a rate twice the highest frequency component of the signal in analog form. When such systems are used on telephone networks, gating occurs 8000 times per second. Each sample is a voltage level encoded in a seven-bit code. For best representation, logarithmic coding is used. Seven bits together with the eighth, indicating the presence of a signal, form an octet.

To reconstruct the message signal, modulation and detection are required, that is, the reverse process. In this case, the signal is converted in a nonlinear way. Nonlinear elements enrich the output signal spectrum with new spectral components, and filters are used to highlight low-frequency components. Modulation and detection can be carried out using vacuum diodes, transistors, semiconductor diodes as nonlinear elements. Traditionally, point-type semiconductor diodes are used, since planar diodes have a noticeably larger input capacitance.

Modern views

Digital modulation provides much greater information capacity and compatibility with a variety of digital data services. In addition, with its help, the security of information is increased, the quality of communication systems is improved, and access to them is accelerated.

There are a number of limitations that developers of any systems face: permissible power and frequency bandwidth, a given noise level of communication systems. Every day the number of users of communication systems increases, as well as the demand for them, which requires an increase in radio resources. Digital modulation is noticeably different from analog modulation in that the carrier in it transmits large amounts of information.

Difficulty of use

The main task facing developers of digital radio communication systems is to find a compromise between the bandwidth of data transmission and the complexity of the system in technical terms. To do this, it is appropriate to use different modulation methods to obtain the desired result. Radio communication can be organized using the simplest transmitter and receiver circuits, but for such communication a frequency spectrum proportional to the number of users will be used. More complex receivers and transmitters require less bandwidth to transmit the same amount of information. To move to spectrally efficient transmission methods, it is necessary to complicate the equipment accordingly. This problem does not depend on the type of connection.

Alternative options

Pulse width modulation is characterized by the fact that its carrier signal is a sequence of pulses, while the pulse frequency is constant. The changes concern only the duration of each pulse according to the modulating signal.

Pulse-width modulation differs from frequency-phase modulation. The latter involves modulating the signal in the form of a sinusoid. It is characterized by constant amplitude and variable frequency or phase. Pulse signals can also be modulated in frequency. The duration of the pulses may be fixed, and their frequency is within a certain range, but their instantaneous value will vary depending on the modulating signals.

conclusions

Simple types of modulation can be used, with only one parameter changing according to the modulating information. A combined modulation scheme, which is used in modern communication equipment, is when both the amplitude and phase of the carrier change simultaneously. Modern systems can use several subcarriers, each of which uses a certain type of modulation. In this case we are talking about signal modulation schemes. This term is also used for complex multi-level views, when additional information is required for comprehensive information.

Modern communication systems use the most efficient types of modulation, thereby minimizing bandwidth to free up frequency space for other types of signals. The quality of communication only benefits from this, but the complexity of the equipment in this case turns out to be very high. Ultimately, the modulation frequency gives a result that is visible to the end user only in terms of ease of use of technical means.

Modulation is a process transformation of one or more characteristics of a modulating high-frequency oscillation under the influence of a low-frequency control 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 the 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 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, resulting in a modulated signal at the output. The condition for correct conversion is considered to be twice the value of the carrier frequency in comparison with the maximum value of the modulating signal bandwidth. 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. With this type of 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 acts as a modulating signal. Since the pulse sequence is characterized by 4 characteristics, there are 4 types of modulation:

— frequency-pulse;

— pulse width;

— amplitude-pulse;

- phase-pulse.

LickSec > Radio communication

The first experiment in transmitting speech and music via 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); to modulate it, a carbon microphone was connected between the generator and the antenna, changing the attenuation of the signal in the circuit. Since 1920, generators based on vacuum tubes began to be used instead of alternators. In the second half of the 1930s, with the development of ultrashort waves, amplitude modulation gradually began to be replaced from VHF radio broadcasting and radio communications 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 question of transferring radio broadcasting to OBP was raised, but this would have required replacing all broadcast receivers with more complex and expensive ones, so it was not implemented. At the end of the 20th century, the transition to digital broadcasting began using signals with amplitude manipulation. Modulation (from the Latin modulation - dimensionality, dimensionality) is a change in time according to a given law of parameters characterizing any stationary physical process. The oscillation parameter changed during the modulation process (amplitude, frequency, phase) determines the name of the modulation. Accordingly, amplitude, frequency, phase. Mixed modulation is also possible, for example amplitude-phase. A modulated signal is the result of the superposition of oscillations of the modulating signal on oscillations of the carrier frequency.

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

In terms of range, AM modulation is inferior 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 AM radio signal, part of the power of the transmitting equipment is used (about 50%). The way out to increase the communication range on AM is to switch to modulation with one sideband, 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 of practical importance.

Signal modulation is the process of changing one signal in accordance with the shape of another signal.
Modulation is carried out to transmit data using electromagnetic radiation. Typically, a sinusoidal signal (carrier) is modified. There are:
- 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 modulation (AM) systems, the modulating wave changes the amplitude of a high-frequency carrier wave. 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 wave is complex, such as a speech signal, which consists of many frequencies, then the sums and differences of the various frequencies will occupy two bands, one below and the other above the carrier frequency. They are called upper and lower lateral. The upper band is a copy of the original conversational signal, only shifted to the Fc frequency. The lower band is an inverted copy of the original signal, i.e. the high frequencies in the original are the low frequencies in the lower side. The lower side is a mirror image of the upper side with respect to the carrier frequency Fc. An AM system that transmits both sidebaud and carrier is known as a double sidebaud (DSB) system. The carrier carries no useful information and can be removed, but with or without the carrier, the DSB signal has twice the bandwidth of the original signal. To narrow the band, it is possible to displace 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 modulation (SSB-SC - Single SideBand Suppressed Carrier).
Demodulation of an AM signal is achieved by mixing the modulated signal with a carrier of the same frequency as the modulator. The original signal is then obtained as a separate frequency (or frequency band) and can be filtered from other signals. When using SSB-SC, the demodulation carrier is generated locally and may not be matched in any way to the carrier frequency at the modulator. The slight difference between the two frequencies causes frequency mismatch, which 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 transmitting data over communications networks.

Modulation types: 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 range it is allowed to use frequency (FM), amplitude (AM) and single-sideband (SSB) modulation. Which 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 correspondent's signal 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 precise setting of the noise suppressor threshold.

AM is used for communication over medium and long distances when the correspondent's signal is too weak to realize the benefits of FM. The maximum communication range when using AM and FM is almost the same.

Radio communication using one sideband has such great advantages over AM and FM that it has completely replaced them in professional and amateur radio communications. SSB appeared in amateur radio bands in the fifties. In 1956 there were only a few dozen amateur SSB radio stations in the world, but in 1961 their number already exceeded 20 thousand. The first Soviet shortwave operator to work on SSB was Georgy Rumyantsev (UA1DZ), one of the oldest Russian radio amateurs, L. Labutin (UA3CR), who began working on SSB in 1958, did a lot to popularize work on SSB.

SSB modulation came to the CB range 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 range is the higher price of SSB transceivers, which exceeds the prices of AM/FM stations by 3-5 times, the second reason is the peculiarities of working on SSB, which require higher operator qualifications.

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, the manual adjustment of which the driver should not be distracted while driving. However, recently quite decent SSB car stations have appeared on the market, but the price is only 1.5-2 times more expensive than AM, FM stations, which have frequency stability quite sufficient to operate on SSB while 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 emitted 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 types of modulation and the peak power with SSB modulation should not exceed 10 W. Where does the winnings come from?

With SSB modulation, the carrier and one of the sidebands are not radiated, allowing all permitted power to be radiated as one 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 do not stop there. AM and FM stations emit carrier power constantly, regardless of whether you speak into the microphone or remain silent. SSB stations do not emit any power during pauses between words. In addition to saving energy and facilitating the operation of the transmitter output stage, this provides additional advantages when operating in a channel overloaded with stations. When using AM or FM modulations, turning on a more powerful station completely “overwhelms” the weaker one, making reception impossible; when using SSB, in the pauses between words of a powerful station, the weaker station continues to be listened to. It is possible not only to follow the station, but also to grasp the meaning of the message. In practically such cases, it is possible to agree on a transition to another frequency. If the signal level of the interfering stations is not much higher than the level of the received one, and the frequencies of all stations are exactly the same, you will understand most of the information of the desired station, just as you understand the interlocutor when talking when surrounded by people talking. In practice, the frequencies of interfering stations always differ from the received one, therefore, due to a violation of the relationships 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, of course, only true in 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 users of the CB range, in which there is no frequency differentiation for working with different types of modulation, agree among themselves on which channels only SSB can be used. Thus, CB users in European countries agreed to preferentially use the D band for working with SSB, leaving the C band for AM and FM.

All of the listed advantages of SSB modulation allow, other things being equal, to obtain a communication range that is 50-75% greater than with AM or FM.

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 the maximum voltage, through a certain 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 oscillations 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 you can see, with 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 does not contain any useful information at all, and one of the sidebands, you get SSB modulation (in Russian: OBP) - modulation with one sideband and no 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 signal frequency on the air 27175000Hz, the frequency of the receptionizer generator 27174000, then the signal 27175000+27174000 = 54349000Hz and 27175000-27174000 = 1000Hz will enter the input of the sound amplifier, of course, the first of them will not be sound, its sound amplifier will not enhance, but here the second, 1000Hz, is an already audible sound and it will amplify it and we will hear “piiiiii” 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. The ratio of the sound of the maximum frequency that the microphone amplifier of the transmitter will transmit 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 - modulation of the carrier phase. Frequency modulation at small indices 1-2 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 with quiet sounds and unpleasant overtones even with loud ones (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 both 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 must be 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 strong sounds are heard: 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 low 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, pulsed noise is much more noticeable, such as the crackling sound of faulty car ignitions, the clicks of lightning discharges, or the rumble from pulsed voltage converters.
The weaker the 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 affected by impulse noise, but the high noise level of the FM detector makes it unbearable to sit without a squelch in the absence of a signal. 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 on the same frequency are turned on at the same time, then you get a mess of carriers, a kind of squeaking and screeching among which you can’t make out anything.
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 spent 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 airwaves in pauses between loud sounds and intelligibility 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 AGC (automatic gain control) and therefore a detector of the level of the received carrier, that is, in fact, a full-fledged AM receiver, only working somewhere there , 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 the 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”.