CTB performance of cascaded externally modulated and directly modulated CATV transmitters

Recently, CATV supertrunk applications with lengths in excess of 40 km have been designed using externally modulated transmitters. The transmission is received at an optical transition node where it is further distributed by conventional DFB CATV transmitters to the rest of the cable plant. These systems are typically limited by the required CTB performance at the subscriber and this paper explores the resultant CTB from cascading these two different types of transmitters. It has been reported that cascading externally modulated transmitters results in a 10 log addition rule on CTB, We show, through experimentation, the "20 log" addition rule traditionally used for CTB in CATV systems still is a worst case rule. However, there may be frequencies where this rule can be relaxed and used to the system designer\´s advantage.