CABLE TELEVISION PRACTICE

A. Objectives:
1. Determining the modulator output attenuation before being distributed to customers.
2. Determine the amount of attenuation along the channel.
3. Knowing the strengthening of the cable television amplifier.

B. Equipment used:

• 1 Spectrum Analyzer
• 1 Modulator 3 channel
• 1 Connector  matching impedance 75 W
• 2 Television
• 2 VCD player
• Connecting cable 75 W (± 2 m)
• Coaxial cable RG-59 75 W (± 140 m)
• Connector  N male to BNC female

 C. Circuit diagram

D. Theory
Edge Head (Head End)
Edge provides event signals (programs) for all channels. Local broadcasting and a much captured by an antenna mounted on top of a very high tower in order to extend the distance limit of view. These signals can be distributed it as their home channel number or frequencies diheterodinkan into different channels.

Distribution Cables
Frequency losses in coaxial cables is high, especially those working in the super frequency region from cable TV. However, loss-loss of channel offset by using a radio frequency amplifier (RF amplifier) with a wide field of frequencies that are placed along the cable network as shown in Figure 2.3.

In distribution systems, main channel is the trunk. From this main line, branch cables extended to groups of customers. Channel for each customer referred to the drop.

Each amplifier channel has a strengthening of the same trunk with a loss channel for the distance between the amplifier. Typical value is 40 dB, or a strengthening of the voltage at 100.

Block diagram of a cable television distribution system

Definition

Cable television or cable television is a television broadcasting system via radio frequencysignals transmitted via fiber optic or coaxial cables are fixed and not through the air like a regular television broadcasts that must be captured antenna (over-the-air). In addition to television, FM radio show, internet, and phones also can be delivered via cable.
This system is often found in North America, Europe, Australia, East Asia, South America, and Middle East.

 Cable television was less successful in Africa because of low population densities in various regions. Like radios, different frequencies are used to distribute many channels through one cable. A receiver box used to select a television channel. Modern cable systems now use digital technology to broadcast more channels than analogue systems.

 How it works In a cable system, the signal may have exceeded 30 or 40 amplifiers before reaching your home, one for each 1000 feet or more, with each amplifier you can get noise and distortion. Plus if one of the amplifiers fails you will lose the image. Posts cable systems do not have good image quality and can not be trusted. At the end of 1970, cable TV amplifiers find a solution to the problem. Since then they also make the technology they can add programming to cable service.

Adding channelsIn the early 1950s, cable systems began to experiment with how to use micro glombang sender and receiver tower to catch signals from distant stations. In some cases, this way to make television available to the people who live outside the area of broadcast standards. In other cases, especially in the northeastern U.S..

That means cable TV subscribers may be able to access to several broadcasting stations that have the same network. For the first time the TV cable is used to increase the spectacle, not only the usual spectacle. This started a trend that started the boom in its cable TV in the 1970s.The addition of the station CATV (Community Antenna Television) and cable distribution system directs the author to add a switch most of the television setting.

People can manage their television channels to choose on the basis of the frequency allocation plan of the Federal Communications Commission (FCC) or they can set all to plan for the use by most cable systems. Two different plans its interests.In both search systems, each television station has provided six megahertz part of the radio spectrum. The FCC has become part of the spectrum of Very High Frequency (VHF) to 12 television channels.

Channel is not contained in one frequency block, but instead split into two groups to avoid interference with existing radio services. After that at the time of the growing popularity of television requires additional channels, the FCC allocated frequencies in the UHF (Ultra High Frequency) of the spectrum.

They create a channel 14 to 69 using a block of frequencies between 470 MHz and 812 MHz.
Because they use a cable instead of antenna, cable TV systems do not have to worry about existing services. Experts can use the so-called mid-band frequency has been passed by television broadcasters as well as for other signals, for channels 14-22. channels 1 through 6 are in a lower frequency while others are higher. CATV / Antenna change notice to the television seekers to look for around mid-band and looking through it.

While we are talking about the search channel, ought to consider why the CATV system does not use the same frequency for stations broadcasting on channels 1 through 6 who used the station used the station to broadcast over the airwaves. Equipment designed to protect cable signal carried on the cable from outside interference, and television is designed to receive signals only through the point of connection to the cable or antenna, but the disorder can still enter the system, especially at the connectors.

When the interference comes from the channel carried by cable, there is a problem caused by the difference in speed between the two signal broadcasting.Radio signals travel through the air at speeds that almost match the speed of light. In a coaxial cable such as that carried by the CATV signal into your house, radio signals running at two thirds the speed of light. When broadcast and cable signals to the television seekers breaking occurs for one second, you will see a shaded image known as ghosting

In 1972, a cable system in Wilkes-Barre, PA, starting with channel system offers pay-per-view first. Customers pay to watch individual movies or sporting events. They named this new service with a name or HBO Home Box Service. A pay-per-view continues as a regional service until 1975, when HBO began transmitting signals to the satellite in geosynchronous orbit and then into the cable system in Florida and Mississippi. Bill Wall said that the satellites in recent years can receive and send back up to 24 channels.

 Cable systems receive the signals using dish antennas 10 meters in diameter, with a separate dish for each channel. With the beginning of program delivery to cable systems, the basic architecture of a modern cable system is placed. As the number of program options continues to grow, the bandwidth of cable systems also increases. Current systems operate at 200 MHz, gain 33 channels. As process technology, the bandwidth increased to 300,400,500 and now to 550 MHz, with the number of channels that can be increased to 91 channels. Two additional technological advances, fiber optic and analog to digital conversion, improve the features and quality of broadcasting while continuing to increase the number of channels available.

E. Experimental Procedure
Prior to testing, calibration done prior to the Spectrum Analyzer. After that test the modulator output can be started.

Note: For each test, before being connected to the Spectrum Analyzer will be  matching impedance from 75 W to 50 W (attenuasion 7,8 dB).

For each measurement of TP, do not connect the entire system to be measured. For example, measurements TP1 just installed only the modulator, modulator and cable TP2 just roll it, and so on.

1. Measure and image frequency spectrum at the measurement point (TP1) to see the modulator output signal level on each channel (± 2 m), with block diagram as shown below. The cable used in this test form coaxial cable 75W.

2.  Repeat the test using a long cable (± 150 m) is TP2, with block diagram as shown below. Image of the frequency spectrum and determine the level., How many dB attenuation that occurred on the cable.

3. Repeat testing for TP3, TP4, TP5, TP6, and TP7 as in steps 1 and 2.
Determine the strengthening of the amplifier, the cable attenuation, attenuation at each port splitter.

4. Repeat to TP 2 and TP 4 with moving the cable roll.

F. Experimental Results

 

G. Data Analysis

Viewed TP1, TP2 and TP3 loss frequency is very large. While at TP4 and TP5 small. This is because at TP4 and TP5 already dimbangi channel using radio frequency amplifier (RF amplifier).

H. Conclusion

           Loss will be a big loss in the channel when the channel is not matched with radio frequency amplifier (RF amplifier).