MATV stands for Master Antenna Television. It is the means by which many
An MATV system is basically a network of cables and specially designed components that process and amplify TV and FM signals and distribute them from one central location . If there were 100 TV sets in a building, it would be extremely expensive to Install and maintain l00 separate antennas. Not only would It be unsightly, but reception would suffer because that many antennas would interact with each other, causing interference problems.
The MATV system concept can be separated into two divisions: the Head End and the Distribution System
The Head End normally consists of an antenna Installation to receive the desired signals to, processing equipment to filter the signals and remove interference, and a distribution amplifier to amplify the signals to the level required to provide an adequate signal to every receiver in the system. Antenna amplifiers, traps, filters, antenna mixing units, and UHF converters are among the equipment used in this portion of the system.
A welt-designed Distribution System is necessary to guarantee that an adequate signal will be delivered to every receiver. It should provide a clean signal to the sets by isolating each receiver from the system and by delivering the proper amount of signal to each set. This portion of the system consists of trunk lines, splitters, feeder lines, and tap-off. Some of the other equipment used includes tine taps, variable isolation wall taps, coaxial cable, and band separators.
It is important to design your distribution system first. Usually, the requirements of the distribution system will determine the type and size of the amplifier that will be necessary at the head end. The actual layout of the system, of course, will depend upon the dimensions and requirements of the building in which it Is installed.
2 HEAD END EQUIPMENT
The antenna is the first component of any MATV system to receive a broadcast signal. And, since the ultimate quality of the TV reception can be no better than the quality of the signal from the antenna, it Is vital that the antenna be selected with care.
Some MATV installations use broadband antennas. However, if the channels to be received lie in different directions or If adjacent channel reception is desired, single channel antennas may be required. (The criteria for selecting antennas will be discussed in a later section.) The number of channels to be received, the directions to the transmitters, the type of signals (UHF, VHF, FM), and the available signal levels all must be considered when designing an antenna installation.
Many Installations will require the use of satellite TVRO antennas. The Installation of a satellite antenna requires attention to potential microwave interference sources, the exact satellites and transponders to be received, and proper mounting techniques to minimize wind load dangers.
A Most antennas have an impedance of 300 ohms, while most MATV equipment has an Impedance of 75 ohms. Therefore, in order for maximum signal transfer to take place, an impedance matching device must be inserted into the line to match the 300 ohm antenna to the 75 ohm coaxial cable and MATV equipment. This is accomplished by a 300-to-75-ohm Matching Transformer called a Balun. The balun is mounted as close as possible to the antenna terminals.
In weak signal areas, it is often necessary to amplify the signal prior to the distribution amplifier in order to get a signal of sufficient strength and acceptable quality. In addition, most MATV preamplifiers act as 300-to-75 ohm matching transformers, eliminating the need for a balun.
Noise is seen on the TV screen as snow, so whenever a preamplifier is needed, It is important to choose a unit with a low noise figure. Because the noise figure of the preamplifier establishes the noise figure of the entire system, the amplifier should always increase the signal much more than it increases the noise. The amplitude of the noise must be kept small in relation to the amplitude of the desired signal.
A modulator accepts any video source and any audio source and combines them onto a single RF channel. Audio and video modulation levels may be adjusted for optimum performance based on the output level desired.
FILTERS AND TRAPS
Channel Rejection `Filters cleanly suppress an entire 6 MHz-wide TV channel so that another video source can be inserted in its place. This reinserted channel may be derived from local origination, VCR, TVRO modulator or any other video source.
Filters and traps are used in the head end to eliminate undesired frequencies and provide interference-free reception. Traps, filters and other head end equipment (except baluns and preamplifiers) are mounted indoors. They should be readily accessible for adjustment and servicing.
ANTENNA MIXING UNITS
If more than one antenna is used, the signals from the various antennas must be combined before they are fed into a broadband distribution amplifier. This can be accomplished by an antenna mixing unit. These units are usually a number of band pass filters in a common housing. The fitters are tuned to pass separate TV channels into a common output.
In addition to combining the signals from various antennas, an antenna mixing unit also filters out interfering frequencies. For example: if channels 2 and 4 are being fed into a low band mixer tuned for channels 2 and 4, and channel 3 is also present, the mixer will filter out most of the channel 3 signal. For this reason antenna mixing units are not recommended when joining adjacent channels. If channel 3 is strong, additional trapping will be required to further reduce adjacent channel interference from channel 3.
If both low band and high band channels are to be received1 the signals from the low band mixer and the high band mixer must be combined into a single line before broadband amplification. This calls for a band separator/joiner. This unit joins (or separates) any VHF low band signals from any VHF high band signal to provide a signal coaxial lead with a minimum of signal interaction.
Antenna mixing units can also be reversed and used to separate the signals from a broadband antenna into separate lines. Stronger signals can then be attenuated to the weaker signal level (equalization) and then recombined before amplification.
Amplifiers are used to increase the strength of received signals to a level greater than the losses in the distribution system. This provides an acceptable level to all sets in the system.
Though the gain of an amplifier (amount of signal increase) is important. The output capability is lust as important. The amplifier's specifications should be checked carefully to make sure that the output level is sufficient to feed the system and that the strength of the input signal plus the gain of the amplifier does exceed its rated output capability. Exceeding the output capability will result in overloading (cross modulation in broadband amplifiers) and overall signal deterioration.
There are two types of amplifiers: broadband and single channel. Broadband amplifiers. The more common type, provide a closely uniform gain across the entire band. Single channel amplifiers allow complete control of both gain and output level of individual channels. This is accomplished by using Automatic Gain Control (AGC) circuitry.
For the most economical installation, the amplifier should be centrally located In relation to the distribution lines. The longer the distribution lines, the more loss in the system and the more costly the system will be to install.
Except for very small residential Installations, MATV systems use 75 ohm coaxial cable to carry the signal throughout the system. Coaxial cable is a concentric transmission line. It consists of a center conductor, a dielectric medium such as polyethylene which fixes the spacing between the center conductor and the outer shield, an outer shield such as copper braid and aluminum foil, and a weatherproof outer jacket, usually vinyl. Coaxial cable has several advantages over 300 ohm twinlead-It can be run through conduit, It stands up better out- doors, it produces less radiation interference, and it virtually eliminates the possibility of direct signal pickup by the center conductor.
Unlike 300 ohm twinlead, coaxial cable cannot be attached to the various pieces of MATV equipment in the system by merely wrapping the center conductor around a terminal. Fittings (connectors) must be employed In most in- stances. All Channel Master 75 ohm MATV equipment uses standard "F" type fittings for faster, easier installations
Most MATV installations use RG-59 cable1 usually the only type of cable used in systems contained within a single building. When used in conjunction with larger size cables (like .412 f or .500) RG-59 is normally used to connect the trunk lines to the individual outlets. RG-59 uses standard 11F" type fittings for easy connection to MATV equipment.
If long spans of cable are needed1 RG-11/U, .412 or .500 cable is usually specified because of Its lower loss. These cables normally have a solid aluminum shield and a flooded jacket which make them an excellent choice for direct burial when running cable between buildings in a multi-building system. Channel Master has cable connectors which allow you to attach these larger cable sizes directly to MATV equipment.
Loss in coaxial cable Is given as attenuation per 100 ft. of cable. A chart indicating attenuation for various cable sizes is found on page 31.
3. Distribution System Equipment
The coaxial cable that carries the signal away from the head end toward the TV sets is called the main trunkline. occasionally MATV systems operate with a single trunkline, but it is usually more efficient to separate (split) the signal into several lines for distribution to the receivers. This is accomplished with the use of a line splitter. Line splitters split the signal into 2, 3 or 4 separate lines. Splitters divide the input signal equally, providing the same amount of signal at each output of the splitter. Backmatched hybrid splitters are recommended for optimum ( performance, as they minimize the possibility of the signal re-entering the system, causing ghosts.
Directional Multi-Taps are used In much the same way as line tapoffs. They are designed for outdoor use, accept cable up to diameter and may be pedestal, or aerial-mounted. All ports are self-sealing, and the housing meets FCC radiation specifications. The unique cover plate may be removed to change Isolation values without removing the tap from the line.
A tapoff is a means of delivering signal from the distribution lines to the TV.sets, while providing enough isolation to prevent the sets from interfering with one another. Tapoffs divide input unequally, sending the smaller portion of the signal to the set. The larger portion is sent further down the line.
Each set In an MATV system should get ap- proximately the same amount of signal. How- ever, because of the losses Involved in any distribution system1 there Is more signal avail- able to sets closer to the amplifier than to sets further down the line. Therefore, tapoffs are made with various values of isolation rather than with a single value in order to achieve a balanced signal distribution.
Channel Master manufactures two types of tapoffs: the wall tap and the line tap. Each performs essentially the same function, but they are used for different installation applications.
A Wall Tap is employed In the same manner as an AC outlet. In new construction the distribution line is run inside the wall and the tap Is mounted in a standard electrical outlet box within the wall. This provides protection for both the tap and the cable and hides unsightly wiring and fixtures.
Wall taps are available with a 300 ohm output, a 75 ohm output or a dual 300/75 ohm output. The signal strength In the area usually determines the type of tap used. Generally, using a 75 ohm tap with a matching transformer is recommended, because in strong signal areas, 300 ohm twin lead tends to pick up signals from the air and can cause ghosting and other forms of electrical Interference 75 ohm coaxial cable prevents this, since It is shielded from direct signal pickup.
Some systems will require an outlet for both television and FM. In this Instance, the dual 300/75 ohm tap can be used. The 75 ohm section is used for the TV and the 300 ohm section is hooked up directly to the FM tuner.
Line Tap off. Are the Ideal units for any system requiring an in-line type tap off. Each tap provides from one to four drop lines. These drop Iines can be connected directly to the set with a matching transformer or run to a "0" dB wall outlet.
Line taps should be used in any MATV installation where losses must be kept to a minimum. They assure low Insertion loss on the thru ports while providing a high degree of isolation between the thru line and the tap ports.
These tapoffs will pass AC/DC voltage on the `thru line to allow for line powered equipment capability, while blocking the voltage at the tap.
Line tapoff units are very effective for use where outlets can be fed from a center run trunkline, such as hallways or attic spaces in schools, motels, hotels, apartment buildings and homes. They are also useful where.a wall outlet type tapoff is not desired, such as a TV sales display area or any area where many sets are in close proximity.
A "0" dB Wall Outlet is used in conjunction with line taps.~lt can be used to match 75 to 300 ohms or for a 75 ohm feed-through. These wall outlets are available as duplex types for flush mounting in standard electrical gem boxes or as flush wall plates.
75 TO 300 OHM MATCHING TRANSFORMERS
Since the characteristic impedance of MATV distribution lines is 75 ohms1 and most TV sets only accept 300 ohms, the signal must be matched to the set. Sometimes this is done with a 300 ohm wall tap. However, it is usually recommended that a 75 to 300 ohm matching transformer be used at the set instead, because it helps eliminate direct signal pickup In strong signal areas.
Band separators are used In all-channel (UHF/ VHF) MATV Systems to separate the UHF signal from the VHF signal before it Is fed Into the TV set. Unlike splitters which divide signal equally, band separators contain circuitry which separates one~band from another band. In addition to UHF/VHF band separators, you may also have occasion to use VHF/FM band separators in VHF/FM Systems.
The end of each 75 ohm distribution cable and any unused port on MATV equipment must be terminated with a 75 ohm resistor to prevent signals from traveling back up the line and causing ghosts. These resistors are called term- inators. Whenever line power is added to a system, a voltage-blocked terminator must be used.
4. The Decibel
The signal levels received on television antennas are usually measured in millionths of a volt or microvolts. Calculations in microvolts are difficult because they often Involve six or seven digit numbers. Therefore1 MATV calculations are made in decibels.
Decibels are used for MAT V calculations because they are convenient. Because they are logarithmic ratios, decibels are added or subtracted instead of multiplied or divided. The number of decibels in MATV calculations is usually less than 100. So by using decibels, calculations become a matter of adding and subtracting two digit numbers rather than multiplying and dividing numbers as large as 1,000,000.
The decibel is 1/10 of a bel-it comes from a mathematical formula used by early telephone and telegraph engineers which has been adapted for television. The formula for the decibel used In MATV is:
Decibel (dB)= 20 log El / E2
However, you don't need to remember the formula to use dB. All you have to do is use the dB tables in this manual.
The decibel has no absolute value. It merely tells how many times greater (or smaller) a quantity Is from a pre-established reference level. lt Is important to realize that the relationship between dB levels Is non-linear. in other words, 40 dB is not merely twice as much as 20 dB It is a lot more. Study these examples:
10dB 3.1 x reference level 40 dB 100 x
20 dB 10 x reference level 50 dB 300x
30dB 32 reference level 60dB 1000x
in the MATV industry the zero reference level-the level to which any plus or minus number of decibels is referred-is 1,000 microvolts measured across 75 ohms of impedance. Why 1,000 microvoits? Well, in the early days of, television, TV sets had very poor noise fIgures.;Through experimentation it was learned that a minimum signal of 1,000 microvolts was required to produce an acceptable picture. This Is why, in MATV, you'll find the dB figure represented as dBmV-a reference to 1,000 microvolts, this figure provides a margin of safety in design, and is still used today.
All MATV amplifier gains, cable losses, Insertion losses and isolation values are expressed in dB. To determine amplifier output and system losses, dB are added or subtracted directly.
Example: An amplifier has an Input signal of 0 dB (1,000 microvolts) and a gain of 50 dB. What is the signal in microvolts available at the last televIsion set if the system has a loss of -36 dB?
Input Signal 0dB (1,000 MIcrovolts)
Amplifier Gain 50 dB
System Loss -38 dB
Available Signal 14 dBmV
What is 14 dBmV? Turn to the chart in Section ii (page 32). Look in the dBmV coiumn on the left until you find 14 dBmV. Now look at the column headed uV and see what the reading is in microvolts. it is 5,000 microvolts. Therefore, the available signal is 5,000 microvolts. The same procedure is applied In calcuiating all system losses. The following section shows how to calculate the losses In dB for a typical system.
5. Designing the Distribution System
Since the losses of the Distribution System, the specific frequencies (I.e. which VHF and/or UHF channels) to be received and the direction of the transmitters determine the requirements of the head end, the distribution system should be designed first. The first step in designing any MATV distribution system is to obtain building plans or a rough layout of the structure and mark the locations of the necessary TV outlets and a central location for the amplifier. You must decide whether the distribution cables are to be run horizontally or vertically. generally speaking1 If the building Is taller than it Is wide1 the cables should run vertically. If the building is wider than it is tall, it is usually more economical to run the cables horizontally. Next determine the number of distribution cable runs necessary to supply every set in the system. Avoid long runs wherever possible- two 400' runs are usually better than one 800' run. Cable runs should be as straight as possible-avoid zigzag runs and loops. One the distribution cables runs have been determined, each tap off and splitter location should be marked. The longest cable run or the one with the greatest number of splitters and tapoffs1 should be used to calculate the distribution system losses. The object Is to use the branch with the greatest loss (in dB), because If you can supply an adequate signal to the last set In that line, you can supply every other set also. When you are in doubt which branch has the highest loss, it is best to calculate the loss on several branches to find the one with the greatest loss.
In general, there are four types of loss that must be considered. They are Cable loss, Splitter loss, Insertion loss, and Isolation loss.
A certain amount of signal will be lost as it travels through coaxial cable. This loss is dependent on two factors: the type, of cable used (refers to the chart of characteristic cable losses, page 31) and the frequency of the signal being carried.
Losses are greater at higher frequencies, the greatest loss occurring at channel 13 in a VHF system or channel 83 In a UHF/VHF system. Always figure the cable loss at channel 13 for a VHF system and the highest frequency that is now being received or will be received in the future for a system including UHF. The Cable Attenuation Chart on page 31 shows losses in dB per 100 feet of various types of coaxial cable.
When a two-way splitter is inserted in the line, the VHF (and/or UHF) signal in each branch leg will be approximately 3.5. dB (4.0 dB) less than that of the main line. If a 4-way splitter is inserted in the main line, the signal in each f branch leg is 6.5 dB (7.2 dB) less than that in'. The main line. The signal sent to each branch of the system will be equal to the signal sent into the splitter minus the splitter loss. That is, an input of 30 dB into a 2-way splitter will deliver a signal of 30 dB minus 3.5 dB splitter loss, or 26.5 dB to each branch of the System.
All tap off devices inserted into the distribution system create signal loss. The amount of this loss is known as the insertion loss of the inserted unit, (sometimes called feed-through loss). The insertion loss of each tap off on the line must be subtracted from the signal carried by that line. When estimating total system losses, the insertion loss of each unit must be added together to find the total insertion loss for that system. For example: If there are 10 tap offs on the line, and each tap off has an insertion loss of .5 dB, the total insertion loss r would be 5 dB.
NOTE: For initial calculations the tap off values and the insertion losses must be estimated because the output of the amplifier will Influence the final selection tap off values.
Each tap off also reduces (attenuates) the signal which it has removed from the line by a specified number of dB to prevent one set from interfering with another.
For example: if there Is a 25 dB signal in the line, and a 23 dB isolation wall tap off Is Inserted in the line, the signal available at the tap off would be 2 dB. The 23 dIEl loss is called Isolation Loss.
In computing the total distribution system losses, we figure the isolation loss of the last tap off only. Since our system design requires that we provide a minimum of 0 dB (1000 uV) to each set, we use the lowest isolation value. For most MATV tap offs this value is 12 dB
. Wall Tap isolation Values with insertion Losses
23 dB isolation-.3 dB insertion loss
17 dB isolation-.7 dB insertion loss
12 dB isolation-.9 dB Insertion loss
SELECTING TAPOFF VALUES
In selecting tap off values, the object is to use ~ taps that will deliver a minimum of 1,000 micro- volts per channel (0 dBmV) to each set in the system1 and provide enough isolation at each set to prevent interference. More than 11000 micro volts at the set will not harm reception, but no set should receive less than 1,000 micro volts per channel. in strong signal areas where there is a possibility of direct signal pickup, it may be necessary to feed more than this to each tap off. Many professional application engineers design their Systems to provide a 10 dBmV signal level at each tap For the sake of discussion and computations here, we will use a 0 dBmV signal level for each tap. The higher the isolation value of the tap, the lower its insertion-so higher value taps mean lower total insertion loss. The insertion loss of a single tap is small compared to its isolation value-but the insertion loss of every tap on the line must be added together to get the total insertion loss for the line. On the other hand1 e as stated earlier, the total isolation loss for the entire line Is determined by only one tap (the last tap off on the line). When you are using line drop taps, you must consider the cable loss between the output of the tap and the TV set. When you are using wall taps, the distance between the tap output and the set is usually so short that cable loss between the tap and the set can be disregarded.
Here is a sample VHF distribution system with losses calculated step by step. We'll go through them one at a time in order to determine the requirements of the head end, and the isolation values at each tap off.
For purposes of illustration, we have selected a system with two equal branches, each having identical isolation values. In a system with unequal branches, we would figure the head end requirements based on the branch with the largest losses.
Our first step is to determine the total system losses...that is the combination of loss incurred through cable, splitter, insertion and isolation. (See page 35 for a chart showing symbols and typical losses for various pieces of equipment.)
Let us assume in this instance that we will be using a low loss, 82 channel coax cable such as Channel Master Super Colorduct which shows losses of approximately 4.2 dB per 100 feet, at channel 13. Note from our diagram that we have:
50' cable leading to the first tap off
30' cable to the second tap off
30' cable to the third tap off
40' cable to the fourth tap off
40' cable to the fifth tap off
190' cable through the entire branch of the system 190' cable @ 4.2 dB attenuation per 100 feet.... gives us 4.2 dB K 1.9 or a total of 8.0 dB cable loss.
The two way splitter used has a loss of 3.5 dB. INSERTION LOSS We have 5 tapoffs In our branch. Note that these are wall tapoffs. Since the losses must,be estimated, we will use the median isolation value....17 dB with attendant i~sertion loss of .7 dIB per tapoff 5 tapoffs K .7 dB=3.5 dB.
Since 12 dB is the least amount of signal we can have at the last tapoff, we will use a tap with an isolation value of 12 dB for this last tapoff.
Our system's losses tally as follows:
Cable Loss 8.0 dB
Splitter Loss 3.5 dB
Insertion Loss 3.5 dB
lsolatiofl Loss 12.0dB
Total System Loss 27.0 dB
The head end must supply at least 27 dB signal to overcome the system's losses1 and deliver a minimum of 0 dB to the last TV receiver on the line.
NOTE: It is generally good practice to allow an additional 6 dB in selecting an amplifier for the system1 however we do not include the extra 6 dB in calculating the isolation values.
Now that we have determined the requirements of the head end, we can determine the isolation values of each tapoff, bearing in mind that each set on the line must receive at least 1,000 uV or 0 dIB input signal
. A rule of thumb In selecting isolation values is to use the highest value possible in order to keep insertion losses to the minimum. For example, where input signals are stronger than 23 dB, we use 23 dB isolation at the tap- off... where signals are lower than 23 dB but greater than 17 dB1 we use 17dB isolation and so forth.
We will proceed through the system step by step, from the distribution amplifier to the final tapoff.
Since the system's losses are 27.0 dBe we will base our calculations on the assumption that we will use an amplifier with a signal output level of 30 dBmV which will provide enough signal to overcome the system losses.
The amplifier sends 30 dBmV of signal to the 2-way splitter, which incurs a 3.5.dS loss.
26.5 dBmV being sent to each branch of the distribution system
NOTE: In instances where it is necessary to locate the amplifier at a distance from the splitter, the cable loss from the amplifier to the splitter will have to be calculated.
FIRST TAPOFF. The splitter sends 26.5 dBmV of signal to the system. This signal must pass through 50 feet of cable to reach the first tapoff. 50' cable at 4.2 dIBI signal loss per 100 feet equals 2.1 dB cable loss.
26.5 dBmV Input Signal
-2.1 dB Cable Loss
24.4 dBmV lnput to First Tapoff
Now, using an isolation value at the first tapoff of 23 dB, we deduct the isolation from the input signal to determine the signal being fed to the set.
24.4 dBmV Input Signal
-23.0 dB Isolation Value
1.4 dBmV Signal being fed to the set. This provides us with more than the required 0 dB signal input to the set.
To figure the input signal to the second tapoff: ~ We have 24.4 dBmV input to the first tapoff9 minus .3 dB insertion loss of that tapoff.
24.4 dBmV Input to the First Tapoff - .3dB InsertionLoss= 24.1 dBmV Signal Level at the Output of First Tapoff
The signal must now pass through 30 feet of cable to reach the second tapoff. 30' of cable at 4.2 dB cable loss per 100 feet equals 1.3 dB cable loss.
24.1 dBmV Output from First Tapoff -1.3 dB Cable Loss = 22.8 dBmV Input to Second Tapoff
SECOND TAPOFF. We now have 22.8 dBmV of signal being fed to the Input of the second tapoff. This requires us to use a tapoff Isolation value of 17 dB9 with an insertion loss of .7 dB
To determine the signal being fed to the set at the second tapoff: We have 22.8 dBmV going into the tapoff, minus the 17dB isolatIon value....
22.8 dBmV Input Signal -17.0 dB Isolation Vatue= 5.8 dBmV Being fed to the set
To determine the Input signal to the third tapoff: We have 22.8 dBmV going Into the second tapoff, minus .7 dB insertion loss....
22.8 dBmV Input to Second Tapoff - .7 dB insertion Loss = 22.1 dBmV Output from Second Tapoff
We now have 22.1 dBmV of signal which must pass through 30' of cable before entering tapoff number 3.30' cable at 4.2 dB loss per 100 feet equals 1.3 dB cable loss.
22.1 dBmV Output from Second Tapoff -1.3dB CableLoss= 20.8 dBmV Input to Third Tapoff
THIRD TAPOFF. We now have 20.8 dBmV of signal at the Input of the third tapoff, allowing us to use a 17 dB isolation value with a .7 dB insertion loss. To determine the signal being fed to the set at the third tapoff:
20.8 dBmV Input Signal -17.0 dB Isolation Value = 3.8dBmV Being fed to the set
To determine the signal level at the input tot he fourth tapoff: we have 20.8 dBmV input to the third tapoff with a .7 dB insertion loss.
20.8 dBmV Input to Third Tapoff - .7dB InsertionLoss = 20.1 dBmV Output from Third Tapoff
The signal now passes through 40 feet of cable. 40' of cable at 4.2 dB cable loss per 100 feet equals 1.7 dB cable loss.
20.1 dBmV Output from Third Tapoff - 1.7dB Cable Loss = 18.4 dBmV lnput to Fourth Tapoff
FOURTH TAPOFF. We have 18.4 dBmV input signal coming to tapoff number 4, which requires our using 17 dB isolation value at the tapoff, again with a .7dB insertion loss. To determine the signal to the set:
18.4 dBmV Input 8 ignal -17.0 dB Isolation Value = 1.4 dBmV Fed to the Fourth Set
To determine the signal being fed to the Input of the fifth tapoff: we have 18.4 dBmV Input to the fourth tapoff, less the .7 dB Insertion loss.
18.4 dBmV Input to Fourth Tapoff - .7 dB insertion Loss = 17.7 dBrnV Output from Fourth Tapoff
We have 17.7 dBmV coming out of the fourth tapoff and passing through 40 feet of cable to reach the fifth tapoff. At 4.2 dB loss per 100 feet, this gives us 1.7 db cable loss.
17.7 dBmV Output from Fourth Tapoff -1.7 dB Cable' Loss =16.0 dBmV Input to Fifth Tapoff
FIFTH TAPOFF. We now have 16.0 dBmV of signal fed to the fifth tapoff, requiring the use of 12 dB isolation. To determine the signal being fed to the set:
16.0 dBmV Input of Fifth Tapoff -12.0dB Isolation Value = 4.0 dBmV Being fed to the Set Since this is the last tapoff in the line, we must terminate the output of the tapoff. In planning your Systems, always remember to include a 16.0 dBmV Input of Fifth Tapoff terminator at the end of each branch tine to erminator at the end of each branch tine to maintain impedance match. Since the other branch of the system has the same number of tapoffs and the same cable lengths, It is a mirror Image of the branch we have just calculated. Therefore1 the same isolation values can be applied. Our system calculations are now complete.
6. Designing the Head End
HEAD END DESIGN Once the distribution system has been designed, you can proceed to design the head end portion of your system. Having calculated the losses (in dB) of the distribution system, you know that the output of the amplifier must equal or exceed that number for each channel. However, you must know the input at the amplifier before you can determine its output.
The first step in designing the head end is to select the proper antenna (or antenna array), because the signals received at the antenna will determine the signal processing compon- ents and the amplifier to be used.
In selecting the correct antenna there are three (3) basic factors to consider: type of antenna required, its gain and its directivity.
The type of antenna to be used will be determined by the number of channels to be received and their directions from the receiving site. If all the transmitters lie in the same direction (or at least several of them), a broad- band antenna may be used. However, if they are in different directions, single channel an- tennas may be necessary. Distance from the transmitter or adjacent channel considerations may als~ require single channel antennas.
If a broadband antenna is used, the channels received will determine the type of broadband antenna. The most common types employed in MATV and home systems are low band VHF (ch. 2-6), high band VHF (7-13), all channel VHF/FM (2-13 plus FM), UHF/VHF/FM, and UHF. Even though the VHF/FM antenna will receive FM signals, a separate FM antenna may be desirable.
The gain of an antenna, like the gain of an amplifier, is an important consideration. The antenna installation should provide at least 0 dB (lOOOuV) of picture signal per channel (each with a reasonably good sound signal) at the amplifier input. In strong signal areas this may be obtained fairly easily. In weak signal areas, however, a larger antenna with a high gain will usually be necessary. It may also be necessary to "stack" two or more antennas. Stacking two antennas will provide an additional 3 dB of gain above the gain of a single antenna. Although a preamplifier may be used1 stacking prior to preamplification is always preferable, since it delivers an initially purer signal to the system.
The directivity of an antenna is another im- portant consideration. Directivity Is a measure of how well an antenna will reject signals from any direction other than its front. The front-to- back ratio is one way of measuring an anten- nas's directivity. It is the ratio of the amount of signal received by the front of the antenna to the amount of signal received by the rear. A highly directional antenna will generally have a high front-to-back ratio (Channel Master Quantum model 111O has a front-to-back ratio of 30 dB on the low band). Stacking an antenna will improve its front-to-back ratio
The determination of exact signal levels (a signal survey) is one of the most important steps in head end design. A signal survey prior to installing the system can help you to avoid many problems before they start. An antenna, several sections of masting, a field strength meter and a portable color TV are the equipment required.
If at all possible, use the type and size antenna that will be installed at the site. If this is not possible, use an antenna that has a known gain so that the actual signal level for the proposed antenna may be determined.
In weak signal areas, antenna location is usually a critical factor. A lateral distance of only 50 feet can produce vastly different signal levels. Antenna height can also make a difference. Although signals normally become stronger as the antenna is raised this is not always true. Optimum height should always be determined by testing.
The field strength meter is used to measure the amount of signal received on each channel.
These levels should be recorded for future use. With these measurements, equalizing signal levels becomes a simple matter, and any need for a preamplifier will be apparent. Since location is important, these measurements should be taken at several points on the site. The point with the best overall signals should be chosen as the location for the antenna installation, if possible.
Signal strength is not the only consideration. Carefully selected antennas can also do much to overcome certain types of interference. The portable TV is used to determine the quality of the signal received on each channel. Several types of interference can be detected by the set.
In extremely weak signal areas and in fringe and deep fringe reception areas, it may be necessary to preamplify the signal in order to provide acceptable quality reception.
There are four important characteristics of preamplifiers to look at before deciding on the correct model to employ in your system: band coverage, gain, noise figure and input capability.
Preamplifiers are available for VHF/FM only, UHF only, or combined UHF/VHF/FM. The channels to be received will determine which type is necessary. Some preamplifiers are available with FM traps if FM reception is not required or creates reception problems.
In extremely weak signal areas, a preamplifier with a high gain figure may be required. The high gain may be required in order to get enough signal to provide sufficient input to the distribution amplifier. The amplifier will then be able to supply an adequate signal for your system. When using single channel amplifiers, a preamplifier may be necessary to provide enough signal for the AGC to function properly.
Noise figure is the single most important specification of a preamplifier. Noise figure is a measure of how much noise an amplifier will generate. The lower the noise figure, the better the signal that will be provided.
Input capability is another important specification. Input capability is a measure of how much signal input the amplifier can accept before it overloads, causing distortion. The higher this figure, the more signal it can handle.
A unit with a high input capability is necessary if you are in a high signal strength area (local stations) and wish to preamplify distant stations in order to get a high-quality signal.
All pre-amps have a power supply that plugs into an AC outlet inside the house that lowers the voltage and sends it up the coax to power the amplifier which is mounted up near the antenna. The benefit is that you get amplification before any line loss or noise and you don't have to run 117 AC up to your roof. NOTE: Don't install a regular splitter between the power supply and the pre-amp or you will short circuit the system and it won't work. You can install a special splitter that is power passive on only one port. Or put a DC Block on the outputs that don't run to the power supply. Also you can't put any type of matching transformers between the power supply and the pre-amp.
SIGNAL PROCESSING AND MIXING
Signal processing equipment consists of filters, traps, mixers, convertors and attenuators. By using band pass filters and traps, interfering signals can be filtered or trapped out. When signals are not received at the same level, attenuators can be used to adjust the signals to a uniform level. If broadband equipment is being employed, antenna mixing units may be used in reverse to filter out unwanted signals or to separate the signals received in order to "pad down" higher signal levels. In systems where single channel antennas are used, mixing units may be required to combine the signals before the input to the distribution amplifier. Converters are used to convert a single UHF channel to an unused VHF channel. In adjacent channel situations, they can be used to convert a VHF channel to another nonadjacent VHF channel.
SIGNAL LEVELS AT THE DISTRIBUTION AMPLIFIER
Once the antennas have been selected, and the signal levels of each channel have been equalized, then the signal level can be measured at the input of the amplifier. When the level of the signal available at the distribution amplifier has been determined, you can then select the distribution amplifier for the system.
CHOOSING AN AMPLIFIER EXAMPLE>>>
The four main considerations in selecting an amplifier are the frequencies and number of channels to be received; the total distribution system losses (the losses caused by cable, splitters and tapoffs); available input signals (the signal levels fed to the distribution amplifier input); and output capability of the distribution amplifier (the maximum signal the amplifier can deliver without overloading).
Channels and frequencies you want to receive are your primary consideration in selecting amplifiers. This refers to the bands of frequencies desired (VHF, UHF, FM) and the specific channels. If many adjacent channels are to be received, each channel must be filtered to prevent interaction, and strip (single channel)
amplifiers are generally required for this purpose. Where no adjacent channel operation is intended, then a broadband ampifier will do the job.
Since the purpose of the distribution amplifier is to compensate for the difference in signals received at the antenna and the losses incurred as the signal passes through the distribution system, you must know these losses to determine the size of the amplifier you will need. For example, where input at the distribution amplifier is 0 dBmV, and the total system losses are 40 dB, a 30 dB amplifier will be inadequate. Amplifiers, cable and other components will normally lose some efficiency over a period of time. Therefore, it is recommended that the output of the amplifier be at least 6 dB more than the distribution system losses. Therefore, the gain should be at least 46 dB.
Remember: Input plus Gain equals Output. So input plus gain must be greater than the total distribution system losses, but not greater than the output capability of the amplifier.
Broadband Amplifiers are available just for VHF bands or just for UHF bands, or for VHF and FM, or for all three bands-UHF, VHF and FM.
Single Channel Amplifiers are used for amplifying individual channels while filtering or blocking out all other channels. Basically there are two types of single channel amplifiers in use....the type that employs automatic gain control, and the type that uses manual gain control. Amplifiers with Automatic Gain Control Circuitry (AGC) maintain a given output signal level even though the input signal level may vary with changes in weather and environment. AGC controlled single channel amplifiers are ideal for use in adjacent channel situations, and in systems where signal levels vary from channel to channel.
Since single channel amplifiers provide complete signal level control they are used in preference to broadband amplifiers.
They also find wide use in large systems where constant output signal levels must be maintained. Moreover, in a single channel amplifier system, if one amplifier should fail, the result is the temporary loss of only one channel. Generally, single channel amplifiers must operate from their own individual power supplies, rather than from a common power supply.
Gain is the amount the amplifier will increase the input signal. A 50 dB amplifier will increase the signal delivered to it by 50 dB. But the output of a 50 dB amplifier will be 50 dB only if the input signal is 0 dBmV (1,000 microvolts). If the input of a 50 dB amplifier is only -12 dBmV (250 microvolts), the output would be 50 minus 12 or 38 dBmV. If input is 20 dBmV (10,000 microvolts), the output would be 50 plus 20 or 70 dBmV which is beyond the amplifiers' output capability. The input signal in dBmV, plus the gain of an amplifier in dB, equals the output signal in dBmV.
A rule of thumb is to select an amplifier with a little more gain than the distribution system losses-but this rule does not always hold true.
If the input signal at the distribution amplifier is considerably less than 0 dB, then the amplifier gain must be considerably greater than the distribution system losses. For example: if the distribution system losses are 28 dB, and the input signal is minus 10 dB, then the amplifier gain must be greater than 38 dB. Following the rule of thumb and installing a 30 dB amplifier in this situation would be a mistake unless the input signal could be increased to 0 dB by preamplification.
If the input signal is considerably more than 0 dB, then the amplifier gain does not have to be as great as the distribution system losses. For example:
If the input signal is plus 20 dB (not uncommon in very strong signal areas) and the distribution system losses are 45 dB-then the amplifier gain must be greater than 25 dB; however the amplifier's output capability must be greater then 45 dBmV (per number of channels). In this case it would be uneconomical to follow the rule of thumb and use a 50 dB amplifier, because the 30 dB amplifier has more than enough gain and its output capability is at least 53 dBmV(per channel).
Output capability is the amount of signal an amplifier can handle without producing cross modulation or sync clipping.
All amplifiers are limited in output by the signal handling capability of their final output amplifying stage. Many multi-band amplifiers have a common output stage for all bands, though others have individual output stages for each band.
In selecting amplifiers it is important to bear in mind that the output capability (per number of channels) may be stated as a total figure for all bands, or as a figure for individual bands. This depends on the design of the amplifier.
Listed below are typical output capability specifications for Channel Master amplifiers: model 7353, VHF/FM which has separate output stages for each band, (lo-band, hi-band) and model 7350 which has a common output stage.
Output capability per channel for Channel Master 50 dB Amplifiers at minus 40 dB cross modulation level:
Low Band 60 dBmV (2 Channels) 57 dBmV (3 Channels) High Band 60 dBmV (2 Channels) 55 dBmV (4 Channels)
Model 7350B 60 dBmV (2 Channels) 54 dBmV (4 Channels) 52 dBmV (7 Channels)
Some amplifiers have built-in Attenuators, Gain Controls and Tilt Controls that permit adjusting the signal input and output levels to the specific levels required by your system.
Attenuators adjust the input by reducing (padding) the signal across the entire bandwidth by a specified amount. For example: Where the maximum input (per channel) of a 50 dB amplifier is 7 dBmV and the input to the amplifier from the antenna is 15 dBmV, we would use the attenuators to reduce the signal to a level below 7 dBmV to prevent overload.
Gain controls adjust the signal level output to the amplifier, with an effect similar to that of adjusting the volume level on a radio. There will usually be a gain control for each band (VHF Lo, VHF Hi, UHF) allowing great flexibility in signal level control.
Tilt Controls adjust the level in which the output is distributed by tilting the output signal level giving you less signal at the low end of the band than at the high end. For example: Higher frequencies incur higher line losses, and, in a system where channels 2 and 13 are to be distributed, you can tilt the gain level to give you more signal on channel 13 than on channel 2 to compensate for the higher line losses at channel 13. This will provide for more uniform signal levels.
7. Interference and Trouble Shooting
CROSS MODULATION INTERFERENCE
Cross modulation occurs in broadband preamplifiers and distribution amplifiers when the amplifier's rated output capability is exceeded by one or rnore signal carriers (picture, sound or FM). This causes two or more signals (TV channels) to beat together resulting in the picture information of one channel appearing superimposed upon another. This interference usually manifests itself as a windshield wiper effect or as a negative image.
Windshield wiper effect is seen as the vertical or horizontal framing bars of the interfering channel appearing on the channel being watched. The negative image appears as a superimposed image in the background of the picture on the channel being watched.
This type of interference can usually be eliminated by highly directive antennas and the use of filters and traps to attenuate and control the offending signals.
NOTE: It is always the strongest signal (channel) being received that causes the interference and it does not normally show up on the interfering channel. The interfering signal may also be an FM signal or combination of FM and TV signals.
Because broadband amplifiers are affected by cross modulation interference long before the amplifier overload point is reached, overload distortion is of importance only when working with single channel equipment. The type of overload we are concerned with is caused by applying such a strong signal to an ampliferthat it operates on an unlinear portion of the response curve of the transistors causing distortion and sync clipping.
The first indication of overload is a picture which is too dark and has an excessive amount of contrast. As overload increases, light irregular lines run through the picture. These are caused by the picture and sound carriers beating together. Extreme overload causes sync clipping which results in the loss of vertical sync and possible horizontal tearing of the picture.
Overload distortion may be reduced or eliminated by reducing the output level of the amplifier. Reducing the input to the amplifier by the use of attenuators may also eliminate this problem.
ADJACENT CHANNEL INTERFERENCE
Adjacent channel interference is caused by strong signals from one channel overriding weaker signals on an adjacent channel, producing a "herringbone" effect. An adjacent channel is one which is right next to another channel. For example: Channels 2 and 3, 7 and 8, 10 and 11 are adjacent channels. Channels 4 and 5 are not adjacent because there is a 4 MHz guard band between them. Also, channels 6 and 7 are not adjacent because the FM band lies between them.
This interference can be eliminated by using a more powerful yagi (or stacked yagis) antenna to increase the weaker signal and by using attenuators to reduce the stronger signal. It may also be eliminated by using "Hi Q" traps to reduce the picture and/or sound carrier of the offending channel.
Co-channel interference is caused by two stations in different cities operating on the same channel allocation. It appears on your TV as two different pictures as though one were placed on top of the other.
The effect can be minimized by using highly directional antennas either singly or, if necessary, by stacking.
POWER LINE INTERFERENCE
Power line interference is caused by radiation from a high voltage power line close to the antenna. To minimize this interference, the antenna should be located as far away from the power line as possible. A balun should be used as close as possible to the antenna terminals to prevent direct pickup of radiation by twin lead. Proper grounding of the system will prevent power line and direct-signal pickup on the transmission lines.
Worn or cracked insulators on the power lines can also cause this interference. If the inter- ference shows up intermittantly (especially during wet weather), it may be due to cracked insulators. Contact your local power company to solve this problem.
There are three common causes of ghosting: pickup of reflected signals by the antenna, direct pickup of a signal by the TV or downlead, or poor installation techniques.
Not all signals reach an antenna directly. They can be reflected by buildings, water towers, mountains, or bodies of water. These reflected signals arrive at the antenna microseconds after the direct signal. This causes a second, fainter image to appear on the TV screen just to the right of the main image. This is called a trailing ghost. It can usually be eliminated by using a highly directional antenna and/or by stacking antennas. Changing the orientation of the antenna slightly may also eliminate the reception of the reflected signal.
A second image appearing to the left of the main image is called a leading ghost. This is the result of the direct pickup of the signal by the short piece of 300 ohm twinlead between the antenna terminals and the tuner, by the tuner itself, or by the downlead from the antenna if 300 ohm twinlead is used. This sometimes occurs in strong signal areas, and what you see is the signal picked up directly, being displayed microseconds before the image picked up by the antenna. Since 300 ohm twinlead is unshielded and can act like an antenna, it should be eliminated and replaced with 75 ohm coaxial cable. A balun should be placed as close as possible to the antenna terminals and the coaxial cable connected to the TV set with a matching transformer. This type of ghosting can also be eliminated by overpowering the unwanted signal. In a large installation, properly chosen taps will ensure that the signal from the antenna is much stronger than the signal picked up directly by the set.
If neither of these methods work, converting the channel to another, non-adjacent channel will eliminate the interference.
Poor installation techniques can also cause ghosts or color smears. If the distribution line installations are improperly terminated, the signal can bounce up and down the line, causing multiple images. Proper use of terminators can eliminate this problem at the time of installation. Poor crimping of "F" fittings sometimes causes an impedance mismatch, and will result in reflected signals in the line. Using a good crimping tool and making sure that all fittings and splices are correctly executed will help ensure troublefree operation of your system.
8. Line Extending
If you ever have occasion to design a very large, complex system, and you find that the distribution losses are greater than the output capability of any amplifier, contact Channel Master. Our MATV applications specialists may be able to redesign the system using different methods or routes in order to reduce losses.
If this is not possible, a system using cascaded distribution amplifiers or line extending amplifiers will be designed. Line extending amplifiers permit you to run feeder lines beyond the capability of the main distribution amplifier. They can operate either from a power source located in the head end amplifier or from a remote power supply located in the distribution system.
LINE POWERED AMPLIFIERS
The powering of amplifiers through the MATV RF distribution lines and the designing of systems incorporating this concept is relatively simple and provides a great amount of flexibility in system design. The following information and basic steps will aid in designing line powered, line extending MATV systems.
Compute Totel System Loss. The distribution section of the MATV system should be designed in the normal manner to determine the total system loss. When the total loss is determined and it is either greater than the output of the head end you prefer to use, or greater than any head end can deliver, then line powered, line extending amplifiers should be designed into the system.
Selecting Head End Amplifiers. Some head end distribution amplifiers provide line power for operating line amplifiers. Some of the Channel Master Titan series broadband distribution amplifiers provide this feature. When single channel amplifiers or other amplifiers not providing line power are required, auxiliary power supplies can be inserted in the lines to provide the voltage for the line amplifiers. When the distribution amplifier(s) have been selected, determine their output signal level settings and the amount of signal they will supply the system.
Design system...Compute Losses. Compute the signal levels throughout the distribution system starting with the signal output level from the head end and going through all the necessary splitters, tapoffs, etc., on all the distribution lines. Compute VHF and UHF signals separately. Compute VHF losses for channel 13 and UHF losses for the highest channel to be included in the system.
Location of Line Amplifier. When the signal level of any distribution line reaches +12 dBmV, add the appropriate line powered, line extending amplifier. The amplifier (depending on its gain setting) will increase the signal level according to the gain of the amplifier. Continue down the line with your signal level computation. Each time the signal level reaches +12 dBmV design in a line amplifier. Inserting the line amplifiers in the line at (or as close as possible to) the recommended input level of +12 dBmV is consistent with system design providing "0" dBmV to each TV set and th'e lowest tapoff isolation value used being 12 dB.
Power for Line Amplifiers. When the entire system has been designed and the line amplifiers are located on the distribution lines where required, then the method of powering the amplifiers can be determined. Channel Master line amplifiers can be powered from either their input or output terminals. The power will also pass through the units allowing the power- ing of additional equipment. Use a power block unit to stop the DC voltage in the line when It is not required beyond a particular amplifier or any particular section of the system's distribution lines.
The power required tb operate the amplifiers can be obtained with auxiliary power supplies. The nurnber of line amplifiers that can be powered from any one power source depends on the total current requirements of the amplifiers to be powered. To find the number of amplifiers (including preampliflers, where applicable) that any one of the power sources will operate1, add the current requirements of all amplifiers to be used and check this against the current available.
Distributon Line Equipment Requirements. All of the passive devices (line splltters, line tapoffs, etc.) that are in a distribution line that will be used for line powering must be capable of passing power and not create a short circuit to the line power voltage. In addition to passing power on their thru line terminals tapoff units must also block voltage on their tap terminals to prevent the DC voltage from getting to any of the TV sets connected to the system. When DC voltage Is no longer needed on the line to power line equipment a DC voltage block should be inserted (see figure 4). This will insure that the DC voltage cannot get to the TV sets. if a voltage block is not used, a terminator that blocks voltage without causing a short ci rcu it to the voltage must be employed. Line Extending in Existing Systems. Adding more outlets and extending distribution lines in an existing MATV system can be accom- plished by inserting a line amplifier at the end of the existing trunk line or by inserting a line splitter Into the system and creating new trunk lines Line power can generally be supplied by an auxiliary power supply. Tapoff isolation values can be calculated in the normal manner.