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TV & SAT TV

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Bazele sistemelor de distribuţie TV-CATV
Transcript of the lecture for antenna installers
Basic requirements on signal parameters
The measurements should be made with signal level meter.
  • 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

2

3

4

5

6

7

8

12

16

20

24

28

36

Level reduction [dB]

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
    • - 3dB (neighboring channels)
    • - 6dB (within any band 60 MHz wide)
    • - 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:
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 (mono)

FM (stereo)

FM (stereo Hi–Fi)

TV I

TV II

TV III

TV IV

TV V

Minimum signal level [dBuV]

43

51

61

53

53

54

55

56

2) Reconnaissance of locations for placing distribution devices (amplifiers, taps) and optimal deployment of cables.
3) Choice of the type of distribution network is determined by accessibility to ducts, power supplying etc, but first of all - the concept of distribution (e.g. program packages).
4) In accordance with the chosen type of the network we have to create its diagram. In a network based on branches and taps, 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). In the case of a network based on distribution of signal from one central point, all taps are situated in this point. The staircases are linked with a splitter, with use of additional amplifiers if needed (for larger number of subscribers in staircases - then it is two-stage system). Location of the splitter should ensure the same or very similar distances to all subscribers.
5) Calculations of attenuation L between building amplifier and subscriber outlets. 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). If Lmin and Lmax do not meet recommendations, the design has to be revised.
6) To the L value we add Uabmin level and 3 dB safety margin that allows for connecting two receivers, having required Uba level in amplifier output.
7) The maximum output level Ubamax of the amplifier is determined by adding to Uba the allowance for reduction of intermodulation distortions (Table 1), and 3 dB margin, which makes it possible to increase the number of distributed programs in the future (up to two times).
8) The last stage is adjustment of the set of channel amplifiers to equalize levels of all programs.
Design recommendations:
  • 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 , i.e. the amplifier only compensates the attenuation in the section, (L dB - A dB = 0)
Illustration of amplifier section
  • due to attenuation of coaxial cable increasing with frequency, the amplifiers used should ensure adequate correction;
  • if the network employs more than one amplifier connected in series, due to intermodulation distortions we have to adequately reduce output level (2 amplifiers - 3 dB, 3 amplifiers - 4.7 dB, 4 amplifiers - 6 dB)
  • it is needed to calculate effective signal-to-noise ratio of the network and compare it with S/Nmin value - correctly designed MATV systems with one amplifier ensure that the ratio is only slightly larger than that of a typical TV set, 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 - 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; taps ensure better separation among subscriber outlets, 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 - they also amplify unwanted signals
  • each program should be received by a separate antenna, unless there is a group of programs that have very similar levels
  • generally, VHF antennas should have lower energetic gain than UHF antennas
  • it is recommended to use one- or two-channel spacing (e.g. 50, 52, 54, 56, 58 or 51, 54 etc.), unless the number of distributed programs is larger than possible in such an allocation. Without a channel spacing there is a risk of interferences coming from the neighboring channels, especially in older TV sets that are not so selective as the modern televisions. The unfavorable phenomenon can be reduced by especially careful signal leveling in the network;
Illustration of channel spacing
  • the best choice is band outlets (separation of FM and TV signals) - broadband solutions are characterized by larger attenuation and low separation;
  • all outputs of network devices have to be closed with terminating resistors, including last outlets in pass-through networks (unless there are used dedicated terminating outlets)
  • all inputs of the devices used have to be closed, especially when the output signal is the sum of the signals from the inputs (e.g. inputs of an amplifier). As an exception, when the unused inputs are equipped with dedicated gain adjustment, it may be enough to set the gain to a minimum;
  • using a number of antenna preamplifiers, it is needed to ensure proper supplying voltage (12VDC or 24VDC) and current output of the power supply;
  • 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;
  • antennas and the masts have to be very well grounded, the same applies to amplifiers, taps, and splitters;
  • it is recommended to set the level of FM broadcasts ca 10dB lower than TV channels;
  • calculations are indispensable for achieving good quality of distribution network;
  • if the network distributes more than two programs, every doubling of the number of programs forces us to decrease output level by 3dB, 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. Unrecommended pairs of channels/bands (1)

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, the separation between close receivers has to be over 54 dB, and in the case of channels listed in Table 4 - over 50 dB.
Table 4. Unrecommended pairs of channels/bands (2)

Interfering channel

Interfered channel/s

1

3, 6, 28, 39, 49, 50, 60

2

5, 8, 23, 35, 47, 59

3

12, 34, 48

4

23, 39, 54

5

27, 44, 60

6

42

7

45

8

48

9

51

10

54

11

57

12

60

21

26

22

27

:

:

The number of interfered channel is higher by 5

in comparison with the interfering channel


:

:

54

59

55

60

Basic terms
Community antenna (MATV system) may be defined as antenna installation where compensation of network attenuation is performed by one (central) set of amplifiers. The amplifiers can be connected in a cascade.
Unlike MATV, cable TV system compensates network attenuation through a number of section amplifier sets, deployed in different parts of the network. Another feature of large cable installations is a multi-layer (tree) structure.
Community antenna (MATV system) - method of receiving and distributing FM and TV signals in apartment houses, condominiums, and public utilities.
Signal power level (also: signal level or level) - the ratio of signal power to the reference level, expressed in decibels (dB). The power reference level 0 dBmV means the power dissipated in 75 ohm resistance when signal of 1mV voltage is applied to. Signal level is then the ratio of the applied voltage to the reference 1mV (across standardized 75 ohm resistance).
So the actual signal level can be determined as:
P=20logU
where: U - the voltage at 75 ohm load in mV.
The level of FM radio signal is defined as rms voltage of the carrier frequency, of TV signal - rms voltage of the video carrier frequency in peaks of synchronization pulses.
Field intensity level - electric field strength expressed in decibels (dBmV/m) - the reference level is 1mV/m.
Maximum output level - maximum level of the signal in the output (each output - in case there is more than one) of active device with rated load, limited by intermodulation distortions.
Kinds of MATV systems:
  • with one amplification stage
  • with two amplification stages
MATV system with one amplification stage - antenna installation up to 200 outlets, based on one amplifier or set of amplifiers - built in small or medium buildings.
MATV system with two amplification stages connected in cascade - antenna installation up to 1000 outlets, based on cascade of two amplifiers (or two sets of amplifiers) that are situated in a considerable distance one from another. This kind of installation is used in large buildings or in the case of groups of buildings e.g. in a condominium.
Topologies of MATV systems:
  • star topology (active and passive elements located in a small number of nodal points) - from small- (10-15 outlets) to medium-size installations (100-200 outlets), guarantying small differences of signal levels among outlets;
Star topology
  • distributed bus (with tap-off branches) - medium-size installations (100-200 outlets) - easy to build and guarantying good isolation among subscriber outlets;
Distributed bus with tap-off branches
Line topology - pass-through network
  • line topology - not used these days, with 100-200 outlets, no possibility of using program packages. The old installations often used outlets with resistors, and their frequency range was limited to 230 MHz or 606 MHz
  • tree topology (two levels) - two amplifiers connected in cascade - up to 1000 outlets.
Tree topology - two levels
  • hybrid topologies - they combine two or more topologies - used in large systems.
Classification of amplifiers used in MATV systems:
- according to frequency ranges:
  • channel amplifier - amplifies one TV channel or the FM band
  • band amplifier - amplifies one TV band
  • multi-band amplifier - amplifies two or more TV bands
  • broadband amplifier - amplifies at least two TV bands and has continuous frequency characteristics within the bands
- according to gain control:
  • no gain adjustment
  • manual gain adjustment
  • automatic gain control (AGC)
- according to location in the network:
  • input amplifier (or preamplifier)
  • line/end amplifier
- according to powering:
  • local powering
  • remote powering (usually via the signal line)
Classification of terminal/subscriber outlets based on application in specific kind of distribution network:
  • pass-through outlet - characterized by high tap-off attenuation, used as intermediate element in vertical line section
  • terminal outlet - characterized by high tap-off attenuation, used as the last element in the series in vertical line section, equipped with terminating resistor
  • low-loss terminal outlet (no loop) - characterized by low attenuation, used e.g. at the end of line in star-shaped network.
Channel conversion - shift of the original channel to any suitable one, e.g. from 28 to 10.
Application:
  • the way of escaping from channels occupied by terrestrial TV transmitters, which lowers interferences and reflections in the network
  • the way of utilizing low channels - the only available in older installations - within I, II, III (and sometimes IV) TV bands
  • in SMATV systems - change of frequency of programs from the first satellite IF (signals from LNB to sat receiver: 950 MHz - 2150 MHz) in order to organize the structure of received channels, as well as to eliminate possibility of collision (the same frequency) of programs from different satellites and polarizations in one-cable installation built without multiswitches.

Channel conversion allows for:
  • 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). Usually the basic set includes terrestrial broadcasts;
  • increasing maximum range of the network through use of lower channels
AM radio programs are not distributed in cable installations, unless some of them are converted (from AM into FM modulation, and shifted into FM band). That's why the networks are capable of providing return channel (below 50 MHz) for data transmission.
Channel conversion
Sources:
  • PN 79/T-05210 Community antenna installations. General requirements and examination methods.
  • PKNMiJ Warsaw 1980
  • Annex No. 21 to directive of Minister of Communications from October 4, 1997: Technical specifications of components used in cable TV; Warsaw 1997
Examples of calculations
Distributed bus with tap-off branches
  • minimum level in subscriber outlet: 62 dBuV (incl. reserve)
  • reserve: 3 dB
  • pass-through attenuation of subscriber outlet:2 dB
  • attenuation of cable linking tap's output and subscr. outlet: 10m*0.2 dB/m=2 dB
  • tap-off attenuation: 16 dB
  • pass-through attenuation: 2*1.5 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 splitter: 4 dB
  • attenuation of distribution cable: 5 m*0.2 dB/m=1 dB
  • minimum level in amplifier's output: 102 dB
  • due to 8 channels (from Table 1) and 7 dB reserve,
  • and 3 dB reserve:
The required maximum output level of the amplifier is 112 dBuV

___
Star-shaped network

  • reserve: 3 dB
  • pass-through attenuation of subscriber outlet:2 dB
  • attenuation of cable linking tap's output and subscr. outlet: 10m*0.2 dB/m=2 dB
  • tap-off attenuation: 20 dB
  • attenuation of lower splitter: 5 dB
  • attenuation of cable in vertical line section: 30 m*0.2 dB/m=6 dB
  • attenuation of upper splitter: 4 dB
  • attenuation of distribution cable: 5 m*0.2 dB/m =1 dB
  • minimum level in amplifier's output: 105 dB
  • due to 8 channels (from Table 1) and 7 dB reserve,
  • and 3 dB reserve:
The required maximum output level of the amplifier is 115 dBuV
 
 
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