CATV / MATV Design Fundamentals Part - 2

Some types of frequency AGILE processors and modulators can also produce broadband noise across the full range of CATV frequencies. When a large number of these low-cost devices are coupled together into a single cable, the result is a noisy or "snowy picture" on all channels before leaving the headend. The best types of AGILE modulators and processors will have some type of "tracking bandpass filter" to minimize the broadband noise output. Fixed channel processors and modulators may contain an internal single-channel bandpass filter to prevent this type of broadband noise problem.

Now that we have television signal sources, all that remains is to combine them using directional couplers and splitters. When combined, the VISUAL RF carriers should all be at the same level. Each associated sound carrier should be set -15 dB in relation to the visual carrier level for that channel. The losses of the couplers and splitters may require a broadband amplifier after all the signals are combined. This will insure that we have enough RF signal level to begin signal distribution to the many television outlets in the RF distribution system.

Depending on the number of channels combined together, the coupler and splitter losses could result in a combined signal level on all channels from +10 dBmV to +30 dBmV on each channel. At +30 dBmV, the signal levels may be strong enough to feed directly into cables leaving the headend to go to other buildings. At lower signal levels, an amplifier is used to provide a signal level from +35 dBmV to +45 dBmV or more to depart the headend. The maximum signal level will vary depending on the number of channels and amplifiers in cascade in the distribution network. This is due to the intermixing of signals when they are passed through the active components of the amplifiers. The most common intermodulation distortion is known as composite triple beat (CTB). You know you have it when you see various types of "lines" in the pictures. This may be called a "busy background" by some individuals.

When the headend amplifier output levels are established, the design of the cable distribution network can begin. For our 750 MHz system, let us assume an output level of +40 dBmV on cable channel 118 and a level of +35 dBmV on channel 2. There are two reasons why the amplifier output is not FLAT (equal level at all channels). The first reason is the coaxial cable that follows the amplifier has lower insertion loss at channel 2 than it has at cable channel 118. The second reason is to provide a lower ratio of CTB distortion from the amplifier with the reduced signal levels at the lower frequencies.

The choice of the type of coaxial cable to be used in the distribution system is based on the distances to be covered and whether or not the cable is aerial, direct buried, or in a conduit or other underground passage ways. Common sizes of outdoor cables are specified by the outside diameter of the metallic outer sheath of the cable. The outdoor cable sizes most used are 0.500" (1/2 inch) and 0.750 (3/4 inch). The larger cable has lower insertion loss per 100 feet of cable.

Systems that are completely inside a single building may use the extruded aluminum cable commonly used outdoors or may require special plenum types of cable in sizes known by their RG numbers: RG-11, RG-6 or RG-59. The RG-11 cable is the largest and has the lowest loss per 100 feet. The RG-59 cable is hardly used anymore in systems with bandwidths above 300 MHz, since it has the highest loss of the three types of flexible coaxial cable.

CALCULATING CABLE AND PASSIVE LOSSES BETWEEN AMPLIFIER

The nominal cable losses per 100 feet of some popular cable sizes is given in the chart below. * The 0.412" and 1.000" cable losses are given as the maximum cable loss per 100 feet. To discover the loss of a given length of cable at specific CATV channels or frequencies, divide the length of cable in feet by 100, then multiply by the appropriate dB figure from the chart below. Doing this at the highest and lowest frequencies to be distributed will provide the losses needed to select a cable equalizer when reaching successive RF amplifiers. This topic is discussed in another article on this web site called Fixed Equalizer Selection. The formula for computing cable equalizer selection is explained and tables of signal level difference solutions are provided.

Similar cable loss data is published by CATV amplifier companies in handy pocket reference books for Times Fiber Cable and Trilogy Communications MC_ (M-C-squared) coaxial cable types. The above chart is given as an example only. The user should refer to the correct cable loss charts for actual signal loss calculations.


The data table on the previous page contains the insertion loss through the tap to the next cable or tap at the frequency ranges specified. The tap loss is the signal level loss that occurs from the input of the tap to each of the output ports to feed subscriber drop cables. This chart indicates the loss differences based on frequency and tap values and will vary from one tap manufacturer to another. Indoor taps that do not contain AC power passing circuitry may have better specifications. Similar charts exist for 2-port and 8-port taps as well as for inline directional couplers and 2-port and 3-port splitters. In the chart above, the 8 dB tap is a 4-way splitter and therefore has no through-port insertion loss, since all of the RF signals are directed to the 4 tap ports.