By Mark Adams, Scientific-Atlanta, Inc.
Distortion parameters have always been the limiting factor within a CATV system, but as the bandwidth has increased from 12 channels to 60 and 77 channels, the characteristics of the limiting distortions have changed. At the beginnings of CATV, cross modulation and noise limited the number of amplifiers an operator could run in a cascade. As the number of channels increased, cross modulation gave way to composite triple beat as the limiting factor, with noise still a prominent element. A strange thing has occurred however. As the number of channels increased further so did the importance of a distortion parameter that caused little if no concern before. This distortion parameter is called "second order" and in its discrete form still presents no problem to the CATV operator. But when this parameter is taken in its composite form, composite second order can compete with composite triple beat as the limiting factor for cascade length and feeder levels, especially in a 77 channel system.
This paper will re-investigate the causes of second order distortion. It will also provide insights into calculating which composite second order beats are present from discrete second order numbers. In addition, it will provide an analytical analysis of a trunk amplifier, bridging amplifier and line extender for composite second order and how this distortion can be a limiting factor within a cable system.