Basics of Designing MATV

March 28, 2018 | Author: Ibrahim Ghannam | Category: Signal To Noise Ratio, Amplifier, Cable Television, Antenna (Radio), Decibel



Basics of designing MATV/community antenna systemsTranscript of the lecture for antenna installers Basic requirements on signal parameters | Basic requirements on signal parameters | Stages of designing distribution network | | Design recommendations | Basic terms | Examples of calculations | The measurements should be made with signal level meter. y y y y recommended signal level at subscriber outlet Uabmin more than 62 dBuV (depends on the band) recommended signal level at subscriber outlet Uabmax less than 80 dBuV (depends on the band) minimum S/N ratio S/Nmin: 43dB (TV), 55dB (FM-stereo) minimum ratio of signal to intermodulation products S/Imin 60dB forces reduction of maximum output signal according to the Table 1 Table 1. Reduction of maximum output signal of an amplifier as a function of the number of programs. Number of programs Level reduction [dB] y y 2 3 4 5 6 7 8 12 16 20 24 28 36 0 2 3.5 4.5 5 5.5 6 8 9.5 10.5 11.5 11.7 12.5 all programs should have the same power level maximum differences between signal levels o - 3dB (neighboring channels) o - 6dB (within any band 60 MHz wide) o - 10dB (within the whole spectrum) minimum isolation between two receivers: -44 dB. all parameters of distributing installation should be kept over 95% of time gain of the antennas used shouldn't change more than 0.5 dB within one TV or FM channel. Stages of designing distribution network: | Basic requirements on signal parameters | Stages of designing distribution network | | Design recommendations | Basic terms | Examples of calculations | y y y 1) Measurements of signal levels in amplifier input/s (antenna output/s). The measurements can be done with e.g. TM 3000 R10502 signal level meter. The aim is to determine whether the levels are sufficient for further processing, i.e. amplifying and distributing the signals to ensure the intended signal levels in all subscriber outlets (Table 2). If needed, preamplifiers should be used. Table 2. Minimum signal levels at inputs of RF distribution amplifiers. Band FM FM FM TV I TV II TV TV TV V g. Design recommendations: | Basic requirements on signal parameters | Stages of designing distribution network | | Design recommendations | Basic terms | Examples of calculations | y within one amplifier section from point A to point B (the amplifier and all dissipative elements to the next amplifier or receiver) the effective amplification factor should be 0 dB . taps) and optimal deployment of cables.the concept of distribution (e. In a network based on branches and taps. If Lmin and Lmax do not meet recommendations. The staircases are linked with a splitter. power supplying etc. and 3 dB margin. 4) In accordance with the chosen type of the network we have to create its diagram.then it is twostage system).(mono) (stereo) (stereo Hi±Fi) III IV Minimum signal level 43 51 61 53 53 54 55 56 [dBuV] 2) Reconnaissance of locations for placing distribution devices (amplifiers. The calculations will allow to find minimum attenuation Lmin (usually in the outlet closest to the amplifier) and maximum attenuation Lmax (usually in the farthest outlet). the design has to be revised. Location of the splitter should ensure the same or very similar distances to all subscribers. i. (L dB . In the case of a network based on distribution of signal from one central point.e. with use of additional amplifiers if needed (for larger number of subscribers in staircases . the amplifier only compensates the attenuation in the section. 8) The last stage is adjustment of the set of channel amplifiers to equalize levels of all programs. but first of all . each story is equipped with a tap characterized by adequate through and tap-off attenuation (taking into consideration differences in signal levels due to different lengths of the cables). 3) Choice of the type of distribution network is determined by accessibility to ducts. which makes it possible to increase the number of distributed programs in the future (up to two times). 7) The maximum output level Ubamax of the amplifier is determined by adding to Uba the allowance for reduction of intermodulation distortions (Table 1). all taps are situated in this point. 6) To the L value we add Uabmin level and 3 dB safety margin that allows for connecting two receivers.A dB = 0) . program packages). 5) Calculations of attenuation L between building amplifier and subscriber outlets. having required Uba level in amplifier output. especially in older TV sets that are not so selective as the modern televisions. VHF antennas should have lower energetic gain than UHF antennas it is recommended to use one.broadband solutions are characterized by larger attenuation and low separation. unless there is a group of programs that have very similar levels generally. the amplifiers used should ensure adequate correction. 56. 52. 54.3 dB.or two-channel spacing (e.6 dB) it is needed to calculate effective signal-to-noise ratio of the network and compare it with S/Nmin value . 54 etc. all outputs of network devices have to be closed with terminating resistors.g. 4 amplifiers .).lack of proper matching within the whole frequency spectrum and interferences injected to the network by the receivers (heterodynes) cause interferences that deteriorate reception quality. which means imperceptible changes of image and sound quality it is not recommended to use splitters for distribution of signal among larger number of receivers .correctly designed MATV systems with one amplifier ensure that the ratio is only slightly larger than that of a typical TV set. taps ensure better separation among subscriber outlets. due to intermodulation distortions we have to adequately reduce output level (2 amplifiers . unless the number of distributed programs is larger than possible in such an allocation. The unfavorable phenomenon can be reduced by especially careful signal leveling in the network.4.Illustration of amplifier section y y due to attenuation of coaxial cable increasing with frequency. if the network employs more than one amplifier connected in series.they also amplify unwanted signals each program should be received by a separate antenna. Without a channel spacing there is a risk of interferences coming from the neighboring channels.7 dB. including last outlets in pass-through networks (unless there are used dedicated terminating outlets) . 50. 3 amplifiers . which practically eliminates these problems FM radio channels are considered as one channel and the calculations only take into consideration the strongest signal it is not recommended to use band amplifiers at the input of the installation . 58 or 51. y y y y y y y Illustration of channel spacing y y the best choice is band outlets (separation of FM and TV signals) . 39. Unrecommended pairs of channels/bands (1) y y y y y y y Interfering channel/band 1 2 FM 6 7 8 Interfered TV channel 4 5 3 10 11 12 If it is necessary to distribute programs in the pairs of channels given in Table 3. 54 27. unless there is another recommendation in the user's manual of the amplifier it should be avoided to distribute programs in the pairs of channels given below: Table 3. 28. Table 4. 50. especially when the output signal is the sum of the signals from the inputs (e. using a number of antenna preamplifiers.y all inputs of the devices used have to be closed. it is needed to ensure proper supplying voltage (12VDC or 24VDC) and current output of the power supply. 60 5. taps. it is recommended to set the level of FM broadcasts ca 10dB lower than TV channels.over 50 dB. and splitters. 49. As an exception. 23. 47. it may be enough to set the gain to a minimum. 39. 59 12. in the case of large networks with long cables it is recommended to use channel converters to avoid distribution of programs in the channels used by terrestrial TV transmitters. every doubling of the number of programs forces us to decrease output level by 3dB. the separation between close receivers has to be over 54 dB. inputs of an amplifier). 60 42 45 48 51 54 57 . if the network distributes more than two programs. 48 23. Unrecommended pairs of channels/bands (2) Interfering channel 1 2 3 4 5 6 7 8 9 10 11 Interfered channel/s 3. 8. 35. the same applies to amplifiers.g. antennas and the masts have to be very well grounded. when the unused inputs are equipped with dedicated gain adjustment. 34. and in the case of channels listed in Table 4 . 6. calculations are indispensable for achieving good quality of distribution network. 44. the voltage at 75 ohm load in mV. The amplifiers can be connected in a cascade. Community antenna (MATV system) .in case there is more than one) of active device with rated load. Signal level is then the ratio of the applied voltage to the reference 1mV (across standardized 75 ohm resistance).electric field strength expressed in decibels (dBmV/m) . deployed in different parts of the network. cable TV system compensates network attenuation through a number of section amplifier sets.the reference level is 1mV/m. The power reference level 0 dBmV means the power dissipated in 75 ohm resistance when signal of 1mV voltage is applied to. Field intensity level . Another feature of large cable installations is a multi-layer (tree) structure.maximum level of the signal in the output (each output . condominiums.12 21 22 : 60 26 27 : The number of interfered channel is higher by 5 in comparison with the interfering channel : 54 55 : 59 60 Basic terms | Basic requirements on signal parameters | Stages of designing distribution network | | Design recommendations | Basic terms | Examples of calculations | Community antenna (MATV system) may be defined as antenna installation where compensation of network attenuation is performed by one (central) set of amplifiers.method of receiving and distributing FM and TV signals in apartment houses. So the actual signal level can be determined as: P=20logU where: U . Signal power level (also: signal level or level) . expressed in decibels (dB). Kinds of MATV systems: y with one amplification stage . and public utilities. Maximum output level . Unlike MATV. The level of FM radio signal is defined as rms voltage of the carrier frequency.the ratio of signal power to the reference level. of TV signal rms voltage of the video carrier frequency in peaks of synchronization pulses. limited by intermodulation distortions. Star topology y distributed bus (with tap-off branches) .y with two amplification stages MATV system with one amplification stage . in a condominium.antenna installation up to 1000 outlets. Distributed bus with tap-off branches . based on cascade of two amplifiers (or two sets of amplifiers) that are situated in a considerable distance one from another.(10-15 outlets) to medium-size installations (100-200 outlets). guarantying small differences of signal levels among outlets. Topologies of MATV systems: y star topology (active and passive elements located in a small number of nodal points) from small. MATV system with two amplification stages connected in cascade . This kind of installation is used in large buildings or in the case of groups of buildings e.medium-size installations (100-200 outlets) easy to build and guarantying good isolation among subscriber outlets.g. based on one amplifier or set of amplifiers .built in small or medium buildings.antenna installation up to 200 outlets. up to 1000 outlets.they combine two or more topologies .two amplifiers connected in cascade .Line topology . with 100-200 outlets. The old installations often used outlets with resistors.two levels y hybrid topologies .used in large systems. y Tree topology . no possibility of using program packages.not used these days. .pass-through network y line topology . and their frequency range was limited to 230 MHz or 606 MHz tree topology (two levels) . amplifies at least two TV bands and has continuous frequency characteristics within the bands .g. as well as to eliminate possibility of collision (the same frequency) of y . e.amplifies one TV band multi-band amplifier . used as the last element in the series in vertical line section. used e. which lowers interferences and reflections in the network the way of utilizing low channels .g. used as intermediate element in vertical line section terminal outlet . II.amplifies two or more TV bands broadband amplifier .within I.amplifies one TV channel or the FM band band amplifier . equipped with terminating resistor low-loss terminal outlet (no loop) .according to gain control: y y y no gain adjustment manual gain adjustment automatic gain control (AGC) . at the end of line in star-shaped network.shift of the original channel to any suitable one. Channel conversion .characterized by high tap-off attenuation.according to frequency ranges: y y y y channel amplifier .characterized by low attenuation.2150 MHz) in order to organize the structure of received channels. Application: y y the way of escaping from channels occupied by terrestrial TV transmitters.characterized by high tap-off attenuation. from 28 to 10.the only available in older installations .Classification of amplifiers used in MATV systems: .according to location in the network: y y input amplifier (or preamplifier) line/end amplifier .change of frequency of programs from the first satellite IF (signals from LNB to sat receiver: 950 MHz . III (and sometimes IV) TV bands in SMATV systems .according to powering: y y local powering remote powering (usually via the signal line) Classification of terminal/subscriber outlets based on application in specific kind of distribution network: y y y pass-through outlet . programs from different satellites and polarizations in one-cable installation built without multiswitches. General requirements and examination methods. Warsaw 1997 Examples of calculations | Basic requirements on signal parameters | Stages of designing distribution network | | Design recommendations | Basic terms | Examples of calculations | . Channel conversion Sources: y y y PN 79/T-05210 Community antenna installations. unless some of them are converted (from AM into FM modulation. That's why the networks are capable of providing return channel (below 50 MHz) for data transmission. 21 to directive of Minister of Communications from October 4. increasing maximum range of the network through use of lower channels AM radio programs are not distributed in cable installations. 1997: Technical specifications of components used in cable TV. and shifted into FM band). PKNMiJ Warsaw 1980 Annex No. Usually the basic set includes terrestrial broadcasts. Channel conversion allows for: y y grouping some programs in order to form program packets (solution used in bigger networks or cable TV systems offering various sets of programs for different prices). and 3 dB reserve: The required maximum output level of the amplifier is 112 dBuV .2 dB/m=2 dB tap-off attenuation: 16 dB pass-through attenuation: 2*1.2 dB/m=6 dB attenuation of splitter: 4 dB attenuation of distribution cable: 5 m*0. reserve) reserve: 3 dB pass-through attenuation of subscriber outlet:2 dB attenuation of cable linking tap's output and subscr.Distributed bus with tap-off branches y y y y y y y y y y y y y minimum level in subscriber outlet: 62 dBuV (incl.2 dB/m=1 dB minimum level in amplifier's output: 102 dB due to 8 channels (from Table 1) and 7 dB reserve. outlet: 10m*0.5 dB=3 dB pass-through attenuation: 3*1 dB=3 dB attenuation of cable in vertical line section: 30m*0. 2 dB/m=6 dB attenuation of upper splitter: 4 dB attenuation of distribution cable: 5 m*0. outlet: 10m*0.___ Star-shaped network y y y y y y y y y y reserve: 3 dB pass-through attenuation of subscriber outlet:2 dB attenuation of cable linking tap's output and subscr.2 dB/m =1 dB minimum level in amplifier's output: 105 dB due to 8 channels (from Table 1) and 7 dB reserve. .2 dB/m=2 dB tap-off attenuation: 20 dB attenuation of lower splitter: 5 dB attenuation of cable in vertical line section: 30 m*0. y and 3 dB reserve: The required maximum output level of the amplifier is 115 dBuV .
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