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band, depending on the size of the bandwidth of RF signals Therefore, for longer distances, the RF band can be split into two or three bands per transmitter Then two or more receivers can have their outputs combined In most instances, high-power transmitters, transmitting through the normal ber-optic distances, can carry the whole RF programming band At one time, it was thought that using FM would be better, but technology in AM laser operation improved to the point that it became the method of choice 7412 The optical receivers used by the cable television industry are the integrated receiver to RF converter type The optical cable is often terminated into the housing, and the optical signal is converted to RF at an appropriate signal level, comparative to a normal bridge ampli er level This signal, as discussed in 4, is distributed to taps by using a ve-ampli er RF cascade Such an optical system from the headend to a receiving point is shown in Figure 7-14
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RF television signals Power Budget Cable loss over 10 km: 20 dB Connector and splice loss: 2 dB Optical power at receiver input: 25 dBm 22 dB 245 dBm if the receiver has a sensitivity of 30 dBm, the margin is: 245 dBm ( 30 dBm) 55 dB, which is adequate
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Figure 7-14 AM ber-optic transmission of cable television carriers
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742 Fiber-Optical Testing
For many technicians, testing optical cable and devices is new and threatening However, it quickly becomes apparent that the optical signal level is measured in terms of power in the quite normal unit of optical dBm Optical power meters have been developed that are small and easy to use Also, the optical time domain re ectometer (TDR) has been developed, which can be used to test optical cable 7421 Optical signal levels can be measured using an optical power meter that measures the level directly in dBm Power meters operate at only one or two wavelengths, depending on whether the instrument is a single- or dual-wavelength type The instrument range is large enough to be connected via an optical jumper directly to the transmitter Reading this level as the input level and testing the level at the receiving point can yield the optical path loss This method is shown in Figure 7-15 The optical level is displayed on a LCD display in numbers of dBm of optical power The 1-mW level corresponds to 0 dBm These instruments are hand held, battery operated, and usually available with a variety of adapters for different optical connectors Also, this instrument is quite rugged and relatively inexpensive, usually about $500 to $1,500 One manufacturer offers a power meter/ light source combination with which a technician can use the light source with voice communications through a spare ber to an accomplice with the same equipment
Figure 7-15 Measurements of optical path loss
Optical power meter
Optical transmitter
1 dBm A receiver should have a threshold 6 dB below 17 dBm or 23 dBm
1 dBm Optical transmitter cable loss Optical-path loss: splice loss connector loss
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at the opposite cable end As a team, the two technicians can test the other bers in the cable 7422 The optical TDR is an important and valuable piece of equipment and every system with any optical cable should have at least one The typical price range for these devices is $6,000 to $16,000, depending on options These instruments have large LCD screens displaying a loss pro le of the ber being tested Cable systems often use cables carrying many bers, where the various bers can branch to other parts of the system at splice locations Optical cable should be tested before it is installed as well as after it is installed and before activation The optical TDR can be used to establish the length of cable on a reel by measuring a ber The end point can be seen and identi ed, and any anomalies along the ber can be seen on the display Some cable operators do not measure every ber, but most systems will measure at least one in each buffer tube for large ber-count cables Optical cable can be either loose-tube or tight buffered-tube construction Some cable systems choose loose-tube cable for aerial plant and tight buffered-tube cable for underground or where it is pulled through a conduit Figure 7-16 shows how some faults are displayed on an optical TDR Because testing of bers is performed before and after installation, the test data has to be stored or led for future reference The TEK Ranger optical TDR has a keyboard option with which cable reel number, cable number, buffer tube identi er (usually a color), and the plastic coating color of the ber can be entered, followed by the loss measurement This instrument can store a large number of screen measurements that can be loaded to a computer for storage and or analysis An optional disk drive is available as well as an RS 232 Port Data can be stored and analyzed by Windows-supported software Most optical TDR instruments are battery powered and built for troubleshooting in the eld 7423 Other types of optical test equipment must have a high-level source of optical power that operates in the visible spectrum Before connecting any optical cables to headend equipment, the cable can be tested by placing a light source to one end of a cable Any bends in the optical cable that are too tight or too sharp will leak light and can be easily spotted visually Many systems are concerned that optical cable will be cut, dug up, or damaged and repairs will have to be made Single-mode optical ber is spliced using a fusion-splicing device that encapsulates the splices in sealed PVC enclosures More and more systems are electing to
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