Magicomm Technologies Ltd

The CATV Return Amplifier

By Allan Sobryan

 

Introduction

 

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).

 

Figure 1:Block diagram of a 2 way CATV amplifier station

 

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.

The hybrid manufacturers try to design universal gain blocks so that RF designers can meet several different circuit criteria. Some points to consider  from an RF engineering viewpoint are:-
 
The insertion loss of the input and output diplex filter in a trunk or line-extender amplifier station. Typically, you can have a roll-off at 42 MHz of 1.5  to 2 dB depending on the filter alignment. Usually, the higher the filter isolation between the low and high pass filters, the high frequency roll-off tends to be greater. This means you can experience a further insertion loss inside of the amplifier housing of 4 dB. Add 1 db of insertion loss for the return equalization and you require an extra 5 db of gain.
 
Some amplifiers like the Magnavox 6TNA (trunk network amplifiers) or 6GNA (Global network amplifiers) have higher insertion losses when compared to line-extenders such as Magnavox 6LE97. This is due to additional splitters on the motherboard used to create 2 , 3 or 4 equal distribution outputs besides the main output port. Instead of splitters some amplifier use directional couplers to create unequal outputs, therefore the return amplifier has to be able to compensate for all these internal losses.
 
Previously,  return amplifiers were designed with AGC (automatic gain control) to compensate for temperature effects on the cable portion of the plant. You would then require another 2  to 3 dB of head room for  AGC operation. Today, the return amplifier is designed as a manual gain amplifier and long loop AGC operation from the head-end controller software is used to adjust the cable modem levels at the subscriber.
 

Allan Sobryan is the President of Magicomm Technologies Ltd. He has spent the last 24 years technically and administratively in several key areas of broadband telecommunications resulting in obtaining a multi-faceted background in RF, voice, video, and data. His present technical responsibilities include design and product development of diplex filters, CATV return/ forward amplifiers and custom CATV products. Allan Sobryan holds a B.tech degree in Electrical Engineering from Ryerson Polytechnic University, Toronto, Canada. His hobbies include performing magic, multimedia, and playing table tennis. He can be reached by e-mail: magicomm@cabletv-4u.com

This information is furnished for guidance and with no guarantee as to its accuracy or completeness; its publication conveys no license under any patent or other right, nor does the publisher assume liability for any consequence of its use; specifications and availability of goods mentioned in it are subject to change without notice. It is not to be reproduced in any way, in whole or in part, without the written consent of Magicomm Technologies Ltd

Copyright 1998-2009 Magicomm Technologies Ltd. All rights reserved.

cabletv-4u.com                           More....Diplex Filter technical paper

MAGICOMM TECHNOLOGIES LTD.

BOLTON,  ONTARIO,

CANADA

Tel: (905) 951-0950             E-mail: sales@magicomm.ca