By William Woodward, Scientific Atlanta
Over the past few years numerous video modulators which allow the user to select their output frequency have been introduced to the CATV market. Initially these frequency agile modulators were only used for back up of fixed channel modulators in CATV Headends, because they were significantly more expensive than fixed channel modulators. With increased interest in SMATV systems, numerous inexpensive frequency agile modulators have been introduced. Some of these "SMATV" modulators made their way into CATV applications, which precipitated the introduction of several cost effective "CATV Quality" frequency agile modulators intended for general Headend use. The CATV operator now has a large field of modulators to choose from. While frequency agile modulators offer numerous advantages, it is important to consider some additional performance parameters that have not historically been reflected in the specifications used to characterize fixed channel modulators. It is possible for two modulators to appear to have the same performance but actually have radically different performance. To understand the causes of these differences, it is necessary to understand how the fixed channel and frequency agile modulator differ in construction.