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برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات. - محاسبات آنتن مرکزی با مثالVideo Distribution Tutorial

برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

یک نکته جالب که اینجا گفته و خودم نمی دونستم اینه که اسپلیترها به ازا هر 2 خروجی یا یک جفت خروجی سه و نیم دسی بل افت دارن
یعنی 4 خروجی میشه 7 دسی بل
6 خروجی میشه 10.5 دسی بل یا 11
8 خروجی میشه 14 دسی بل

A tap is used when a cable needs to feed TVs in one location and then continue downstream to more locations. Hallways in schools are a good example. The cable (called a "trunk" at this point) will hit a tap to feed a block of four rooms. The cable connected to the output side of the tap will run down the hall to the next block of four rooms where another tap will be inserted, and so on to the end of the hall. The closest taps have the highest attenuation, while taps at the end of the hall have the lowest attenuation.


A splitter divides the input between two or more outputs. It is a "dead-end" device. In the above example it would be used at the end of the hall to feed the last rooms.


Taps are rarely used in homes. Most home systems use a single splitter near the demarc (point at which the cable enters the house) to feed all of the drops.


A splitter will cut the signal by 3.5 dB at each output for every doubling of ports. IOW, a two-way will cut -3.5dB. A four-way = -7 dB, and so on. For this reason, a broadband amplifier is used before the input of the splitter to compensate for the splitter insertion loss.




So a Tap somehow splits the signal, but not evenly? It allows more signal to continue down the trunk... is that it? You say the closest ones have highest attenuation... is that due to different specs on the taps in different locations or just a natural outcome of siphoning off a small amount of signal at each location?


A broadband video system is designed by starting at the end of the line and working backwards.

Different sizes of cable (RG59, RG6, RG11, etc.) attenuate the signal at different rates. The loss is specified in dB per hundred feet.


Installing a tap in the line at any point causes an insertion loss of roughly 1dB between its input and output. Insert ten taps right next to each other and you'll see a difference of 10dB between the input of the first and the output of the last. This has nothing to do with the dB rating of the tap or the number of TV drops on it. (Or whether there are any TVs connected to it.) Input-to-output is simply a pass-through.


The ratings of taps range from 3dB to 30dB. This is the attenuation that will be found between the input side of the tap and each port on the tap (plus 1dB insertion loss).


In terms of dB, it's simple addition and subtraction.


In the example of the school hallway, we're told that we need 12 TVs on three four-port taps, the hallway is 100 feet long, and it's 100 feet from the beginning of the line (the "head end") to the first tap. The cable loss is 6dB per 100 feet. Each TV needs a signal of +9dB, but we'll ignore that for now.


200 feet of cable = 12dB loss. Three taps = 3dB loss. There will be a total loss of 15dB at the end of the line. The smallest 4-port tap available is a 7dB, so we need to have a total of +22dB at the head end to overcome all of the losses at the end of the line.


This is accomplished by inserting an amplifier at the head end with enough power to make up the difference: +22dB.


Now that the EOL is done, we work backwards to determine the ratings of the taps at 150 and 100 feet.


At 150 feet, we have a difference of one splitter and 50 feet of cable = 4dB less attenuation. We need a bigger tap value to match the tap at the EOL. 4dB + the EOL tap of 7dB = 11dB.


At 100 feet we have a difference of two splitters (2dB) and 100 feet of cable (6dB) = 8dB. 8dB + the 7db tap at EOL equals 15dB. Our choices for tap ratings are 14dB or 17dB. 14 is closer.


To this point we haven't addressed the +9dB we need at each TV. All we've done is balance the system for the insertion losses. At this point it's a simple matter of increasing the signal by 9dB at the input or the output of the amp (or both).


One more thing before I close this book: Cable loss specs include the frequencies at which the attenuation is specified. This is because the signal is attenuated more at high frequencies than it is at low frequencies. To make up for this lop-sided loss, most amplifiers have a "slope" control which equalizes the signal. Calculating slope over distance gets a bit more complicated, but uses the same methods.
  • Thanks Rick, that's very helpful and educational, no pun intended.

    Regarding insertion loss, is that spec'd or is that counted for any device (including a cable extender thru-connector)? For a splitter is it just the db rating on the device or is there also this -1db loss just for having a device?


    Also, on the splitter, each leg is marked 3.5db, am I correct in thinking this splitter causes a -7db loss in terms of signal attenuation?


    Finally, in your example you refer to all the three distribution points as "taps" is the last device actually a splitter as your first reply indicated?


    Thanks again
  • When you split a signal you cut the power in half. 1/2 the power = -3dB.

    A two-way splitter therefore splits the signal in half, but it's rated at -3.5dB at each output port because of the insertion loss of the device itself. The ports are not cumulative. You'll see -3.5dB at each, not -7.


    An F-to-F coupler will have some insertion loss, but it's negligible and can be ignored.


    Although a splitter is an end-of-line device, a tap is normally used at the end of the line in applications like the one I described. The output side of the EOL tap is terminated with a 75-ohm resistor to prevent noise from entering the system. This allows some leeway for expanding the system.


    A different "star" distribution method is most commonly used in homes: Everything is split (not tapped) at the demarc where the cable enters the home. This head-end is simply a single splitter with enough ports to handle every TV drop in the house. All TV connections are home-run to the head end.


    If a single splitter can't handle it, additional splitters are added in a balanced configuration until all of the TV drops are accommodated.


    For example, a typical subscriber drop carries a signal of +10 to +15dB from the cable company. There are 16 TVs in the house. 16-way splitters are rare, so a two-way is used to feed two 8-way splitters. The insertion loss of a two-way is 3.5dB, and the insertion loss of an 8-way is roughly 12dB.


    The entire signal from the cable company is attenuated to -.5dB at each output of the 8-way splitters. (Remember, the loss is at each port and is not cumulative.)


    An amplifier is added before the splitters to make up the difference. That difference also has to make up for the cable loss for the longest run of cable in the house. This usually isn't a major factor because TVs can produce a good picture with between +5dB and +15dB of signal. So the amp needs to make up the loss of the splitters and give each TV a decent signal to work with. A 15dB amplifier will do the job nicely.


    This "star" distribution pattern is not only easier to install, it's a whole lot easier to troubleshoot than a distributed system with taps all over the place.


    Going back to your original question, a two-way splitter should feed (1) the cable modem, and (2) the rest of the home's cable system. The amplifier should be after the split on the TV side of the system.


    Another vote for the star system: If you ever move the cable modem to another room, you can simply swap that line to the first splitter.


    Here
    is a PDF that has a chart of the losses of splitters. An example of cable attenuation for RG6 is in a chart in this PDF.

  • We had to do this in the Navy when running CATV to the berthing and lounge areas from up in the electronics shop where the VCR's, switches and amplifiers were. Normally 2 trunks, one down each side of the ship, probably 500 ft each,(with maybe 20-30 taps off each side (small ship...lol).
    You must have been in the more recent, modern Navy. In the 80's we didn't have 20 tv's on either of my ships. I was an IC. You?
  • by Jeff Fisher

    محاسبات آنتن مرکزی با مثالVideo Distribution Tutorial

    Your video distribution need may be as simple as getting a decent quality cable TV hookup to each TV. Or you may be planning to have several video cameras that you want to view from any TV, and want to "share" VCRs, satellite receivers, DVDs, etc. among all the TVs in the house.

    In either case, if you're going to be wiring the video distribution system in your new or remodeled home, you need to come up to speed on what it's all about and how its done. It isn't terribly difficult or complicated. In fact, I think its actually quite fun! Its pretty much a science until you get to the "tuning" part at the end.

    So don't be scared. Dive in and enjoy! You can do a much better job than your electrician or local  "cable guy."


    A Little History

    Residential video distribution has come a long way since the days of a roof-mounted antenna connected to a TV by a length of 300 ohm twin-lead flat-cable snaking through a window screen. Back in those days just about anybody could hook up a TV. And when a home had more than one TV (which was rare) the other sets made do with rabbit ears!

    Then came cable. At first the cable companies were more than happy to hook up your TVs for free, just to get your long-term business. If you had more than one TV, they just put a splitter on the side of your home. They controlled the signal level coming down the cable so if they needed more signal strength, they handled it "up at the pole."

    After a while, the cable companies became the de-facto masters of video distribution and, thus, were able to charge for installations. Over the last few years, there has been a trend for the homeowner to take charge of the low-voltage wiring within the home. This trend was driven, for the most part, by the phone and cable TV companies' attempt to make installations a "profit center." When phone and cable installations got expensive enough (and sloppily done, at that) the homeowners started doing it themselves.

    Today, it seems, neither the phone or cable companies care who does the installation. Which suits us just fine, because we know that we can help you do a whole lot better job than they would. All the cable company cares about (besides you paying your bill) is that you don't mess up the signal going to neighboring "drops" (in other words--don't send any signals back up the cable) and that you don't get them in trouble with the FCC by "leaking" their signals into the air.

    Introduction To Video Distribution

    Video distribution is all about getting a strong clear signal, of all channels (regardless of the source), to all video destinations within the home. This involves three general functions: Gathering, and in some cases creating, the signals in one area; Combining, conditioning, and amplifying the signals; And distributing the signals to their destinations.

    The first thing your need to know about video distribution is that what you are really distributing is Radio Frequency (RF) signals. These signals, given the opportunity, would happily fly through the air to your TVs. But this method of video distribution is frowned upon by the FCC because they would rather let the "licensed broadcasters" handle that method of distribution. Instead, we force the RF signal to go down shielded coaxial cables. Apart from the distribution task itself, the two most important parts of creating a video distribution system are to keep your signals inside the cables, and to keep other signals out of the cables!

    A single coaxial cable can carry 130, or more, standard channel frequencies. Each channel includes video and audio components. With MTS encoding, each channel can even have stereo audio.

    Contrary to an RF channel, which can coexist on a coaxial cable with many other channels, "baseband" video takes the whole cable, and doesn't even include sound! Baseband video and audio is what comes out of the RCA (a.k.a. "phono") jacks on the back of your VCR. Since it takes two coaxial cables to transport a single baseband video and audio source, you generally want to convert baseband into RF as "early" as possible. Which means as close to the source as possible.

    You convert baseband to RF with a device called a modulator. Most modulators today are simple little digital boxes that are similar to, but the reverse of, a cable box. They have inputs for video and audio, and an output for RF. (RF is almost always an "F" type connector.) You can use one or more modulators to create your own "in-house" channels. In effect, you create your own cable TV company.

    A cable coming in from your antenna or the cable company contains many RF channels and needs no modulation.

    From a "block diagram" standpoint, there are four key pieces to any video distribution system: The coaxial cables themselves, which serve as a conduit for the RF signals and allow interconnecting the other key pieces; RF Amplifiers that "boost" the RF signals to make up for the signal losses the other pieces impose; Combiners which "add" two or more cables together to create a single output that contains all channels from both input cables; And splitters that take a single input cable and distribute the same signal to two or more output cables.

    Planning a video distribution system is not difficult. You need to understand what the key pieces do, have a blueprint of your home, and be able to do a little very simple math. That's it!

    Ready to Go On?

    This application note covers video distribution in four sections:

    • The first section describes how to design (and spec out) a system. This section will help you determine what equipment to buy.
    • The second section covers prewiring the system. This section will tell you what to do while the walls are still open.
    • The third section details the installation of the system components.
    • The fourth section describes how to "tune" and troubleshoot your system when you've got everything hooked up.

    Please note that this application note is not about home theater systems, although it obviously describes how to get RF signals to and from the home theater system, and how to share home theater video sources with the rest of the home. It does not, however, describe how to interconnect baseband video/audio between the home theater components, or how to carry baseband or s-video signals throughout your home. (A practice we do not recommend.)

    Nor does this application note cover whole-house audio distribution. See our Whole-House Audio Tutorial. Although video source components may supply audio to the whole-house audio system, we will treat whole-house audio distribution as a separate system.

    This document also does not cover DSS/Satellite signal distribution. See How Do I...Connect Multiple DSS Receivers. To integrate DSS distribution with your video distribution system, design the video distribution system first, then "piggyback" the DSS signals to the appropriate locations on the video distribution cable as described in the DSS document.

    Throughout this document you will see highlighted part numbers. These are hyperlinks to more information elsewhere on our website about specific products. Unlike other "sterile" documents that claim to educate you about whole-house audio without actually naming any products or even brands, this document lets you click your way all the way through to actual product specifications, pictures, and prices!

    Video Distribution Tutorial - Design

    Section 1: Designing a Video Distribution System

    video1e.gif This section of the tutorial describes how to design a video distribution system for your needs.


    Misconceptions

    Before beginning, I want to take a moment to dispel a couple of common misconceptions about video distribution. There are two kinds of things you can do when it comes to video distribution: The very easy and not-too-expensive; And the pretty hard and very expensive. In residential video distribution, we have always stayed with the former. Here are some examples from the "pretty hard and very expensive" category:

    Combining Coaxial Cables With Common Channels I once said: I'll just combine my in-house UHF channel 22 with the antenna feed. There doesn't seem to be a channel 22 in my area. My channel 22 will surely drown out any little bit of any 22 than is coming in from the antenna. Silly me. It is astounding how little of a signal it takes to screw up a perfectly good signal...when they are on the same frequency. When you "combine" two coaxial cables containing RF signals, you have to be absolutely sure the cables have no frequencies (channels) in common. Combiners are very simple little devices: Essentially they are transformers that simply "sum" the two input signals.

    Removing a Channel Let's say that you wanted to remove a channel, or a group of channels from a cable. Either just because you didn't want them in your house, or because you wanted to insert your own channels in their place. The cable companies do this with very precise, and obscenely expensive "notch filters." We don't do it in residential installations at all. The closest we come is with low-pass filters that remove all stations above a certain point. And they are not all that precise: They begin "attenuating" stations at one channel, and as you go up the channels get worse and worse, until you reach the rated cutoff, where the channels are completely gone. This spread can encompass ten or more channels.

    Adjacent Channel Modulation Let's say that your local area doesn't have a channel 3 being broadcast. Why can't you just take the RF output of your VCR, which is channel 3, and combine it with your antenna signal so that everyone can watch the VCR? Well, you can...so long as nobody wants to watch channel 2 or channel 4. Modulators (and the channel 3 output from the VCR is coming from a little modulator built-in to the VCR) produce an RF signal that is much "wider" in frequency range than one channel. In order to keep this signal from interfering with the adjacent channels you have to use an extremely precise (and expensive) "narrow-band filter." The cable companies use the filters. In residential applications, we just make sure there is an unused channel above and below any "in-house" inserted channel.

    If you keep these three limitations in mind, you won't "design yourself into a corner."


    RF Video Distribution Concepts

    Integrity of RF Signals

    There are two parts to maintaining the integrity of RF signals; The first part is keeping your signals from leaking out of the cable. The second part is keeping outside signals from leaking into the cable (called "ingress".) When your signals leak out of the cable, it can cause interference on your neighbor's TVs. Besides, it annoys the FCC. If a local broadcast channel leaks into your cable, it will interfere with that channel (though not necessarily the same program) in your distribution system. Both problems are avoided by using good quality RG6 coaxial cable and properly installed high-quality connectors.

    Splitters, Combiners, & Taps, Oh My!

    go?imagesm=be-1189aRF cables are designed to carry RF signals from one point to another, not from one point to many. In other words, you can't run RF signals to multiple locations by wiring all the destinations in parallel. The reason is that the residential RF distribution scheme is based on 75 ohm terminated transmissions. Meaning that the transmitting side expects to see one, and only one, 75 ohm load on the other end of the cable.

    hs2.jpg A splitter is a small device that has one input (the 75 ohm load) and 2 or more outputs, each driving a separate 75 ohm load. Essentially they are transformers that split the power in the input signal to multiple outputs, while maintaining the 75 ohm impedance. However, there is no free lunch! Every time you split an RF signal with a splitter, you drastically decrease the signal's strength. An RF signal only has so much power. Logic dictates that splitting this signal in two with a "passive" device will result in two signals that each haveat mosthalf of the original signal's strength.

    A combiner is simply a splitter hooked up backwards. It combines the channels on two or more separate cables onto one cable. The only drawback to this piece of magic, is that the cables being combined cannot have any channels in common with each other. The resulting signal on that channel would be trashed.

    Combiners make some neat things possible. Let's say you have cable TV that has channels 2 through 63. And you have a DSS receiver that you would like to be able to see on any TV in the house. You can hook up a modulator to the DSS receiver, set the modulator to channel 65, then combine this new channel back in your wiring closet with the cable TV coming in! Now any TV can watch DSS by simply changing to channel 65. This concept of "in-house" channel generation, together with the new cheaper and more reliable digital modulators, is opening up many new possibilities in residential video distribution.

    tp12db.jpgTaps are similar to splitters, but are "wound crooked" so that the outputs are not equal in signal strength. The "through" output of a tap may only reduce the signal level by a very small amount, while the "tap" output is a small fraction of the signal level. Taps are primarily used in complex commercial distribution installations.

    2503.jpgAttenuators are simple "one in, one out" devices that reduce the signal strength. Attenuators come in various sizes and are useful when tuning up the video distribution system.

    Calculating RF Signal Loss and Gain

    Table 1 Rule-Of-Thumb Losses
    Device Loss (-dBmV)
    2-Way Splitter/Combiner 4.0
    3-Way Splitter/Combiner 6.5
    4-Way Splitter/Combiner 8.0
    8-Way Splitter/Combiner 12.0
    100 ft RG6 4.0
    This table gives some rule-of-thumb losses for various splitters and cable lengths. Better quality splitters, such as the Channel Vision line, have slightly less loss than shown.
    The RF signal looses strength as it passes down the cable and through combiners and splitters. To counter this loss (or "attenuation") we use RF amplifiers. In the ideal RF distribution system, the signal level at each wall-plate should be about the same as the signal level coming in from the cable TV system or antenna. This ideal is called "unity gain." By applying a little math, and the table below, you can calculate the approximate losses and gains in your system to approach this goal.

    RF signal levels are measured in dBmV which is a logarithmic scale of signal relative to one millivolt. Since decibel values represent power levels, and are logarithmic, they can be calculated with simple addition and subtraction. The main thing to remember about dB (for short) values is that if the level drops below 0 dB (into the negative dB range), you are loosing actual signal information and no amount of amplification will be able to recover this lost information (picture quality.) In fact, amplifying a signal that is below 0 dB will usually make the picture worse since the noise is now being amplified and picked up. So you must insure that your signal levels never drop dangerously near 0 dB anywhere in your distribution system. This is why the main RF amplifier us usually connected near the input side of the distribution system; so the signal is boosted early, and never drops precariously low.

    Example 1 1-Input, 8 Output Design
    video1a.gif
    Let's say that you want to take a cable TV signal to 8 TVs over up to 100 feet of cable. An 8 way splitter has 12 dB loss, and 100 feet of RG6 has around 4 dB loss. This total loss of 16 dB (12 + 4) must be offset by an amplifier before the splitter. An inexpensive 15 dB amplifier just fills the bill. With such a setup, a 15 dB signal coming in will reach each TV with at least 14 dB of signal strength. Close enough! And note that the signal never drops below zero. Working up a diagram such as this one, with the gains and losses noted at each point, is an easy way to design a system.

    The only way to actually measure the signal level is with an RF signal level meter specifically designed for this task. We ended up buying one (they go for $1000 up) that we rent out to our local customers that are having trouble tuning up their very complex systems. But most folks get by just fine by just doing the calculations up front.

    Cable TV companies are supposed to deliver around 15 dB of signal strength at the side of the house, but I've seen this range from below 0 to well over 25 dB. An antenna can deliver a wide range of signal strengths depending on the strength and distance of the stations.

    The optimum level at the wall-plate is between 8 and 15 dB.

    Example 2 4-Input 12-Output Design
    video1b.gif
    This is an example of a more complex system design. It has one cable input, and three modulated signal inputs. (We assume the modulators put out around 25 dB.) These signals are combined, amplified, and distributed to 12 destinations. Four of the destinations are longer 100 - 200 feet, and the rest are 100 feet or less. The math shows that the signal strength, before the amplifier, is around 11 dB, and that there will be a 20 dB drop in the splitters and coax runs. Since we want the signal at the TVs to be around 15 dB, we can do the math backwards to see that we need around 25 dB of amplification.


    Counting INs and OUTs

    Before you can get to the business of designing the headend, you need to know how many inputs and outputs you need. Cable TV coming in, or the Antenna feed coming in, counts as one (you can't use both simultaneously). Other inputs to the headend will come from modulators generating in-house channels. Modulators are often remotely located (by the equipment they get their A/V signals from) and send their modulated signals to the headend over the "upstream" coaxial cable. Two or more modulators at the same location should be treated as one at the headend since their signals will be combined at the remote location.

    Usually, all inputs at the headend are combined separately, then combined with the main cable TV or antenna input. This is so the main input is not attenuated any more than necessary before being amplified. Besides, you have more control over the level of the in-house signals than you do over the cable TV or antenna signals. Also, you can add additional inputs later without greatly affecting the overall video distribution system design.

    Outputs are the "downstream" cables that carry the signals to each of the wall plate "drops". It doesn't matter if there is actually a TV connected...the load on the system is the same. (Unused downstream drops should be capped with a terminator.)

    If you end up with a lot more drops than you think you will ever have TVs, you can design your headend to handle only a limited number of connections at one time, and switch cables when you move TVs. Most folks, however, design the headend to drive all the drops, whether used or not. Note: Although you should have as many upstream cables coming back to the headend as you have downstream cables, most of the upstream cables will not be connected at any given time. Only the upstream cables that are connected to modulators should be combined into the system.

    Now count the number of "in"s that you will have connected to the headend at any one time, and the number of "out"s you want to connect. These numbers will be used later to choose or design the headend.



    Choosing Drop Locations and Calculating Cable Lengths

    Next you need to choose a location for your cable drops. Initially, we just use the cable drop locations noted on the electrical plans. Although these are usually placed by the architect, not the owner, the countif not the exact locationsare usually pretty close. The locations can be refined as the construction date nears. Approximations are fine at this stage. The main thing is to make sure you have cable drops in the rooms where you will need them!

    In bedrooms, the cable drop locations are generally opposite the bed location. In the main TV room, the location should be behind the TV/Home Theater. Don't forget to put a drop near the computer in the study! TV on the PC is increasingly easy and popular, as are cable modems. Also run some coax cable and some 4+ conductor telephone wire to the front door location for a security camera. Along with security cameras come the need for a cable drop in the kitchen and other often occupied spaces, so that the owner can easily see through the cameras.

    You need to determine a location for the headend. Basements and garages are OK. Although under stairs and closets are a more common location. Attics are out due to excessive heat in the summer. A "central" location is nice, but the vagaries of architecture often don't allow this. Just make sure the location has access, through the walls, to all parts of the house.

    One more "drop" will need to run to the cable entrance ("Point Of Demarcation") and/or the antenna location. If yours is new construction and you don't know where your cable will be coming in, check with other homes already built in the area to get a clue. If you have underground service, most often the entrance will be just inside the garage or on an outside wall nearest the street. If you have overhead service, the entrance will be on an outside wall nearest the closest pole. The cable company will run coax and install a grounding block at this point. You will connect to the other side of the grounding block.

    Once the approximate locations are determined, you can estimate the cable lengths you'll need. With your ruler, measure the distance from each drop to the headend, always turning at right angles. Multiply by the scale to get the number of feet, then add ten feet to each run to account for ups and downs at each end. Once you've done this for all drops, add the lengths together and add a 20% fudge factor. (Why the fudge factor? Experience. Trust me.) Then multiply by two to account for upstream and downstream runs. Is your number somewhere between 500 and 1000 feet? Most of the time, it is. If less than 500 feet, you must have a home with very few rooms. If over 1000 feet, you're building a pretty impressive place! One 1000 foot spool of coax is the average purchase for our customers.



    Designing the Headend

    Now that you've figured out how many ins and outs you have, where they go, and how far they go, its time to think about the headend. There's two very different ways to go about this. The first is to choose a preconfigured video distribution panel that is appropriate for your needs. The second is to design your own video headend with what we call "video plumbing." This is where you screw a bunch of components to at piece of plywood on the wall and interconnect them with short pieces of coax. While this is good clean fun, it isn't always the prettiest, or the neatest solution. However, it may meet your needs much better than a preconfigured panel.

    Designing the Headend with Discrete Components

    For designing a structure wire cabinet see our (How do I wire a structured wire panel?) in our knowledge base

    Example 3 A three input front-end
    video1c.gif
    This is a typical front-end to a video distribution system. Notice how the antenna/cable feed only passes through one two-way splitter.
    When designing a headend with discrete components, start at each end and work towards the middle, with the amplifier being the last component selected.

    Inputs

    Bring in the cable TV or antenna feed into a two input combiner. Then combine the upstream cables together in such a manner that the output of this combination is in the 12 - 19 dB range. (Assume the output of modulators is 25 dB, most are this high or higher, and are adjustable.) Run this signal into the other input of the main combiner. You now have a single cable with balanced blend of CATV/antenna signals and in-house signals in the 8 - 15 dB range.

    Minimizing the drop on the main feed is a primary concern since you don't have much control over the strength of this signal. If it happens to be very weak, you don't want it to go below 0 dB in your input stage.

    Outputs

    Example 4 Equal Outputs
    video1f.gif
    For a headend system with all outputs equal, design everything in a "symmetrical" manner. For 12 outputs, figure on 3 4-Way splitters in the final drive stage, then a 3-Way splitter driving the finals. Total attenuation: 18.5 dB.
    Now see if you can group the "drop" (downstream) feeds into longer and shorter runs. Think in numbers like 2, 4, and 8 for the number of cables. And lengths like <100, 100-200, and 200+ feet.
    Equal Outputs

    If most of your runs are within one of these categories, you should design a system with all outputs at the same level. An equal-output system is achieved by using a "symmetrical" approach. All splitters at each stage should be the same size. An equal-output design with eight or less outputs needs only a single splitter stage (see Example 1). But for more than eight outputs, you will need two or more splitter stages (Example 4).

    Unequal Outputs
    Example 5 Unequal Outputs
    video1d.gif
    This is an example of a design with unequal outputs. Four of the outputs have 4 dB higher output than the other eight due to the difference in loss between a 4-way splitter and an 8-way splitter. The extra 4 dB will drive a signal an extra 100 feet and end up with the same level.
    If, however, you have some runs that are longer and some that are shorter, you should split these up (2, 4, 8!) and design a system with some outputs stronger than others. Figuring a 4 dB loss per 100 feet of coax, a 200 foot run should have 4 dB higher output at the headend than the 100 foot runs.

    Remember that the smaller the splitter, the stronger the output. Thus, use a single large splitter to drive all the short runs, and one or more smaller splitters to drive the longer runs. In an unequal-output system, you will always end up with another level of splitter that drives the final splitters.

    Amplification

    Now that you have the numbers for the total loss in your system, you can determine how big of a main amplifier you need to counteract these losses.
    Main Amplifier
    35ia.jpgAdd up the total loss of your system by adding the loss in the input side to the loss in the output side. For average systems, this number is usually around 25 dB. Choose an amplifier that has at least that much gain. Amplifiers with variable outputs are nice since they let you easily adjust the gain to meet your exact requirements.

    The main amplifier will connect your input stage to your output stage. (See Example 2.)

    Isolation Amplifier
    15pia.jpgIt is good practice to plan for an isolation amplifier on your main antenna/cable input. This amplifier takes the input antenna/cable signal and amplifies it a little, and the resulting output is then fed into the main antenna/cable input to your distribution system.

    The isolation amplifier serves two purposes: It may be needed if the input signal is very low, in order to boost it to a level that is roughly equal to the other inputs so that it can be successfully combined in your input stage. And it keeps your in-house modulated signals from going back up the cable or out of your antenna!

    That having been said, in practice I usually find that an isolation amplifier is not needed: The input signal is usually strong enough, and the attenuation between the two inputs of the main combiner is quite high, so your in-house channels shouldn't "leak" back up the cable. There are always, however, exceptions. So at least keep in mind that you may ultimately need an isolation amplifier. (All the pre-configured panels include an isolation amplifier. Apparently the manufacturers believe it is important.)

    Once you have the headend designed, you can put up a sheet of plywood and screw the components to it, and interconnect the components just like your diagram!

    Designing the Headend with a Distribution Panel

    dp3_12.jpg The pre-configured distribution panel approach is much easier and cleaner. Now that your know how many outputs you need, just choose a panel with enough outputs. If the panel you choose doesn't have enough inputs, you can add an external combiner to get all your signals in.
    Example 6 Adding an Extra Output
    video1g.gif
    This example shows how you can ad an extra output to a distribution panel. You loose one long run output and gain two short run outputs.
    You can also mix a pre-configured panel with a custom design. For example, you could take one of the outputs, amplify it, and split it eight ways to add a total of 7 outputs. Example 6 shows how you can turn one long run output into two short runs without even adding an amplifier!



    Choosing Modulators

    Modulators are devices that take video and audio signals and turn them into an RF channel. Normally, modulators reside near the device that they are creating the channel for, such as a satellite receiver. Although several cables are needed to connect the source to the modulator, a single coaxial cable can carry the audio and video signal to the headend. Exceptions to this include video cameras an other such sources where you might not want the modulator located out by the camera. In these cases, the modulator is usually placed at the headend and the camera's signal is carried in its "baseband" form over coax to the headend.

    The exact modulator(s) you choose is up to you. If you have two or more sources at a single location, you can save some money by using a double- or triple-channel modulator.

    The NetMedia modulators are smaller and a little less expensive than the Channel Vision line. The Channel Vision modulators are easier to set up, come in more versions, and have a slightly stronger output than the NetMedia line. Our newer product line now includes the CELabs modulators that are small, have channel displays like the Channel Visions. They also have the least expensive one channel stereo modulator.On the high-end Channel Plus offers 3 & 4 channel stereo versions with  S-Video inputs. All these modulators are digitally tuned, are drift-free, and perform equally well.

     

    Video Distribution Tutorial - Install

    Section 3: Installing a Video Distribution System

    under.gif HomeTech Solutions newest update is currently part of our  "how to install combo cable" which covers some of the installation for video distribution. Structured Wire Tutorial
    Installing Combination Cable

    How Do I...Install Combo Cable

    This application note describes how to install HomeTech's speed-wrap or combination cable.

    This application note only covers installations where the sheet-rock is not yet up. Installing combo cable after the sheet rock is installed is a more complicated process.

    -->purple.gif
    What You'll Need
    Tools
    Materials
    Miscellaneous

    -->purple.gif
    Procedure
    Preparation
    Do's and Don'ts
    Do It!

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    Related Links
    Main Tools Page
    Combo Cable Page

    What You'll Need

    Tools

    • A pair of wire cutters capable of handling the combination cable. Good Better
    • A sharp pair of scissors for the fiberglass strands, if you have fiber.
    • A helper. The spools weight over 70 pounds and the whole job goes much faster.
    • A cable reel. You can improvise with a pole or rod and two chairs.
    • A drill.
    • A 3/4" bit for drilling the combo cable holes.
    • A phillips screwdriver or screwdriver bit for screwing in mud-rings.

    Materials

    • The spools of combination cable. (Approx. one spool of 500 feet for every 8 runs.)
    • A Single-gang (MP1S)or double-gang (MP2S)mud ring for each location.
    • Miscellaneous cable hangers.(See below.)
    • A box of #6 x 1/2" pan-head phillips sheet-metal screws. (For mud-rings.)
    • A few cable-ties.

    Miscellaneous

    • Marked up floor plan with each drop and the headend location noted.
    • For all headends except the FutureSmart Pro panels, you'll need the headend install can.

    Procedure

    Do's and Don'ts

    • Do wait for the electrical and plumbing to be roughed in before installing low-voltage wiring.
    • Don't run combo cable within 18" of AC wiring for more than 18".
    • Don't run combo cable and AC wiring though the same stud cavity (for more than 18").
    • Don't put a mud-ring on the same stud (even the other side) as an electrical box.
    • Do cross AC wiring at a right angle.
    • Don't kink or step on the combo cable.
    • Don't bend the combo cable with a radius of less than 4".
    • Don't jerk the cable or pull at an angle to holes. (Doing so could tear the outer jacket and expose the inner cables to damage.
    • Do watch for sharp edges that could damage the cable.
    • Don't use staples or other securing devices unless you are sure that they will not crush or kink the cable. (Use our cable hangers, cable ties applied loosely with random spacing, or plastic staples larger than the cable.)
    • Don't use securing devices inside stud cavities if it can be avoided. (This makes it easier to replace the cable in the unlikely even it is damaged.)
    • Do label cables on each end. A marker and masking tapes works fine for temporary use.
    • Don't try to splice or repair a cable. Instead, remove it, use the pieces somewhere else, and pull a new cable.
    • Don't let the cable lay on the ground in the crawlspace or on the ceiling joists in a main access area in the attic. Instead, support the cable from the roof or floor-joists with J-hooks.
    • Do drill through center of stud. This spacing protects the cable from drywall screws. If you must drill closer to one side, use a nail-plate on the face of the stud to protect the cable.
    • Do not drill through fire-walls, shear-walls, or specialized members like glue-laminates without first checking with the builder or local codes.
    • Do drill holes straight and even when passing cable through several studs at once...the cable will pull much easier.

    Preparation

    • Install the headend enclosure per the manufacturer's instructions.
    • Place a combo cable spool on your cable reel immediately below the headend enclosure.
    • Walk through the structure with your plans and a brightly colored crayon. Mark the studs where the mud-rings will be placed, drawing an arrow to the right or left.

    Do It!

    • Install the mud-rings. Use at least two screws, one on each face to hold the plate in place. Measure the height your other electrical boxes, from floor to center, for reference.
    • Starting from the headend, feed the cable through the top or bottom of the stud cavity of the headend (no need to pass through the headend knock-outs at this point.) Pull the cable through each hole about 10 feet at a time. Keep working the cable through, 10 feet at a time, until you reach the mud-ring. You'll want about 18" of cable past the mud-ring.
    • Bend the center tabs back and secure the cable inside the stud cavity with a tie-wrap. Double-check that the tie-wrap can be cut through the opening, the cable can be fished out, and that there will be at least 18" of slack cable available.
    • Pull back any remaining slack towards the headend.
    • Cut the cable long enough at the headend so that the end reaches just past the furthest point of the headend box. This will insure that the cable will be able to connect anywhere inside the box.
    • Fish the cable through the knockout in the headend can, secure it inside, and label it.
    • Repeat for other runs.

     

     

    Glossary

    Baseband Video



    An unmodulated video signal. Depending on the regional standard, it may be NTSC, PAL, SECAM, etc. format. NTSC is used throughout North America. This signal does not carry any audio component and "takes up" the entire coaxial cable. Baseband video can be transported over the same kind of cable (I.E. 75 ohm RG-6 dual or quad shield coaxial) as RF video, but never at the same time. Runs of several hundred feet are possible without amplification, but amplification and distribution of baseband video is very different than RF video/audio and is not covered in this document. You can tell a baseband video input or output jack from an RF jack because the baseband jacks are usually non-threaded RCA style connectors.


    CATV


    Short for Cable Access Television. The method for distributing RF signals via coaxial cable rather than radiated through the air.


    MATV


    Short for Multiple Access Television. The method for distributing RF TV signals by broadcasting them through the air.


    MTS Encoding


    A method of encoding stereo audio along with the video signal on an RF channel. Many TVs and VCRs can decode this stereo signal. Those that can't simply get the mono audio signal.


    RF Video/Audio


    One or more video/audio signals modulated to Radio Frequencies. As in TV channels. You can tell an RF input or output jack from a baseband video or line level audio jack because the RF jacks are usually threaded "F" style connectors.

     

    Tuning & Troubleshooting

    video2a.gif

    Section 2: Troubleshooting a Video Distribution System

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    Please be patient...I'm working as fast as I can...

    And, please, no flames about incomplete information. This stuff is free! I'm doing this all on my own time, just because I really love to share information (isn't that what the internet used to be all about?). So I don't take kindly to complaints about unfinished sections.


    در ساختمان هایی که تعداد زیادی گیرنده تلویزیونی وجود دارد (مانند هتل ها و برج های مسکونی) در صورتی که بخواهیم برای هر گیرنده یک آنتن مجزا نصب نماییم مشکلاتی مانند موارد ذکر شده در زیر بروز خواهند کرد: 
    *محدودیت فضایی پشت بام برای نصب تعداد زیادی آنتن 
    *اثر انعکاسی و القا یی آنتن ها بر یکدیگر 
    *هزینه بالای نصب آنتن برای تک تک گیرنده ها و سیم کشی آنتن تا گیرنده
    *از بین رفتن زیبایی ظاهر ساختمان و به وجود آمدن جنگلی از آنتن ها
    *حجم بالای سیم کشی آنتن ها تا گیرنده نیز مشکلاتی به وجود خواهد آورد.
    با توجه به موارد ذکر شده راه کار پیشنهادی این است که از یک آنتن برای تمام گیرنده ها استفاده گردد و چون سیگنال در یافت شده توسط این آنتن برای تمام گیرنده ها کافی نخواهد بود لذا از تجهیزاتی برای افزایش مقدار سیگنال و توزیع آن بین گیرنده ها استفاده می کنیم.عملی کردن این راه کار با استفاده از تجهیزات سیستم ها ی آنتن مرکزی (MATV) (MASTER ANTENNA TV) انجام می پذیرد. از این سیستم ها به عنوان (CATV) (COMMUNITY ANTENNA TV) نیز نامبرده می شود. یک سیستمMATV مجموعه ای از تجهیزات اولیه سیگنال تلویزیونی و تجهیزات پردازش و تقویت سیگنال و توزیع آن از طریق کابل های کواکسیال بین گیرنده های تلویزیونی است و هدف از برقراری آن مهیا کردن سطح سیگنال مناسب را برای هر گیرنده جهت دریافت تصویری با کیفیت قابل قبول می باشد.

     
    از دیگرامکانات آنتن مرکزی قابلیت نصــب دور بین مدار بسته جهت پارکینگ ویا درب ورودی روی این سیستم است که می توان این دوربین را مستقیمأبه آنتن مرکزی نصب وبرای کلیه واحدها نمایش داد .

    تجهیزات سیستم MATV به دو دسته اصلی صفحه بعد تقسیم می گردد: 
    1- تجهیزات ابتدایی تهیه سیگنال ( HEADEND equipment ) 
    این تجهیزات شامل آنتن و تقویت کننده فیلترها ، مبدل های فرکانسی ، تله موج ها و مچینگ ها می باشد که برای پردازش سیگنال تلویزیونی و رساندن آن به اندازه و کیفیت مطلوب برای گیرنده ها به کار می روند . 

    2- تجهیزات توزیع سیگنال ( DISTRIBUTION equipment ) 
    شامل قطعاتی چون تقسیم کننده های انشعابی یا مقسم انتهایی (SPLITTER) و تقسیم کننده عبوری یا میانی (TAP OFF) و مقاومت های انتهایی (TEMINATOR) و غیره برای تحویل سیگنال به گیرنده ها و جدا سازی ( ISOLOTION ) هر گیرنده از سیستم می باشد.


    دسی بل ( db ) : 
    مقدار سیگنال تلویزیونی را عموماً با واحد میكرو ولت اندازه می گیرند و برای سادگی محاسبات وكم شدن اعشار از دسی بل ( db ) استفاده می گردد كه مقدار آن از رابطهdb = 20 log( E1/E2) محاسبه می گردد. 
    در حقیقت دسی بل چند مرتبه بزرگ یا كوچك بودن سیگنال را نسبت به یك سطح مبنا نشان می دهد . در سیستم های MATV این سطح مبنا (E2 ) را برابر1000 میكرو ولت میگیرند لذا برای خروجی 1000 میكرو ولت بهره برابر صفر دسی بل میشود. تمام مقادیر ضریب تقویت آمپلی فایر ها و افت های سیستم و مقادیر ایزولاسیون به db بیان می شود . در محاسبات بر حسب دسی بل به راحتی می توان مقادیر را جمع یا تفریق كرد. در ادامه بحث ما مبنای بالا را در نظر میگیریم.لازم به ذكر است در بعضی سیستمها ولتاژ مبنا(E2 ) را برابر یك میكرو ولت میگیرند و از رابطه db = 20 log E1 مقدار بهره را به دست می آورند و بر حسب دبی میكرو ولت بیان میكنندكه برای ولتاژ خروجی (E1) یك میكرو ولت مقدار بهره برابر صفر دبی میكرو ولت بدست می آید. در این صورت برای مقدار مبنای 1000 میكرو ولت كه در حالت قبلی صفر دسی بل به دست می آمد 60 دبی میكرو ولت بیان می شود.
    كابل های مورد استفاده در MATV:
    در كابل كشی سیستم های MATV از كابل كواكسیال 75 اهمی استفاده می گردد . این كابل ها كه به آن ها كابل هم محور هم اطلاق می شود دارای یك هادی مركزی از جنس مس می باشند كه وظیفه حمل سیگنال را به عهده دارد و یك شیلد به صورت بافته مسی كه دور كابل را گرفته واز اثر القا و تداخل روی سیگنال توسط عوامل خارجی جلوگیری می كند و امكان جذب مستقیم سیگنال توسط هادی مركزی را از بین می برد . برای اتصال كابل های كواكسیال به تجهیزات MATV از كانكتور های نوع F استفاده می گردد كه بسته به نوع كابل سایز آن انتخاب می گردد . كابل های مورد استفاده در سیستم MATV برای خطوط اصلی RG6 – RG11 – RG59 می باشد كه تفاوت آن ها در مقدار افت كابل به ازای طول مشخص می باشد . برای فواصل طولانی ( بین چندین ساختمان ) و یا برای مواردی كه نیاز به خاك كردن كابل باشد كابل RG11/U استفاده می گردد . در داخل ساختمان نیز معمولاً برای تمام مسیرها به طور یكسان كابل RG59 به كار می رود . برای اتصال پریزها به سیستم بین تپ آف و پریز و یا بین اسپلیتر و پریز بسته به فاصله و تعداد پریزها ی مسیر از كابل های 3C-2V و 4/5C-2V و 5C-2V استفاده می گردد هرچه ضریب حرف C بالاتر باشد افت كابل كمتر است.
    طراحی سیستم MATV:
    طراحی سیستم توزیع 
    از آنجا كه افت سیستم توزیع آنتن مركزی در انتخاب تجهیزات اولیه ( HEAD END ) موثر است لذا باید ابتدا سیستم توزیع را طراحی و محاسبه نمود . قدم اول تهیه نقشه ساختمان و علامت گذاری محل پریزها و محل آمپلی فایر است . نحوه توزیع كابل ها نیز از نظر عمودی یا افقی بودن نسبت به شكل ساختمان باید تعیین شود وسپس كابل های لازم تعیین شود . از كابل كشی طولانی و كابل كشی زیگزاگ و حلقوی باید اجتناب كرد و كابل ها را حدالامكان به طور مستقیم كشید . بعد محل تپ آف ها واسپلیتر ها را تعیین می كنیم . طولانی ترین كابل یا كابل با بیشترین تعداد تپ آف ها و اسپلیتر ها را باید برای محاسبه افت سیستم درنظر گرفت . در صورت عدم اطمینان در مورد شاخه با بیشترین افت باید در چندین شاخه افت را محاسبه كرد وشاخه با بیشترین افت را انتخاب نمود.
    افت های سیستم توزیع:

    1- افت كابل ها : مقداری از سیگنال در حین عبور از كابل كواكسیال افت خواهد كرد مقدار این افت به نوع كابل مورد استفاده وفركانس سیگنال عبوری بستگی دارد در فركانس های بالاتر افت بیشتری وجود خواهد داشت . بهتر است افت كابل را برای بالاترین فركانس موجود یا فركانسی كه ممكن است در آینده دریافت شود محاسبه نمود . 

    2- افت اسپلیتر ها (INSERTION LOSS) : مقدار افت در اسپلیتر عبارت است از مقدار ورودی بر حسب db منهای مقدار خروجی. به عنوان مثال این مقدار برای اسپلیتر دو راه حدود 5/3 db وبرای اسپلیتر 4 راه حدود 5/6 الی 2/7 دسی بل خواهد بود . معمولاً كارخانجات سازنده مقدار این افت را برای فركانس های مختلف در جدولی ارائه می كنند.


    3- افت جداسازی ( ISOLATION LOSS ) (TAP LOSS) : هر تپ آف برای ایزولاسیون گیرنده ها از یكدیگر سیگنال ورودی را مقداری كاهش می دهد وآن را به خروجی فرعی می دهد این افت را افت جداسازی (ایزولاسیون ) می نامند مثلاً اگر یك سیگنال 25db به یك تپ اآف با افت ایزولاسیون 23db اعمال شود در خروجی فرعی مقدار 2db سیگنال قابل دسترس خواهد بود . 

    4- افت عبوری (Trough loss) INSERTION LOSS)) : هنگام عبور سیگنال از داخل تپ آف از ورودی اصلی به خروجی اصلی مقداری افت ایجاد می شود كه باید مقدار آن را در محاسبات مد نظر قرار داد . مقدار این افت برای فركانس های مختلف فرق می كند وتوسط كارخانه سازنده جدولی ارائه می گردد ولی معمولاً تپ آف های با مقدار ایزولاسیون بالا افت عبور ی كمتری دارند . 

    نحوه انتخاب تپ آف:
    باید در یك سیستم MATV تپ آف هایی انتخاب شود كه حداقل 1000 میکرو ولت را برای هر گیرنده تامین كند وایزولاسیون كافی بین گیرنده و سیستم جهت جلوگیری از تداخل ایجاد كند دریافت سیگنال بیش از 1000 میكرو ولت ( صفر دسی بل ) به گیرنده آسیبی نمی رساند و بسیاری از طراحان سیستم های MATV سطح خروجی های فرعی را تا 10 db نیز در نظر می گیرند . در طراحی سیستم افت ایزولاسیون آخرین تپ آف قبل از آمپلی فایر را در نظر می گیرند ودر صورت طولانی بودن مسیر بین تپ آف و دستگاه تلویزیون باید افت كابل آن را نیز در نظر گرفت . در صورت استفاده از تپ آف های دیواری ( wall tap ) به علت كم بودن فاصله بین تپ آف و تلویزیون می توان از این افت صرف نظر كرد . 


    انتخاب آنتن:
    سه فاكتور اساسی باید در انتخاب آنتن در نظر گرفته شود : 
    1- نوع آنتن 2- بهره آنتن 3- جهت آنتن 

    نوع آنتن با توجه به تعداد و فركانس كانال های مورد در یافت تعیین می گردد. جهت آنتن نیز نسبت به فرستنده تلویزیونی‌تنظیم می شود. اگر تمام فرستنده ها یا تعدادی از آن ها در یك جهت باشند از آنتن پهن باند (‌BROAD BAND ) استفاده می شود و اگر در جهت های متفاوت باشند از آنتن تك كانال استفاده می گردد . انواع معمول آنتن ها عبارتند از : 
    VHF/UHF/FM , VHF/UHF , UHF , VHF 
    البته برای دریافت سیگنال FM بهتر است از آنتن جداگانه FM استفاده می گردد .بهره آنتن یك مساله مهم است باید آنتن حداقل سیگنال 0 db رابرای ورودی آمپلی فایر مهیا نماید . در محل های با سیگنال ضعیف باید از آنتن با بهره و اندازه بزرگتر استفاده كرد . در صورتی كه باز هم سیگنال مناسب به دست نیامد مجبوریم از پری آمپلی فایر استفاده كنیم . جهت آنتن نیز باید به دقت تنظیم شود . اگر آنتن خوب تنظیم شده باشد نسبت سیگنال هایی كه با قسمت جلو آنتن دریافت می گردد به سیگنال هایی كه با عقب آنتن دریافت می گردد بیشتر خواهد بود.


    بر آورد سطح سیگنال:
    تعیین دقیق سطح سیگنال برای طراحی صحیح سیستم مهم و اساسی است . لذا با استفاده از یك آنتن با بهره مشخص ( در صورت امكان همان آنتنی كه نصب خواهد شد ) و یك تلویزیون رنگی قابل حمل و نقل و یك میدان سنج میتوان مقدار سیگنال را در محل نصب آنتن تعیین كرد . در محل هایی كه سیگنال ضعیف است محل آنتن بسیار حساس است . ممكن است در یك محدوده 15 متری تفاوت های فاحشی در مقدار سیگنال وجود داشته باشد . ارتفاع آنتن نیز در مقدار سیگنال موثر است . ولی این مطلب را باید در نظر داشت كه همیشه ارتفاع بالاتر باعث ایجاد سیگنال بیشتر نمی شود بلكه باید مناسب ترین ارتفاع را با آزمایش به دست آورد . میدان سنج نیز برای اندازه گیری سیگنال دریافت شده برای هر كانال به كار می رود.این تست باید در چند جای سایت انجام گیرد و بهترین محل برای آنتن انتخاب گردد . در صورتی كه آنتن به دقت انتخاب شود حتی می تواند بعضی تداخل ها را از بین ببرد . با استفاده از تلویزیون رنگی می توان كیفت سیگنال را در هر كانال مشخص كرد ودر صورت وجود تداخل امواج اثر آن را روی تصویر مشاهده نمود.


    انتخاب پیش تقویت كننده ( PRE AMPLIFIRE ) : 
    در محل هایی كه سیگنال ضعیف است ممكن است تقویت اولیه سیگنال لازم شود . در انتخاب پری آمپلی فایر باید چهار نكته را در نظر گرفت : 
    1- پوشش باند فركانسی 2- بهره  (GAIN) 
     3-مقدار نویز 4- توان خروجی 
    پری آمپلی فایر ها به صورت UHF یا VHF یا VHF/UHF ساخته شده اند بعضی از آن ها دارای مسدود كننده های موج FM هستند تا اگر دریافت FM باعث ایجاد نویز شود آن را بلوكه كنند . پری آمپلی فایر باید سطح سیگنال كافی برای آمپلی فایر توزیع را فراهم كند . هنگام استفاده از آمپلی فایرهای تك كانال هم ممكن است یك پری آمپلی فایر لازم شود . تا سیگنال كافی برای عمل كرد صحیحAGC فراهم گردد . مقدار نویز تولید شده توسط پری آمپلی فایر یا همان عدد نویز(noise figure ) نیز باید پایین باشد تا كیفیت سیگنال حفظ شود . تغذیه پری آمپلی فایر كه در نزدیكترین فاصله از آنتن نصب شده است از طریق یك منبع تغذیه در داخل ساختمان نیز ممكن است و پس از كاهش دادن ولتاژ به مقدار لازم توسط خطوط سیگنال به پری آمپلی فایر اعمال می شود . توجه كنید بین منبع تغذیه و پری آمپلی فایر یك اسپلیتر معمولی قرار ندهید چون باعث اتصال كوتاه منبع تغذیه می گردد . از مبدل تطبیق امپدانس نیز نباید استفاده نمایید.


    پردازش و تركیب سیگنال : 
    عمل پردازش سیگنال توسط فیلترها – مسدود كننده ها – تركیب كننده ها و تضعیف كننده ها انجام می گیرد . در صورت لزوم از مبدل فركانس UHF به VHF نیز می توان استفاده كرد .

    انتخاب آمپلی فایر : 
    در انتخاب آمپلی فایر باید 4 مورد را در نظر گرفت : 
    1- فركانس و تعداد كانال های مورد دریافت 
    2- افت كل سیستم 
    3- نوع سیگنال ورودی 
    4- قابلیت خروجی ( مقدار خروجی ) 

    اگر كانال های هم جوار زیادی در یافت شود هر كانال برای جلوگیری از تداخل باید فیلتر شود و برای این منظور معمولاً از آمپلی فایرهای تك كانال ( STRIP ) استفاده می گردد . مقدار ورودی به علاوه بهره تقویت كننده باید از افت كل سیستم بیشتر شود ومعمولاً 6 db نیز به این مقدار اضافه می كنند . آمپلی فایرهای تك كانال بعد از فیلتر كردن و بلوكه كردن تمام كانال های دیگر به كار می روند و دارای 2 نوع كنترل بهره اتوماتیك ( AGC ) و دستی هستند . كه نوع AGC در شرایط آب و هوایی و محیطی مختلف سطح سیگنال را ثابت نگه می دارند . آمپلی فایرها با ورودیUHF/VHF , VHF ساخته شده اند . در ضمن مقدار سیگنال ورودی به علاوه بهره تقویت كننده نباید از توان خروجی آمپلی فایر بیشتر شود . قابلیت یا مقدار خروجی آمپلی فایر مقداریست كه تقویت كننده بدون برش و یا مدولاسیون عرضی می تواند تحویل دهد . بعضی از آمپلی فایرها دارای كنترل بهره و اعوجاج و نوسان وتضعیف كننده قابل تنظیم می باشند تا سطح سیگنال یكسانی را برای تمام كانال ها ایجاد كنند .

    صفحات جانبی

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      لطفاً نظرات خود را درمورد وبلاگ با اینجانب در میان بگذارید.(iman.sariri@yahoo.com)نتایج تاکنون15000مفید و 125غیرمفید. با سپاس


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