TV transmitter. TV radio transmitter
The television transmitter 10 has a power of several tens of kilowatts.
Television transmitters operate in the meter and decimeter wavelength ranges. A television signal, consisting of signals transmitting image brightness, synchronizing and canceling pulses, occupies a band of 6-5 MHz according to the USSR standard; amplitude modulation is used to transmit it. Sound signals are transmitted by a special transmitter using frequency modulation.
The television transmitter must be capable of transmitting both black and white signals and a color program. To achieve this it is necessary that the 31 5 kHz generator can be controlled automatic circuit, which would synchronize it with the signals arising in the studio.
TV transmitters present a special case due to the height, antenna type and long emitted wavelength. Basic elements of complex TV antenna the usual type consists of a double lens complemented by flat reflectors that change the radiation pattern.
| Block diagram of a pulse-modulated radar transmitter.| Block diagram of a coherent radar transmitter. |
The video path of a television transmitter must pass modulating frequencies from 50 Hz to 5 5 MHz. The path carries out correction of linear and nonlinear distortions, limiting the white level, and adjusting the modulation depth. In the output stage, the DC component is restored, which does not pass through the amplification path.
The lens of a television transmitter casts the image of a freely falling object located in front of it at a distance of d5 m onto the photosensitive layer of the transmitting tube.
For relay television transmitters and special purpose transmitters operating on one receiver, the antenna system must have a radiation pattern compressed into a narrow solid angle. Such compression of the radiation pattern is energetically beneficial, since it allows one to concentrate all the emitted power of high-frequency oscillations in the direction of the radio receiver. The ratio of the power radiated per unit solid angle in the desired direction to the total radiated power divided by the full solid angle - 4i is called the antenna gain.
Television transmitters are subject to strict requirements for the uniformity of amplitude-frequency and linearity of phase-frequency characteristics, deviation from which leads to fringing and multi-contour of the image. This can also be caused by a lack of good coordination with the antenna.
Television transmitters use modulation circuits at high (in the output stage of the transmitter), medium and low power levels. When modulating to high level power, the radio frequency channel before the modulated stage is a master oscillator and an amplifier of unmodulated oscillations, and the modulation channel is a powerful video amplifier. Grid modulation is usually used, because other, more efficient modulation methods (anode, anode-screen) cannot be used due to the need to use a broadband anode transformer, which is technically impossible. At the output of the modulated stage, a lower sideband suppression filter is installed, which, at high transmitter powers, is a complex radio engineering device. To increase the efficiency of the modulator, it is designed as a broadband amplifier direct current, since multiple (at the output of each stage) restoration of the average component of the signal leads to significant distortion of the latter.
In this article we will talk about the television and radio transmitters circuit of this device.
In my youth, there was a period in my life - military service “at the ends of the Earth.” This place was where steam locomotives did not go, ships visited once a year, and planes flew over our heads along the air corridor provided by the glorious and formidable (at that time) air defense system, vigilantly monitoring the crossing of aircraft by the state border of our Motherland. The most “representable” of our friends were dogs, and among our neighbors - polar bears, deer, walruses and whales. There we only had one television program, received satellite dish called "Moscow".
Officers and warrant officers at that time had video recorders, and when they returned from summer vacations, they carried countless videotapes with films. These cassettes were then passed around until they became unusable. The most effective video recorder that spoils video cassettes was in our barracks. The front panel was missing and the recording mode lock did not work. And everyone was too lazy to press the bare micro buttons, trying to play or rewind. Often, instead the desired button the record button was pressed, and a piece of the film was simply erased. After such abuse at the cinema, the officers stopped giving videotapes to the barracks. To “satisfy the demand” of my comrades, I assembled a simple television transmitter, which broadcast a full-fledged television signal using VIDEO and AUDIO signals. The power was small, but within a radius of 100 meters there was confident reception on the second television channel. This was enough to broadcast films to the entire town. The broadcast was made from one video recorder “based on telephone requests” from residents of the town.
The television transmitter described in this background requires an RF permit, so I will not publish its circuit diagram. Modern VCRs have the ability to connect to a display device (TV) through special connectors (via HF or LF channels). In some cases, it can be extremely inconvenient to use connecting cables to play a video on a TV. The proposed circuit makes it possible to amplify the power of the high-frequency signal generated by the VCR and broadcast it over the air to remote consumers.
The operating radius of the device, which maintains guaranteed high image quality, is 50-60 m without taking into account attenuation (for example, a solid reinforced concrete wall of a house reduces its operating radius by 8-10 m). The circuit is powered from any stabilized source with a voltage of +9...18 V (recommended voltage value 12 V). The device operates in the frequency range 300…600 MHz.
Fundamental electrical diagram devices is shown in the figure.
Transistors VT1 and VT2 assemble two stages of an RF amplifier with negative feedback (R-type). The total gain of both stages is 20 dB. Transistor VT3 is the final stage of the power amplifier. The gain of this stage is 6-7 dB.
The low noise level of the entire circuit (0.4-0.8 dB) with a total gain of at least 2.6 dB is ensured by the use of negative feedback on strip elements, which also eliminates self-excitation of amplifier stages and signal distortion.
The absence of adjustment elements makes the design accessible for repetition and eliminates difficulties when setting up high-frequency stages. The printed circuit board and the arrangement of elements on the board are shown in the figure below.
The board is made of double-sided foil fiberglass. The housing pads of both sides of the board should be connected to each other with through solder jumpers (1 jumper is installed on approximately 2 cm 2 of the board).
Holes for the terminals of structural elements are not needed. Between the cascades of the device, it is necessary to install shielding partitions connected to the circuit housing. It is advisable to supply power to the cascades through feed-through capacitors with a capacity of 1000-3000 pF installed in the holes of the shielding partitions.
It is not advisable to maintain the printed conductors of the board; solder should only be in the places where radio elements are installed. Before installing elements on the board, it must be sanded with fine-grained sandpaper and then polished with a polishing paste (for example, GOI). It is advisable to use leadless passive elements for surface mounting. To ensure the above electrical parameters It is necessary to accurately copy the printed circuit board and follow all recommendations.
The device uses powerful KT939A microwave transistors. Good results are obtained by using other transistors with similar parameters with a cutoff frequency of about 1 GHz, for example, KT610A and KT916A.
Coils L1, L2 are frameless and contain, respectively, 5 and 10 turns of enameled wire with a diameter of 0.3 mm. The inner diameter of both coils is 4 mm.
The antenna of the device is an aluminum or copper pin with a diameter of 10 mm and a length of 120 mm, soldered directly to the board at point “A” (see figure). The recommended antenna dimensions are given for channel 37 of the UHF range (600 MHz). This is the average frequency of any VCR RF output.
However, to increase the transmission range, without increasing the power of the signal amplifier, you can make an antenna consisting of two half-wave vibrators standing crosswise with a signal phase shift of 90 degrees. Thus, we will get a high-quality transmitting antenna with horizontal polarization and a circular radiation pattern. This is exactly the antenna I used in my first television transmitter. The transmission range is guaranteed to triple.
After assembly, the device is placed in a housing made of conductive material (for example, copper), which is connected around the perimeter by soldering to the housing pads of the board on both sides.
The device is connected coaxial cable directly to the high frequency output of the VCR. An indoor UHF antenna is connected to the TV, and by changing its location it is adjusted to the best reception quality. If the distance between the device and the TV is small (5-6 m), the antenna is replaced with a piece of wire 10-15 mm long.
Addressing in X.25 networks. CCITT Recommendation X.121 defines an international address numbering system for data networks common use. If an X.25 network wants to communicate with other X.25 networks, then it must adhere to the X.121 addressing standard.
X.121 addresses (also called International Data Numbers (IDN) have different lengths, which can be up to 14 decimal places. The first four digits of the IDN are called Data Network Identification Code (DNIC). DNIC is divided into two parts; the first part (3 digits) determines the country in which the network is located, and the second - the number of the X.25 network in this country. Thus, only 10 X.25 networks can be organized within each country. If it is necessary to renumber more than 10 networks for one country, the problem is solved by giving one country several codes.
Technology Frame Relay(FR) was designed specifically for transmitting uneven computer traffic through “good” communication channels. FR networks, like X.25 networks, operate on the basis of virtual channels, but each such channel is now associated not only with path parameters (labels), but also with QoS parameters. The X.25 network is characterized by high redundancy of the protocol stack; in them, error detection and correction procedures, as well as data flow control procedures, are provided on both the link and the network levels. The FR node performs only two functions when receiving a frame:
Frame integrity check. If not, the frame will be discarded;
Checking the correctness of the address. If the address is unknown, the frame will again be discarded.
When an error is detected, the FR node simply discards the failed packet without worrying about the need to resend it - this is the responsibility of the end user equipment. Compared to X.25, FR takes significantly less time to process one frame of information, resulting in a latency of only 3 ms/node, while X.25 latency is up to 50 ms/node.
There are two types of virtual circuits that can be used in an FR network: switched (SVC) and permanent (PVC). FR operates primarily with permanent virtual circuits (PVCs). Switched virtual connections are used quite little in FR.
FR network components There are devices of three main categories (Figure 8.2):
DTE (Data Terminal Equipment) devices;
DCE (Data Circuit-Terminating Equipment) devices;
FRAD (Frame Relay Access Device) devices.
Figure 8.2 - Network structure with the Frame Relay protocol
The basis of the FR network is formed by specialized switches - FRAD (FrameRelay Access Device, frame relay network access device)
Disadvantages of Frame Relay. Frame Relay technology does not guarantee frame transmission delays, leaving this service to ATM networks.
ATM - Asynchronous Transfer Mode - Asynchronous transfer mode. The essence of ATM technology. Transporting all types of information in fixed-length packets (cells), when cell streams from different users are asynchronously multiplexed in a single digital path. As a protocol unit, ATM accepts a fixed-length packet, including a header (5 octets) and an information field (48 octets), the total length of the packet is 53 octets.
Because of the possibility of output collisions, the ATM switch must have the ability to buffer ATM packets. Both virtual circuits (VCs) and virtual paths (VPs) are defined as virtual connections between adjacent routing entities in an ATM network.
The logical connection between two end users consists of a number of virtual connections, if n switching nodes are switched, the virtual path is a bundle of virtual channels. Because virtual connection marked by a hierarchical VPI/VCI (Virtual Path Identifier/Virtual Channel Identifier) key in the ATM cell header, the switching circuit can use either full VC switching or VP-only switching.
The first case corresponds to a full ATM switch, while the last case refers to a simplified switch node with reduced processing, where the minimum switching object is a virtual path. Therefore, the VP/VC switch reassigns a new VPI/VCI to each switched virtual cell, considering that only the VPI is reassigned in the VP switch.
Lecture 12. Radio channels. Radio wave range. Radio systems
Radio channels. Radio engineering – scientific- technical area, whose tasks are:
1) study of the principles of generation, amplification, radiation and reception of electromagnetic oscillations and waves related to the radio range;
2) practical use these vibrations and waves for the purposes of transmitting, storing and converting information.
At the initial stage of its development, following the invention of radio
(A.S. Popov, 1895 and G. Marconi, 1901) and radio engineering primarily solved the problems of telecommunications using electromagnetic oscillations with wavelengths of several hundred or a thousand meters. Currently, the range of applications has expanded enormously. Radio communications, satellite and mobile radio communications, television, radio control, radiolocation, radio navigation, radio engineering methods in various fields of science, technology and other sectors of human activity.
The science that studies the physical foundations of radio engineering is called radiophysics. Radiophysics is a rapidly developing branch of applied natural science, closely related to such fundamental areas as quantum mechanics, solid state physics, etc.
The penetration of radio engineering into related fields (electronics, computer technology) led to the emergence of a broad scientific and technical field, collectively called radio electronics.
Communication theory is currently the most dynamically developing discipline. Communication systems include all the basic devices used in most radio systems. In communication theory, the following definition is accepted: a set technical means for transmitting messages from source to consumer is called a communication system.
Let's consider general principles construction of radio communication systems (radio channel). Conditionally everything existing systems Radio communications can be divided into two large classes: simplex (one-to-all communication) and duplex (one-to-one communication) communication systems.
Under simplex connection understand communication between two points, in which in each of them the transmission and reception of messages are carried out alternately on the same carrier frequency. Often simplex communication is used to transmit information in one direction, for example, radio broadcasting, television, public address, etc. Duplex communication– two-way radio communication between two points, in which the transmission and reception of messages is carried out simultaneously on different frequencies. Currently, a type of simplex communication is used, such as half-duplex communication (dual-frequency simplex), when the system alternately transmits and receives information on two different carrier frequencies using repeaters. Note that the repeater (from re... and lat. translator– carrier) – a radio device used as an intermediate transceiver point of a radio communication line.
Depending on the number of channels used, they are distinguished single-channel and multi-channel communication systems. The task of multichannel communication systems is the simultaneous transmission of messages from many sources, i.e. increasing throughput (often used term capacity). In such systems, one path (channel) is used to transmit messages from many sources. To increase the throughput of most communication systems, time and frequency multiplexing of signals are used.
With time multiplexing, due to the fact that signals are not transmitted continuously, but only in their samples (samples) in very short time intervals, a number of signals can be transmitted on the same carrier frequency various signals. To do this, different signals U 1 (t), U 2 (t), ..., U n (t), reflecting a group of transmitted messages, are connected to the transmitter through an analog multiplexer (selector or analog switch; from lat. multiplex complex, multiple). Time-sampled message signals are transmitted using a type of pulse modulation.
Information is a set of information about objects, considered from the perspective of transmitting this information in space and time.
Let's consider the general scheme for transmitting and receiving information:
Figure 12.1 - General scheme for transmitting and receiving information
Here AI is the source of information (message); PS - conversion into an electrical signal; Kd – coding; M – modulator; GN - carrier generator; RU - recording device; DO - decoding, processing, separation from interference; ULF - low frequency amplifier; D - detector (demodulator); UHF - high frequency amplifier, IVC - selective input circuit. An information transmission channel is a set of devices used to transmit information from a source to a recipient, as well as the medium separating them.
Let's consider analog communication system. One of the important links in any communication system is message source to be transferred. In general, the original message is not electrical and therefore needs to be converted to an electrical (primary) signal using electrophysical signal converter(EFPS), easier signal converter(PS). So, for example, when transmitting speech and music, such a transformation is carried out by a microphone, when transmitting an image - by transmitting television tubes, in telegraphy - using a telegraph apparatus, the sequence of message elements (letters) is replaced by a sequence of code symbols (0,1 or dot, dash), which is simultaneously converted into a sequence of direct current electrical pulses, when transmitting information about any non-electrical processes or quantities - by special sensors. Recently, in block diagrams of a radio channel, the message source and the signal converter are combined into one link, called source of primary messages.
As already noted, the transmitted (primary) signal is low-frequency. However, the term low-frequency is rather arbitrary here; in particular, a television signal has a spectrum with a bandwidth of the order of 0...6 MHz. Therefore, in some cases, the primary signal is directly transmitted over the communication line. This is what they do, for example, in an ordinary city telephone communication. For transmission over long distances (via cable, optical fiber or radio channel), the primary signal is converted into high-frequency.
The conversion of the message into an electrical signal must be reversible. In this case, the output signal can be used to restore the input primary signal, i.e. receive all the information contained in the transmitted message. Otherwise, some information will be lost during signal transmission.
The transmitting device includes, in addition to the signal converter, transmitter(containing modulator, carrier frequency generator And amplifier) And transmitting antenna(Fig.1). To transmit a message, the signal must first be introduced into a carrier high-frequency electromagnetic oscillation. This is done in the transmitter modulator. The carrier vibration is generated carrier frequency generator.
The process as a result of which one or more parameters of the carrier oscillation changes according to the law of the transmitted message is called modulation(from lat. modulation– regularity). Modulated high-frequency oscillation is referred to as secondary signals and call radio signal. One of the famous popularizers of radio engineering A.A. Kharkevich so figuratively assessed the importance of modulation in the transmission of information: “The emission of radio waves without modulation is like a blank page; the modulated vibration is like a page on which signs and letters are written.”
When transmitting a message over a radio channel, several types of modulation are used: amplitude, frequency, phase, pulse, pulse-code, etc. Amplitude modulation is the simplest and most common way of introducing a transmitted message (modulating signal) into a high-frequency carrier wave. At amplitude modulation According to the law of the transmitted message, only the amplitude of the carrier vibration changes, with its other parameters unchanged.
Digital (discrete) communication systems. In digital (discrete, pulsed) transmission systems, signal energy is not emitted continuously (as with a harmonic carrier), but in the form of short radio pulses. This allows, with the same total radiation energy as with a continuous carrier, to increase the peak (maximum) power in the corresponding pulse and thereby increase the noise immunity of reception. A periodic sequence of video and radio pulses is used as a carrier of the primary signal e(t) in pulsed communication systems.
Continuous messages can be transmitted over discrete (digital) communication systems. To do this, they are converted into digital form using the operations of time sampling, level quantization and encoding. Coding in a broad sense refers to the mapping of a message into a signal for transmission over a channel. Coding in the narrow sense is understood as the operation of transforming messages from a discrete source to transmit them over discrete channel. In this case, discrete messages are converted into a sequence of code symbols. Unless otherwise specified, the word coding will henceforth mean coding in the narrow sense. A coding system is a set of rules for coding objects.
In the transmitting device of a digital radio communication system, the encoding of the transmitted signal is performed by a digital chip called coder(Fig.1). At the output of the encoder, the transmitted primary signal has the form of a digital code - a certain sequence of pulses (“ones”) and pauses (“zeros”), usually having the same duration.
In the transmitter modulator, the carrier wave is modulated by the pulse sequence received in the encoder. Most often in digital communication systems the so-called pulse code modulation(ICM). In the case of PCM, discrete values of a continuous signal are transmitted in the form of code combinations. When using a binary representation, the codeword can express an integer equal to the corresponding level of the continuous signal at the time of its discrete sampling.
So, in a digital transmission system, the transformation of a message into a radio signal is carried out by three operations: conversion, coding and modulation (in an analog system, two operations - conversion and modulation). Note that encoding determines the mathematical side, and modulation determines the physical side of turning a message into a signal. Essentially, coding is the conversion of a message into a sequence of symbols, and modulation is the conversion of these symbols into signals suitable for transmission over digital channel. Using coding and modulation, the message source is matched to the communication channel.
In the receiver, after amplification at radio frequency, a sequence of code symbols (primary signal) is extracted from the intermediate frequency signal (received secondary signal) using a demodulator. These symbols are then decoded into decoder. Decoding consists of reconstructing the message from the received code symbols. Restored from decoder output analog signal goes to message recipient.
Modern digital information transmission systems use two groups of relatively independent analog-digital devices combined into separate microcircuits - codecs and modems. A codec is a pair of encoder-decor converters(usually these are logical devices), and modem– a pair of converters modulator- demodulator
Modems perform a set of different functions and, depending on the principles of their implementation, are divided into wired or telephone modems, cellular modems, packet radio modems, high-frequency radio modems, digital modems, fax modems, etc. Wired modems are included in communication systems between the public telephone network and computer manager.
Propagation and range of radio waves. The following division is generally accepted electromagnetic waves by frequency:
Myriameter:l =100km, 10 km; VLF (very low frequencies) f = 3 kHz ¸ 30 kHz;
long: l = 10 km ¸ 1 km; LF (low frequencies) f = 30 kHz ¸ 300 kHz;
average: l = 1000 m ¸ 100 m; MF (mid frequencies) f = 300 kHz ¸ 3 MHz;
short: l = 100 m ¸ 10 m; HF (high frequencies) f = 3 MHz ¸ 30 MHz;
meter: l = 10 m ¸ 1 m; VHF (very high) f = 30 MHz ¸ 300 MHz;
decimeter: l = 1.0 m ¸ 0.1 m; UHF (ultra high frequencies) f = 300 MHz ¸ 3 GHz;
centimeter: l = 10 cm ¸ 1 cm; Microwave (ultra high frequencies) f = 3 GHz ¸ 30 GHz;
millimeter: l = 10 mm ¸ 1 mm; EHF (extremely high frequencies) f = 30 GHz ¸ 300 GHz;
decimmillimeter: l = 10 mm ¸0.1 mm; HHF (hyper high frequencies) f = 300 GHz, 3000 GHz;
optical: l = 100 µm, 0.01 µm; optical range f = 3 THz, 30000 THz.
The choice of a particular wave range for each specific communication system is influenced by the following factors:
a) Features of the propagation of electromagnetic waves of a given range, the state of space in which the wave propagates. Long waves are strongly absorbed by the ground, short and ultra-short waves do not bend around obstacles. Long, medium and short can be reflected from the upper atmosphere.
b) Specifications: radiation directivity, use of an antenna system of appropriate dimensions, generation of powerful oscillations and their control, circuit diagram of the receiving device.
Radiation directionality can be ensured if antenna device in size significantly exceeds the wavelength. The focus has great importance in radar, radio navigation. Greater oscillation power is required at long waves due to absorption by the earth, and at other ranges - for ultra-long-distance space communications. The development of new ranges requires new technical means, as a result of which the transition to the short-wave region occurred gradually as generating devices were mastered.
c) The nature of noise and interference in this range. Research is regularly carried out on the transmission of radio waves of various ranges.
d) Nature of the message (amount of information and associated spectrum width (frequency range).
Thus, television, due to the large amount of information transmitted, must have a wide range of frequencies, so it is only possible on VHF.
Thus, when transmitting and receiving a message over a radio channel, the following operations are performed:
1. The message can be in the form of signs (print), sound signals(speech, music), light image or signal, etc.
2. Conversion of speech and music into an electrical signal is carried out using a microphone, conversion of images - using television transmitting tubes. A written message is first encoded by replacing each letter of the text with the combination standard characters(dots - dashes, zero - one), which are then converted into standard electrical signals (for example, pulses of different durations, polarities, etc.). All messages can be encoded, and they can be encrypted at the same time.
3. Generation of high-frequency oscillations. Basic requirements: frequency range, frequency stability, power (up to millions of watts).
4. Modulation - changing one or more parameters of a high-frequency oscillation according to the law of the transmitted message. The frequencies of the modulating signal must be small compared to the carrier frequency.
5. Isolation of the desired signal in the receiver from all oscillations on the air is carried out by an input selective circuit using resonant oscillatory systems Df/f up to 10 -6.
6. Gain weak signals in the receiver. The antenna receives a signal with a power of 10 -10 ¸10 -14 W (~ 10 -6 V). At the receiver output, for reliable signal registration, a power of the order of several watts is required, i.e. it is necessary to increase the power to 10 10 ¸ 10 14, and the voltage to 10 7. This is achieved using multi-stage amplifiers for high, intermediate and low frequencies.
7. Detection (demodulation) - separation of a low-frequency message (electrical information signal) from a modulated high-frequency signal. It is carried out using various types of detectors (synchronous, amplitude, quadratic).
8. Decoding - restoration of the original form information message from electrical signals of a standard form after detection. Encrypted signals are decrypted. IN the simplest system There may be no connection between the encoding and decoding devices. When transmitting a message by wire (telegraph), there may be no radio transmitting or receiving devices.
Literature:
1. What types of messages are available?
2. Define signal.
3. Megameter wavelengths belong to what frequency range?
4. Centimeter wavelengths belong to which frequency range?
5. What is the main part of the radio system?
6. What blocks does it consist of? radio transmitter?
7. What blocks does a radio receiver consist of?
Lecture 13.Types radio signals. Deterministic and random signals. Analog and discrete signals
When transmitting information over a distance using radio systems, various types of radio signals are used. Traditionally radio engineering It is customary to consider any electrical signals related to the radio range. From a mathematical point of view, any radio signal can be represented by some function of time u(t), which characterizes the change in its instantaneous values of voltage, current or charge.
A mathematical (analytical) model of a signal is its description using mathematical objects (functions, vectors, distributions, etc.), which allow one to draw conclusions about the characteristics of the signal by applying formal procedures (for example, mathematical transformations) to its description.
It is convenient to consider radio signals in the form of mathematical functions specified in time and physical coordinates.
A signal is a physical process that carries information. The signal can be sound, light, in the form postal item etc. The most common signal is in electrical form in the form of voltage versus time u(t). A signal is a physical process propagating in space and time, the parameters of which are capable of displaying (containing) a message. Under the signal s(t) we will understand the change in time of one of the parameters of the physical process.
General classification signals is shown in Fig. 13.1.
According to the peculiarities of the structure of time representation, all radio signals are divided into analog, discrete (from Lat. discretus divided, intermittent) and digital.
If the physical process generating the signal can be represented as a continuous function in time u(t) (Fig. 13.2, a), then such a signal is called analog ( continuous or, more generally, continual, when it has jumps, breaks along the amplitude axis). The concept of an “analog” signal is due to the fact that any instantaneous value of it is similar to (repeats) the law of change of the corresponding physical quantity over time.
Figure 13.1 – Signal classification
In radio electronics and communications technology, pulsed systems are widely used, the operation of which is based on the use of discrete signals. For example, a signal reflecting speech. It is continuous both in level and in time, and the temperature sensor, which produces its values every 10 minutes, serves as a source of messages that are continuous in value, but discrete in time.
The simplest mathematical model discrete signal is a sequence of points on the time axis, at each of which the values of the corresponding continuous signal are specified (Fig. 12.2, b). Discrete signals can be generated directly by the information source (for example, discrete reports in control systems).
One of the types of discrete signals is digital signal. A signal with a finite number of discrete levels is often called digital, since the levels can be numbered with numbers with a finite number of bits implemented in binary code“0” and “1” or a digital signal is a combination of narrow pulses of the same amplitude, expressing discrete signal samples in binary form.
 |  |
 |  |
G  digital |
d) Fig. 13.2-Signal graphs
According to the mathematical representation (according to the degree of presence a priori information) the entire variety of radio signals is usually divided into two main groups: deterministic (regular) and random signals.
Deterministic is a signal that is precisely defined at any time (for example, specified in analytical form). Deterministic signals can be periodic or non-periodic. A signal is called periodic for which the condition s(t) = s(t + kT) is satisfied, where k is any integer, T is a period, which is a finite period of time. An example of a periodic signal - harmonic oscillation:
Any complex periodic signal can be represented as a sum of harmonic oscillations with frequencies that are multiples of the fundamental frequency. A non-periodic signal is usually limited in time. Random signal (Fig. 13.3, b) is a function of time whose values are unknown in advance and can only be predicted with some probability. The following are taken as the main characteristics of random signals: a) the law of probability distribution (the relative time the signal value remains in a certain interval); b) spectral distribution of signal power.
The forms of representation of deterministic and random signals are shown in Fig. 13.3.
Rice. 13.3. Signals: A- deterministic; b- random
Recent times have been characterized by an accelerated transition from analog to digital methods of processing, transmitting and storing signals, which have a number of significant advantages.
Literature: 1, 2; 6[ 46-61].
Control questions:
1.What frequencies and wavelengths belong to the VLF and LF ranges?
2.What frequencies and wavelengths belong to the mid and high frequency ranges?
3.What frequencies and wavelengths belong to the VHF and UHF bands?
4.What frequencies and wavelengths belong to the microwave and EHF ranges?
5.What is the spectrum (what does it look like) of a harmonic vibration? Write down the expression for harmonic vibration.
6.Draw a simple block diagram of a radio communication system.
Television radio broadcasting is carried out in the meter wavelength range, occupying the bands: 48.5, 74 100, 174 230 MHz (channel numbers from 1st to
12th), and in the UHF wavelength range in the band 470 958 MHz (channel numbers from 21 to 81). A television radio transmitting device consists of two independent radio transmitters, one of which transmits an image signal and the other an audio signal. The image transmitter uses amplitude modulation with a partially suppressed lower sideband, and the audio transmitter uses frequency modulation.
The modulating signal of the image transmitter contains the video luminance signal - converted into an electrical signal optical image, color signal and synchronization signals - horizontal and vertical. The frequency spectrum of such a complex signal occupies a band from 0 to 6.5 MHz. The lower frequency value in this spectrum is associated with the slowly changing illumination of the transmitted image. With such a modulating signal after amplitude modulation, the radio signal would have to occupy a frequency band of 13 MHz. However, to narrow the width of the spectrum of the emitted signal, the lower side band is partially suppressed and, in general, the spectrum of the radio signal of the television transmitter occupies the 8 MHz band (Fig. 14.1).
Fig. 14.1. Spectrum of a radio signal of an image and a radio signal of a sound of a television radio transmitter
The parameters of the radio signal of the audio transmitter correspond to the parameters of the radio signal of VHF FM broadcasting and occupy a band of 145 kHz. The carrier frequency of this transmitter is located above the spectrum occupied by the image transmitter (Fig. 14.1).
The power of terrestrial radio image transmitters, depending on the broadcast conditions and coverage of the serviced territory, ranges from several hundred watts to 50 kW, and the power of radio sound transmitters is correspondingly 10 times less, i.e. no more than 5 kW.
Block diagram of a television radio transmitter. Each radio transmitter - image and sound - consists of two semi-sets, the powers of which are summed up using bridge devices. Thus, in general, a television radio transmitter, the general block diagram of which is shown in Fig. 13.2, contains: four RF or microwave power amplifiers operating on a common antenna; signal adders; General filter duplexer; AMP exciter of the image transmitter and FM exciter of the sound transmitter.
If one of the semi-sets fails, the power of the corresponding radio transmitter is reduced by 4 times. But by switching, the power of the operating half-set is sent directly to the antenna, bypassing the adder, and then the radiated power is reduced by only half. After the bridge devices, a duplexer filter is turned on, which has two inputs with different frequency bands and one common output, which allows you to send two signals with different frequencies to one antenna.
Fig. 14.2. General block diagram of a television radio transmitter
It is also possible that a structural diagram of a television radio transmitter, different from that shown in Fig. 13.2, is in which, first, using a filter - duplexer, half-sets of image and sound radio transmitters are combined, and then their powers are summed up using a common bridge device. With a power of up to 1 kW, a meter-wave television radio transmitter can be entirely semiconductor; at higher powers, electric vacuum devices are used in the output stages. The radio sound transmitter is practically identical in design and design to the VHF FM radio transmitter.
Main literature: 6[ 434-447];9.
Control questions
1. In channel numbers 1 to 12, television radio transmitters operate in what frequency range? 2. In the rooms with channel numbers from 21 to 81, television radio transmitters operate in what frequency range? 3. Why does a television broadcasting device consist of two separate radio transmitters?
4. What is the duplex filter used for? 5. What blocks does the block diagram of a television radio transmitter consist of?
In television, the image is transmitted using the method of amplitude modulation of the carrier, as in conventional AM radio transmission. Frequency modulation is used to transmit audio signals. The difference between the frequencies of the image carrier and the sound carrier is 4.5 MHz (see Fig. 5.14, a).
When transferring black and white image It is also necessary to transmit signals for synchronizing vertical and horizontal scans. However, in color television, when modulating the carrier, in addition, color signals and additional synchronizing signals are used.
In a black-and-white television receiver, the master oscillator produces oscillations of the fundamental frequency, from which signals are obtained for the scanning circuits. The oscillation frequency of the master oscillator is 31.5 kHz. To obtain a horizontal scanning frequency of 15750 Hz, it is divided by two, and to obtain a vertical scanning frequency of 60 Hz, it is divided by 7, 5, 5 and 3. In the case of color image transmission, these frequencies are slightly different due to the characteristics of the spectrum width and synchronization. In color transmission, a sub-carrier needs to be generated and modulated to obtain the chrominance side components, and then the carrier needs to be suppressed due to the limited bandwidth available for transmission.Therefore, at the receiver, the carrier must be recovered and mixed with the side components for subsequent demodulation of the color difference signals. signals.
Thus, the horizontal scanning frequency in a color television receiver is 15734.264 Hz, and the subcarrier frequency is 3.579545 MHz (3.58 MHz). The frame rate in a color television receiver is 59.94 Hz. Since the horizontal and vertical scanning frequencies in a color receiver are close to the corresponding frequencies in a black-and-white receiver, under normal operating conditions there are no problems when switching from receiving a black-and-white image to a color one.
The main blocks of the color television transmitting device are shown in Fig. 15.5. A color television transmitting camera with a special transmitting tube and lens system perceives the three primary colors of the image. Based on the principle of color additivity, these colors are red (R), blue (IN) and green (G).
As follows from the diagram shown in Fig. 15.5, the amplification and scanning circuits generate three components (signals of red, green and blue) of the transmitted image at the output. Signals R, G And IN then they are fed to three matrix circuits, two of which contain bass reflexes. The output signals of the matrices are designated Y, 7 and Q. The Y signal, as noted above, is called the luminance signal. It is obtained by adding three signals of primary colors - red, green and blue - in a ratio of 0.3: 0.59: 0.11. Maintaining this ratio is necessary to compensate for the unequal sensitivity of the human eye to different colors.
Rice. 15.5. Block diagram of a color television transmitter.
The two main color difference signals consist of an I signal (in phase) and a Q signal (quadrature). Signal I contains 0.6 signals of red, 0.28 signals of green and 032 signals of blue. The ratio of these components for the Q signal is as follows: R : G : B = 0,21: 0,52: 0,13.
The I and Q signals are fed to balanced modulators, where they modulate two 3.58 MHz subcarriers, 90° out of phase, with the I signal leading the Q signal. In balanced modulators, the subcarrier and the I and Q signals are suppressed, and only the I and Q signals pass to the output lateral oscillations of the subcarrier. The Y signal is fed through a filter to the adder, where the output signals from the balanced modulators are also supplied.
The color burst generator, which receives signals from a 3.58 MHz oscillator, produces a 9-cycle 3.58 MHz signal, which is transmitted at the back edge of the horizontal blanking pulse and serves to synchronize the subcarrier oscillator in the receiver (see Section 4.6 ). All signals, including clock signals and line and field blanking pulses, are added to the adder. The complete television signal thus generated is fed to an amplifier-modulator, where it is amplified if necessary, and then goes to the final modulation stage operating in class C amplification mode. As in other AM transmitters, a crystal-stabilized oscillator is used here. Signals from this generator are frequency multiplied, amplified and fed to a class C amplifier. A separate FM transmitter is used to transmit audio signals. Thus, a television transmission device uses two transmitters: one with amplitude modulation and the other with frequency modulation.