The age of high-speed data communications and multimedia application have dawned on cable television. It has brought with it a myriad of services such as high-speed data, video on demand, IP telephony, status monitoring, impulse pay-per-view and internet access to name a few. A cable plant must be two-way ready to carry this high-speed data traffic from a variety of sources.

Traditionally, in a two-way cable television plant, return amplifiers are employed to carry the reverse path signals (5 – 42 MHz). Diplex filters are used to separate the reverse signals from the forward path. A diplex filter is a Low-pass and High-pass combination. In a frequency division multiplexing (FDM) arrangement, without the use of diplex filters which are located between the input and output ports of an amplifier station, interference and oscillations between the forward and reverse path signals could occur.

In a typical CATV trunk amplifier station, there are three diplex filters, (trunk in, trunk out, and Bridger out) forward amplifier, Bridger amplifier and reverse amplifier present. In a line-extender, an arrangement of two diplex filters with forward and reverse amplifiers is employed. In North America, a modern diplex filter Low-pass section would pass frequencies between 5 Mhz through 42 Mhz (reverse path) and the High-pass filter section would pass frequencies between 54 Mhz through 860 Mhz (forward path). 

In a cable TV plant the forward path signals (54 MHz – 860 MHz) carries the entertainment television channels. This signal is said to be going downstream from the headend (origination) to the subscriber’s television set. RF data signals coming back from the subscriber’s cable modem to the headend is known as the upstream or return path signal (5 MHz – 42 MHz). Note in Figure 1 block diagram the direction of the forward amplifier’s output is opposite to the return amplifier’s output.

The primary purpose of equalization in a cable TV plant is to compensate for the slope generated in coaxial cable between the high and low TV frequencies. When various TV carriers with respect to frequency are transmitted through coaxial cable they are subjected to increasing attenuation (loss). Simply put, as the frequencies of the TV carriers are increased, attenuation loss (dB) increases. An equalizer is installed in an amplifier to compensate for this slope in cable resulting in a flat frequency response at the output. The amount of compensation in a reverse path application is related to the cable slope at 5 MHz with respect to 42 MHz signal. Return equalizers are chosen to compensate for this slope so that the output levels are flat.

Video and data carriers suffer from the effects of cable and flat losses in a cable plant. Forward and reverse path amplification are necessary at predetermined locations, in order to compensate for these losses. The return amplifier ( 5 – 42 MHz) has typically an operational gain of approximately 18 dB. The unity gain concept is utilized in cable TV to compensate for the cable spacing and flat losses between amplifiers. This means that for a certain cable spacing (loss) at the highest frequency, when compensated by an amplifier with equal gain will produce a unity gain outcome. For example, if a length of coaxial cable at 42 MHz frequency exhibits a 6 dB loss and is in series with a flat loss from a directional coupler of 12 dB, the total losses would be 18 dB. A return amplifier with 18 dB of operational gain would be needed to compensate for these losses: -18 dB (loss) + 18 dB (gain) = 0 dB or unity gain. The logarithm of 0 dB = 1, this is condition is repeated for each section of cable as the line technician balances the return path.

The hybrid return amplifier is usually a 25 dB or 30 dB low current gain block with superior 1 dB compression point compared to a two stage transistor design. The cable equalizer can be either fixed or variable . The pick-up circuitry is normally 1 dB to 3 dB pick-up at the corner frequencies to compensate for diplex filter and main motherboard roll-off. The pad is usually an attenuator type pad (JXP-A)/7A series type with a flat loss ranging from 1 dB to 20 dB in 0.5 dB steps.