Approved by: Ministry of Construction of People's Republic of China (PRC).
Implementation date: 1 991August1.
Notice on Issuing the National Standard "Protection Distance Standard between Medium and Short Wave Broadcasting Transmitting Station and Cable Carrier Communication System"
(90) build a standard word. The 673rd time
According to the requirements of documentNo. [1986]2630 issued by the former State Planning Commission, the Protection Distance Standard between Medium and Short Wave Broadcasting Transmitting Station and Cable Carrier Communication System jointly formulated by the Ministry of Radio, Film and Television and relevant departments has been jointly reviewed by relevant departments. The Standard of Protection Distance between Medium and Short Wave Broadcasting Transmitting Station and Cable Carrier Communication System (GBJ 142-90) is now approved as the national standard, and will come into force on June 20 10, 5438+099 1 day.
This standard is managed by the Ministry of Radio, Film and Television. The specific explanation and other work shall be the responsibility of the Institute of Standardization Planning of the Ministry of Radio, Film and Television. Publication is organized by the Institute of Standards and Quotas of the Ministry of Construction.
Ministry of Construction
1990 65438+February 22nd
Preparation instructions
This standard is compiled by the Institute of Standardization Planning of the Ministry of Radio, Film and Television according to the requirements of Document No.2630 of the former State Planning Commission [1986] in conjunction with relevant units.
During the compilation process, the compilation team collected a large amount of data, collected and measured the broadcast electric field intensity of the medium and short wave broadcast transmitter widely used in China, and also tested and verified the interference of overhead cables to broadcast signals. After extensively soliciting the opinions of relevant national units, it was revised several times and finally finalized by our department in conjunction with relevant departments.
This standard is divided into three chapters and an appendix. The main content is the protection distance between the medium and short wave broadcasting transmitting station and the cable carrier communication system.
In view of the fact that this standard is formulated for the first time, during the implementation process, it is hoped that all units will combine engineering practice and scientific research, conscientiously sum up experience and pay attention to accumulating data. If you need to revise or supplement, please send your comments and related materials to the Institute of Standardization Planning of the Ministry of Radio, Film and Television (postal code: 100866) for reference in future revisions.
Radio, film and television department
1June 990
Chapter I General Provisions
Article 1.0. 1 This standard is specially formulated to ensure the normal operation of medium and short wave broadcasting transmitting stations and cable carrier communication systems.
Article 1.0.2 This standard is applicable to the site selection of medium-wave broadcast transmitters with the frequency of 526.5 ~ 1606.5 kHz and short-wave broadcast transmitters with the frequency of 2.3 ~ 26. 1 MHz, and is applicable to the new cable carrier communication system working in this frequency band.
Article 65438 +0.0.3 protection distance refers to the distance from the center of transmitting antenna of medium and short wave broadcasting transmitting station to wired carrier communication line or office (station).
Article 1.0.4 Cable carrier communication lines and equipment shall comply with the current national standards "Anti-Radio Broadcasting and Communication Interference Index of Coaxial Cable Carrier Communication System" and "Anti-Radio Broadcasting and Communication Interference Index of Symmetric Cable Carrier Communication System".
Chapter II Protection Distance between MW Broadcasting Transmitting Station and Cable Carrier Communication System
The electric field strength of the wave broadcast signal in Article 2.0. 1 can be determined by field measurement or calculation.
Article 2.0.2 The electric field intensity of the built-in radio broadcasting transmitter should be determined by field measurement. In actual measurement, broadcast field strength meter should be used, and the distance between the receiving antenna of the field strength meter and the ground is1~1.5m. Rotate the antenna to find the direction with the strongest electric field strength for measurement.
Article 2.0.3 When the earth conductivity is included, the electric field intensity must be measured. When the earth conductivity is ignored, the electric field strength of the medium wave broadcast signal can be calculated according to the following formula:
Where e is the electric field strength (db μ v/m) of the medium wave broadcast signal;
P—— Effective radiation power of medium-wave broadcasting transmitting station (kW);
R—— the distance (km) to the transmitting antenna of the radio station.
Article 2.0.4 When the electric field intensity of the medium wave broadcast signal is greater than or equal to 120B μ/m, the protection distance between the medium wave broadcast transmitter and the cable carrier station with terminal and manned repeater can be calculated as follows:
Where p refers to the effective radiation power (kW) of the medium wave broadcast transmitter;
D—— protective spacing (m).
Article 2.0.5 When the electric field intensity of the medium wave broadcast signal is 140dBμV/m, the protection distance between the medium wave broadcast transmitter and the wired carrier unmanned transponder and the underground cable can be calculated according to the following formula, but the minimum protection distance shall not be less than 20m outside the edge of the transmitter grounding grid.
Article 2.0.6 When the electric field intensity of the medium wave broadcast signal is 125dBμV/m, the protective spacing between the medium wave broadcast transmitter and the aluminum sheathed overhead symmetrical cable can be calculated according to the following formula, but the minimum protective spacing shall not be less than 20m outside the edge of the grounding grid of the transmitter.
Chapter III Protection Distance between Short-wave Broadcasting Transmitting Station and Cable Carrier Communication System
Article 3.0. 1 The protective distance between the short-wave broadcast transmitter and the wired carrier communication system shall meet the following requirements:
First, the strong directional antenna emits the half-power angle direction of the main lobe, not less than 300m, and the strong directional antenna emits the half-power angle direction of the non-main lobe, not less than 50 m;
Second, the non-directional antenna shall not be less than 200 m in all directions;
Three, for short wave launch center, shall not be less than1000 m. ..
Article 3.0.2 The electric field intensity of short-wave broadcast signals of cable carrier stations and stations with terminals and manned repeaters should not be greater than100 dB μ V/m. ..
Article 3.0.3 The electric field intensity of the short-wave broadcast signal at the location of the cable carrier unattended repeater shall not be greater than110dB μ V/m. ..
Article 3.0.4 The electric field strength of short-wave broadcast transmission signals where underground cables and overhead symmetrical cables are located shall not be greater than140dB μ V/m. ..
Appendix Description of Terms Used in this Standard
1. When implementing the provisions of this standard, the strict words are described as follows, so as to be treated differently in implementation.
1. Very strict, if it must be done:
The positive word is "must"; The negative word is "forbidden".
2. Strict requirements, what should be done under normal circumstances:
The positive word is "should"; Negative words are "should not" or "should not".
3. Words that allow a little choice should be done first if conditions permit:
The positive words are "Yi" or "Ke"; The negative word is "inappropriate".
Two, the provisions that must be implemented in accordance with other relevant standards and norms shall be written as "should be in accordance with" or "should meet the requirements or regulations".
additional information
List of editing units, participating units and main drafters of this standard
Editor: Institute of Standardization Planning, Ministry of Radio, Film and Television
Participating unit: Design Institute of Ministry of Posts and Telecommunications
Main drafters: Wu Lengqian, Cheng Gang, Wei Sufen and Xie Feng.
Basic knowledge of satellite TV! !
Basic knowledge of satellite TV! !
1. Do you want to receive satellite TV? Just: 1. Pot 2. Tuner 3. Receiver.
In addition to these three types, there are tuners and coaxial cables for connection. If you want to watch encrypted programs, you need a decryption card.
Second, is the cardholder a distributor or a non-distributor? Do you have to insert the card into the card dispenser? Can I watch encrypted TV without buying pirated cards? Can I watch free TV without inserting a card machine? Do non-card machines have to watch free TV?
The receiver consists of a simulator and a digital computer. Digital machines can be divided into ordinary digital machines (non-card inserting machines) and card inserting machines. Card inserting machine can only watch unencrypted (free) TV programs without inserting cards. You can watch encrypted (free) TV programs without paying the annual fee when buying pirated cards. The card inserting machine can only watch unencrypted (free) TV programs without inserting the card, but sometimes the encryption station will decrypt and play them for a short time.
Can I use a 3.60cm or 75cm pot? I want a pot under 75 cm. I wonder if the viewing accuracy and the number of channels are less?
The size of the pot depends on which satellite you want to receive. If the satellite has high power and strong signal, you can use a small pot instead of a bigger one. For example, the satellite's 75CM antenna can fully receive 76.5, and if the digital program is too small, Massek (like a VCD dial) will appear, and the performance of analog programs is more than that of Xue Huadian. As long as the digital clarity of the signal is enough for the receiver, the minimum signal value is clear, otherwise, Massek will appear until there is no satellite signal, and the simulation program is Xue Huadian.
How many channels can a pot below 4.75cm receive How many countries can you accept? Can all domestic satellite TV channels and local stations receive it? Can you receive the channel in Taiwan Province Province? For example, Phoenix Chinese Channel, chanal V and so on?
The 75CM pot depends on which satellite you receive. There are programs from different countries on different satellites, and the signal power of satellites is also different. Satellite TV channels all over China can receive 75cm pots for free, while satellite TV channels in Taiwan Province Province can receive them. Hong Kong's Chinese and other 75 cm pots are not available, and Hong Kong's cauldron takes 1.35m- 1.5m (see 3S in Asia). None of these programs are available. Of course, at present, 1 10.5 degree programs cover almost all local programs, but it is impossible to watch them all.
5. Can I watch the pot under 75cm at home with the window closed? What if the window is open? What if it is installed outside the window?
If you watch 76.5-degree KU TV programs, you can put a 75-cm pot at home, but depending on the direction of your window, you can close the window, but the signal attenuation is great. It is possible that some stations will have Massek or no signal. Single glass windows are better, but double glass windows can be much worse. And if it is insulating glass, it is totally unacceptable. It would be nice to replace the glass with plexiglass, but it is absolutely no problem to install it outside the window. Generally, it is ideal to put it on the balcony, and the anti-theft net has little effect. Put the 60CM antenna on the balcony with anti-theft net on the south coast to see if it is stable.
6. Does the recipient need to constantly upgrade the software and authorize it? Do you need to constantly upgrade the software and authorize the card collection?
Machine upgrade, card authorization. As an ordinary user, there is no need to upgrade the receiver at all. The higher version of the software is not necessarily better than your original software. Now the machine hardware is developing rapidly. Even if the manufacturer introduces new software, your machine hardware may no longer support it. Card authorization is automatic. Each authorization time is 10-20 minutes, and the authorization time of different satellite TV providers is not necessarily one month, one week and so on.
How much will it cost to complete this set of equipment? (Good quality, pot below 75cm)
You can refer to our discount package station.
8.ku is a free program, and c is a paid program?
No, satellite band. KU band works at 10.7- 12.75Mhz, and C band works at 3.4-4.2Mhz.
76.5, what does 166 mean?
Is the degree of the satellite, that is, the position of the satellite, 76.5 is probably in the middle of the south and west, and 166 is a little east-south
10. What is the tuner used for? How many brands are there and what is the price?
The function of the tuner is to receive the converged satellite signal reflected from the cauldron, then convert it into an intermediate frequency signal between 950-2 150, and then transmit it to the receiver through the cable. There are many brands of tuners, ranging from 100 yuan to hundreds or even higher.
XI。 What do analog programs and digital programs mean?
Analog programs are ordinary analog signals downloaded from satellites, which are the same as ordinary TV programs we are watching now, while digital signals are transmitted by MPEG2-2 coding, similar to DVD coding.
12. What do simulators and digital computers mean?
Simulators can only receive analog signals, digital machines can only receive digital signals, but there are also two-in-one machines, but the price is more expensive than buying these two separately.
13. What does the authorization information mean? Do I have to be authorized to watch satellite TV?
Authorization is to verify your card. Only card machines with inserted cards are authorized, and ordinary digital machines do not need it.
14. What do you mean by encryption program?
Encrypted programs can only be watched by special receivers or card machines, which are invisible to ordinary machines.
15. What is Huawei?
Huawei is the name of a satellite live TV company, namely: China Live TV. If you have money, you can rent a satellite, find some TV programs to upload, and then encrypt the charges, so you can sit at home and count the money. Of course, it is best not to touch pirated cards.
What do you mean by pirated cards? How much is the pirated card? How much is the genuine card? Can pirated cards watch many TV programs? Are all cards issued by Huawei?
The pirated card is fake. You have to ask the dealer how much it costs, about 1000 RMB. If you buy a good stolen card, you can receive more programs, but it may be more expensive, on the contrary it will be cheaper. Some stolen cards are good and have strong vitality, while those that are not good will die.
17. What are the reception parameters?
The reception parameters are downlink frequency, symbol rate, polarization, etc.
Eighteen, what is HBO?
HBO is the name of a TV station, that is, Hollywood movies and American home theater channels, both of which are blockbusters.
Please explain the meaning of the sentence "166 E pan am -8 satellite 12366 H 27500".
12366 is the downlink frequency, h is the polarization, and 27500 is the symbol rate. Only when these parameters are set, H is horizontal polarization and V is vertical polarization, can your receiver receive programs.
In addition, some novices suggest releasing some explanations of satellite reception terms. Here are three basic concepts: azimuth, elevation and polarization angle.
1. Azimuth: Usually, the result we get through calculation software or data should be to adjust the satellite antenna pointing to the east or west by an angle based on the due south direction, which is the so-called azimuth. As for whether it is east or west, it depends on the longitude relationship between the receiving place and the satellite to be received. Take our northern hemisphere as an example. If the longitude of the receiving site is greater than the longitude of the satellite to be received, the azimuth will be rotated from south to west by a certain angle. Instead, you should turn to the east at a certain angle. The due south direction is measured by a compass, but because the geographical south pole and the geomagnetic south pole are not completely coincident, it is possible to receive the strongest satellite signal after selecting the azimuth.
2. Elevation: it is the included angle between the antenna axis and the horizontal plane. The axis of the feed antenna is very clear, which is the connecting line between the position of the tuner and the center of the antenna; The axis of the offset feed antenna is not clear. I carefully observed the structure and shape of the offset feed antenna, and came to the conclusion that the axis is basically parallel to the L-shaped rod supporting the KU head. Later, I adjusted the elevation angle of the offset antenna according to this conclusion. As a result, I couldn't get the 76.5 Asia-Pacific 2R after two days' adjustment. I've always wondered if the tuner is broken. I'm going to mail order a new tuner. But that afternoon, I had a whim to use sunlight to check the focus position of the offset feed antenna, so I aimed the L-shaped rod at the sun (adjusted the antenna position so that the shadow of the L-shaped rod on the ground converged to one point) and found that the sunlight reflected by the antenna did not converge to the position of the tuner. But a little above it (where you can receive the concentrated solar rays by hand, you can also judge the focusing performance of the antenna), and then lower the elevation angle of the antenna to make the light convergence point just where the tuner is located. The angle between the L-shaped rod and the horizontal plane is about 10 degree. At this time, I suddenly realized why I couldn't find the damn Asia-Pacific 2R: I received that the elevation angle of the star should be 30 degrees, so the angle between the L-shaped rod and the horizontal plane should be adjusted to about 20 degrees (I adjusted it like this: tie a rope, hang a heavy object on the rope, and measure the angle θ between the rope and the L-shaped rod with a protractor, then the angle between the L-shaped rod and the horizontal plane must be 90-θ, that is, just adjust θ to make it equal to 70 correction angle. What a "cool" feeling at that time!
3. Polarization angle: At present, most of the satellite signals we can see are transmitted in the so-called linear polarization mode, and two signals that are perpendicular to each other and do not affect each other can be transmitted in the same transponder. Generally, these two directions are horizontal (H) and vertical (V). Because the longitude of the satellite over the equator is generally different from that of the receiving place, the polarization direction of the horizontal or vertical polarized wave emitted by the satellite will change after it reaches the receiving place, and the changed angle is called the so-called angle. The commonly used dual-polarization tuner corresponds to this, so when receiving different satellites, it is necessary to rotate LNB, that is, change the polarization angle to obtain the best signal. For example, in Asia-Pacific 2R at 76.5 degrees east longitude, the horizontally polarized signal of a transponder is to be received, and the polarization angle in Suzhou is about 45 degrees. The zero scale on the original tuner should coincide with the zero scale on the tuner clip. At this time, the tuner should be rotated 45 degrees counterclockwise (facing the pot), and the signal lead of the tuner is in a horizontal right state. If the longitude of the receiving satellite is greater than the longitude of the receiving place, the rotation direction becomes clockwise! The polarization angle should be adjusted in advance, and then slightly after receiving the satellite signal, so that the received signal quality is the best.
Then introduce some basic knowledge to beginners:
First, the composition of the satellite TV receiving system:
Satellite TV receiving system is a complete satellite ground receiving station composed of parabolic antenna, feed, tuner and satellite receiver.
1. Parabolic antenna reflects the satellite signal energy from the air and converges it into a point (focus).
2. Feed is to set a horn that is afraid of satellite signals at the focus of parabolic antenna, which is called feed, which means the source of feeding energy and requires to collect all the energy gathered at the focus. Feedforward satellite receiving antenna basically adopts large-angle corrugated feed.
3. The high frequency head (LNB) is used to down-amplify the satellite signal sent by the feed and then transmit it to the satellite receiver. The lower the noise level of tuner, the better.
4. The satellite receiver demodulates the satellite signal sent by the tuner, and demodulates the satellite TV image signal and sound signal.
2. Basic configuration:
Receiving C-band analog programs: feed antenna +C-band tuner+analog satellite receiver.
Receiving C-band digital programs: feed antenna +C-band tuner+digital satellite receiver.
Receiving KU-band digital programs: KU offset feed antenna +KU-band tuner+digital satellite receiver.
Polarization mode of TV signal;
There are four polarization modes of satellite TV signals: right-handed circular polarization, left-handed circular polarization, vertical polarization and horizontal polarization. Because the first two kinds of polarization are not commonly used, only vertical polarization (V) and horizontal linear polarization (H) are introduced.
The reception of vertical polarization and horizontal polarization is determined by changing the direction of rectangular waveguide feed port. When the long side of a rectangular waveguide is parallel to the ground, it receives vertical polarization, and when it is perpendicular to the ground, it receives horizontal polarization. The polarization direction (polarization angle) varies from place to place. Because the earth is a sphere, but the downlink beam of the satellite signal propagates horizontally and linearly, which causes the same polarized signal received at different azimuth angles to be different, so the polarization direction of the signal received at different geographical locations is also different. The feed long waveguide port (polarization direction) will not be completely vertical or horizontal to the ground. Pay attention to this when adjusting the polarization direction.
Azimuth of four common satellites:
Check the relevant information. 120 E is roughly due south, and the direction of the sun at noon 12 is 120 E, or the compass is roughly positioned 120 E is due south.
The azimuth of Asia-Pacific 1A is 134 E, and the elevation is slightly different from place to place. 134 E is easterly.
The azimuth of Asia 2 is 100.5 E, and the elevation is slightly different from place to place. 100.5 E is west.
Asia's 3S azimuth 105.5 E, with slightly different elevation angles, 105.5 E is west.
The azimuth of CIGNA 1 is 1 10.5 E, and the elevation is slightly different from place to place. 1 10.5 E is west.
The 2R azimuth of Asia-Pacific is 76.5 east longitude, and the elevation angles are slightly different from place to place, and 76.5 east longitude is west.
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Shanxi Jo Jiang
Basic knowledge of satellite TV (2)
The cross section of the spot beam is circular or elliptical, covering a certain area of the earth's surface, which is smaller than the global beam.
STV (pay TV) Time-based pay radio and TV is a pay radio and TV service.
The carrier frequency of a television signal, used to transmit sound and other information.
Synchronization is a technique to keep the signals of receiver and transmitter synchronized in time.
Conference call Many people who are not in the same place use some electronic devices, such as telephones and televisions. Computer terminals, etc.
Teletext teletext“teletext "is transmitted at the same time as the TV signal, which can display characters and images on the TV screen. There are usually hundreds of "pages", each of which contains 20 lines of Chinese and English characters. "Teletext" is transmitted by data subcarrier, using the frame blanking time in TV signal. Ordinary TV sets can't watch teletext, and TV sets can only receive teletext with a special teletext decoder. At present, many TV networks and satellite TV systems have increased the "electronic newspaper teletext" signal.
Terrestrial TV Terrestrial radio and television are traditional TV transmission in a limited range. The transmission range is about160km in diameter, and the working frequency band is VHF~UHF.
Threshold extension Threshold extension is a technology to improve the signal-to-noise ratio of satellite receivers by nearly 3 decibels.
The function of Translator's TV differential unit is to receive TV signals from far away, and then convert the TV signals to another channel and transmit them to the local area to increase TV coverage.
Repeater Repeater consists of receiver, transmitter and antenna, which is a part of satellite entity. The transmission power of a typical communication transponder is 5 ~ 8.5 watts (at present, the transmission power of satellite TV transponder is about tens of watts ~ 100 watts), the working frequency of C-band is 4 ~ 6 GHz, and the bandwidth is 36MHz. Ku band is 12 ~ 14 GHz with a bandwidth of 54MHz. A group of communication satellites usually has 12 ~ 24 transponders.
Double-core parallel feeder The transmission line commonly used to connect antenna and TV set in 1950s has been replaced by coaxial cable with better performance.
UHF (ultra high frequency) ultra high frequency band frequency is 300 MHz ~ 3 GHz. The frequency band of terrestrial radio and television is 470 ~ 890 MHz.
Uplink uplink channel A channel through which a satellite earth station transmits information to a satellite.
Vertical blanking The process of blanking a frame to eliminate the retrace lines between frames.
Very high frequency (VHF) VHF frequencies range from 30 MHz to 300 MHz.
VHS format VHS format is one of the two most common formats for home video recorders, and the other is Beta format.
Video monitor has no TV set with tuner, and can only receive baseband TV signals, but can't receive VHF and UHF broadcast TV signals. The resolution and image quality of the monitor are higher than those of ordinary TV sets.
Rectangular or elliptical waveguide metal tubes are used for microwave signal transmission.
Wind load Wind pressure load The pressure exerted by the wind on the receiving antenna of the satellite ground station. A well-designed parabolic antenna should be able to withstand the wind load with a wind speed of 64 km/h, without obvious damage, and should even be able to withstand the wind speed of 160 km/h.
Surface acoustic wave filter (SAWF)
Surface acoustic wave filter is made of quartz, lithium niobate and barium titanate crystals with piezoelectric effect. The so-called piezoelectric effect is the phenomenon that when the crystal is subjected to mechanical action, it will produce an electric field proportional to the pressure. Crystals with piezoelectric effect will also deform elastically under the action of electrical signals, and emit mechanical waves (sound waves), which can convert electrical signals into acoustic signals. Because this kind of sound wave only propagates on the crystal surface, it is called surface sound wave. SAWF is short for SAWF. SAWF has the advantages of small size, light weight, reliable performance and no need for complicated adjustment. Key equipment for realizing adjacent frequency transmission in CATV system. SAW filter is characterized by a flat (1) frequency response, with an unevenness of only 0.3- 0.5dB and a group delay of 30-50 ns. (2)SAWF has a good rectangular coefficient, and the out-of-band rejection can reach more than 40dB. (3) Although the insertion loss is as high as 25-30dB, the amplifier can be used to compensate the level loss. Surface acoustic wave filter includes surface acoustic wave TV image intermediate frequency filter, TV sound filter and TV channel residual sideband filter. Typical technical indicators of SAW filter are shown in the following table.
comb filter
Comb filter consists of many passbands and stopbands arranged at a certain frequency interval, and only allows signals within a certain frequency range to pass. Comb filter is called comb filter because its characteristic curve is like a comb. Comb filter has many applications in TV technology. Comb filter is used to separate two orthogonal components of chroma signal, namely U color difference signal and V color difference signal. Comb filter generally consists of delay, adder, subtractor and bandpass filter. For still images, comb filtering is carried out between frames, that is, three-dimensional comb filtering. For moving images, comb filtering is carried out within frames, that is, two-dimensional comb filtering. Except for special requirements, most digital TV equipment or high-quality digital TV receivers can obtain satisfactory image quality by cascading a line delay comb filter and a bandpass filter to form a Y and C separation scheme. The use of comb filter can obviously improve the image quality. The interference spots and patterns caused by bright color strings and bright color strings are solved. Eliminate the color edge peristalsis caused by the overlap of u and v; Eliminate bright and colored edges.
attenuator
A circuit that introduces a predetermined attenuation in a specific frequency range. It is generally expressed by the number of decibels that introduce attenuation and the number of ohms of its characteristic impedance. Attenuators are widely used in CATV systems to meet the requirements of multi-port level. Such as controlling the input and output levels of the amplifier and controlling branch attenuation. There are two kinds of attenuators: passive attenuator and active attenuator. The active attenuator is combined with other thermal sensitive elements to form a variable attenuator, which is installed in the amplifier and used for automatic gain or slope control circuit. Passive attenuator has fixed attenuator and adjustable attenuator. Fixed attenuators are composed of resistors, which do not affect the frequency characteristics, and usually consist of T-type or π-type networks. (For the resistance data of attenuating different T-type and π-type impedances, please refer to the book Antenna Television System); The adjustable attenuator consists of potentiometers and is used for debugging and level adjustment. It is required that the input and output impedance of the attenuator match the interface terminal, which is 75 ohms in the cable TV system. The frequency characteristics of the attenuator should meet the requirements of the system frequency range, and the attenuation of the attenuator should be independent of the frequency within the frequency range. Therefore, it usually consists of resistive elements. The form of attenuator varies with the frequency range. Useful coaxial line as attenuator; In waveguide system, the diaphragm that absorbs electric field energy is often used as attenuator; There are also electrically adjustable attenuators, which use solid-state diodes (such as PIN diodes) to make waveguide or coaxial system in microwave band. Attenuators are usually used in various telecommunication equipment and electronic instruments.
equalizer
In telecommunication equipment, it is used to correct attenuation (transmission loss) caused by networks with different frequencies and different phase differences. It can correct the relationship between attenuation and frequency, and is called "attenuation equalizer". The one that can correct the relationship between phase difference and frequency is called "phase equalizer". Equalizers are often used in cable TV systems. An equalizer is usually connected in series in the amplifier circuit. The equalizer is set to balance the inconsistent attenuation of high-frequency and low-frequency signals caused by cable transmission. Because the attenuation characteristics of cables increase with the increase of frequency. Commonly used attenuation equalizer, also known as amplitude equalizer. Generally, it consists of coils, capacitors, resistors and other components. The characteristic impedance of attenuation equalizer is equal to a fixed value, and its balance value is the decibel difference of attenuation between high and low frequency reference points of cable. The frequency characteristic of equalizer is just the opposite to that of cable, but the low frequency attenuation is large and the high frequency attenuation is small, which plays a balancing role. Equalizers are often made into small printed board plug-in structures, which are divided by the average measured size.
blender
The device for mixing more than two groups of radio frequency programs (signals) with different frequencies to form a broadband multi-channel radio frequency program output is a mixer. At the front end of CATV system, the mixer is the distribution point of system signals, that is, all the technically processed multi-channel RF signals are concentrated at the input end of the mixer, and then the signals are output at the output end of the mixer and distributed to the system network for users. Main technical requirements of mixer. Operating frequency: If the mixer is broadband, the frequency should meet the requirements of the whole frequency band in the system. If the mixer is channel type, the frequency should meet the requirements of each channel to be mixed; Access loss: When a signal passes through a passive network, it is always hoped that the access loss (insertion loss) will be as small as possible. The ratio of input power to output power of mixer is called the access loss of mixer. Access loss is usually expressed in decibels. When expressed in decibels, it is the difference between input level decibels and output level decibels. Different mixers have different access losses; Input and output impedance: In order to match all interfaces in the whole system, the input and output impedance of the mixer should be 75 ohms; Mutual isolation between input terminals; Ideally, when a signal is added at any input of the mixer, the signal cannot appear at other inputs, and when any input is open or short-circuited, the other inputs should not be affected. But there is always some influence. When the two terminals are matched, a signal is applied to one input terminal, and the difference between the signal level and the signal level appearing at other inputs is the mutual isolation between the input terminals of the mixer, which is generally expressed in decibels. There are different requirements for different mixers, generally above 20 decibels.
Intermodulation index
In cable TV system, when the amplifier amplifies TV signals of multiple channels, the transmitted signals are mixed with each other due to the nonlinear effect (mainly quadratic term) of the amplifier, and the generated sum frequency or difference frequency falls within the frequency range of the channel to be received, which will enter the TV receiver together with the useful signal and cause interference. It is called intermodulation for short. Intermodulation is closely related to frequency. Intermodulation interference, resulting in ripple or twill interference. The intermodulation ratio is defined as IM=20lg carrier level rms/intermodulation product rms. According to the national standard, IM≥57dB, which should be taken as 58dB in design.
Cross modulation index
When an amplifier amplifies TV signals of multiple channels in a cable TV system, the mirror carrier of the required channel is interfered by the amplitude change of modulation waves of other (interfering) channels due to the influence of nonlinear devices (mainly cubic terms) in the amplifier, which is called crosstalk modulation or crosstalk.