CATV / MATV Design Fundamentals Part - 1

This section is for those of you who need the fundamentals to build an indoor (or outdoor) RF distribution system. If you are an "Old Pro" in the cable television industry, this will be a boring review. For the "first timer" this will open your eyes to the reasons why your TV signals have snow or lines in the pictures. This article is just the "fundamentals" of cable television (CATV) and master antenna television (MATV) signal distribution.

If your RF distribution network requires more than one amplifier in series, the number one cable TV rule applies. Cable television systems consist of UNITY GAIN building blocks. What that means is every length of cable (and the passive devices in that length of cable) is followed by an amplifier with an equal amount of RF gain and equalization to overcome the differing losses at all desired frequencies.

A piece of coaxial cable is nothing more than a piece of round wire surrounded by an insulating material (usually foam) that is then surrounded with an outer metal jacket which may consist of overlapping vinyl/foil layers, braided wire, or extruded (or welded) aluminum tubing. The object of coaxial cable is to keep electromagnetic RF carrier waves inside the cable from getting out and similar waves from the outside world from getting inside the cable. And by the way, the term COAXIAL means the center conductor must stay in the CENTER. If you kink, or flatten the cable, it does not work as well. Damaged cable creates reflections and it will radiate and absorb signals to and from the outside world. DO NOT DAMAGE THE CABLE.

Electromagnetic waves of different wavelengths (frequencies) can flow through a single cable at the same time. The higher the frequency of a wave, the greater the insertion loss of the cable for a given length of cable. This loss is usually expressed in decibels (dB) per 100 feet of cable here in the United States. Common cable television test equipment and signal level meters are calibrated with decibel ranges. A table of losses for popular cable types is included in this article for quick reference. The signal level losses will vary with the temperature of the cable. These data tables are published for ambient cable temperature of 70 degrees Fahrenheit. For cables exposed to the weather, when the temperature is COLDER, the losses of the cable are LESS. On the hottest day in the summer, the cable losses are the greatest. Cable that is buried in the ground, or indoors, will usually remain at the same temperature year round.

For ease of use and recognition, each of the different television frequencies has been assigned a channel number. The channel numbering scheme applied to these frequencies is different in overthe-air TV transmissions as received with an antenna when compared to the channel numbers and frequencies used in a cable television system. There are several different frequency plans used in cable. We will only be concerned with "standard CATV" channels in this article and not the IRC or HRC CATV RF frequency plans. For more information of channel numbers versus actual frequencies, see the article ÒUS Broadcast & CATV Channel Assigned Frequencies.Ó

The passive splitters and directional couplers used in cable systems are called flat loss devices. Below 450 MHz (CATV channel 61) they are essentially flat in their loss characteristics. With the newer 750 to 860 MHz RF distribution systems, the high-frequency insertion losses of these devices needs to be addressed when totaling the signal losses between amplifiers. Passive losses for splitters, taps and directional couplers are published by the various manufacturers. Pocket-sized reference books are published by the major CATV manufacturing companies containing this type of data. A brief excerpt of this type of data is included in this article.

A splitter is a passive device with from 2 to 8 ports with equal loss to each and every port. The most common type of splitter has one input port and two output ports. It is commonly referred to as a "two-way splitter" - - meaning it has two output ports. In a perfect world of 100% efficient devices, an equal split of RF energy would result in a signal loss of 3 dB to each output port. The "real world" loss of the splitter is typically 3.5 dB to 3.8 dB at lower frequencies to as much as 5 to 5.5 dB at the highest frequencies used in CATV / MATV systems. For practical purposes, the loss of a 2-way splitter may be assumed to be 4 dB, a 4-way splitter loss can be 7.5 to 8 dB, and an 8-way splitter can be from 11 to 12 dB insertion loss to each port.

A directional coupler is a three-port or four-port passive device that does NOT evenly divide the RF signals at the output ports. Depending on the RF bandwidth of these devices, they typically have tap loss values of 8 dB, 12 dB, or 16 dB. The higher the loss to the "tap port", the lower the insertion loss to the "through port". When a directional coupler is connected directly to a splitter in a single housing, it is called a directional tap or multi-tap. Common tap loss values for these devices vary from 8 to 32 dB depending on configuration and number of tap ports (up to 8).

The third part of the CATV / RF distribution system consists of the active devices like modulators, signal processors and broadband amplifiers. Here are brief descriptions of each type of device: Modulators take video and audio signals from satellite receivers, video tape machines or cameras and produce a standard modulated television channel for viewing on a television set. The signal output level from a modulator is constant and can be as much as +60 dBmV. This value is 60 decibels above ONE millivolt as measured across a 75 ohm source impedance. This value is ONE volt of RF carrier at the peak amplitude of the signal.

Signal processors (or strip amplifiers) are single channel amplifiers designed to receive a single television signal from an antenna and control its output to a steady level. This automatic gain control (AGC) function is essential for the success of the distribution network of broadband amplifiers. Strip amplifiers do not alter the input frequency of the received television signal - - what goes in, comes out on the same frequency. Signal processors convert received television signals to an intermediate frequency ( I.F. ) for filtering out unwanted adjacent television signals that may be present at the antenna input. Precise signal level control (AGC) is accomplished at this point inside the signal processor. The sound carrier RF level is reduced from the 6 to 10 dB level difference transmitted by the TV station to a level between 13 and 17 dB lower than the associated visual RF carrier.

This allows adjacent TV channels to be received by a television set with no visual impairment on the viewing screen. The final stage of a signal processor converts the I.F. signal to a standard or CATV channel for distribution to the cable system. A typical maximum RF output level for a processor may also be as much as +60 dBmV.