By Eugene C. Walding, CATV Engineering, OAK Electro/netics
INTRODUCTION Set top converters have been on the market for over five years. For the most part, they have performed their intended function admirably. However, due to changing requirements and more stringent specifications, we are moving into an era in which second generation converters are required.
BACKGROUND Set top converters evolved as a means of solving the direct pickup problem prevalent in strong signal metropolitan areas. The method used was to down convert the cable signals to a 44MHz I-F and then up convert to a channel that was unoccupied by a broadcast signal. The RF circuitry to perform this function was enclosed in a well shielded metal compartment. The basic tuning methods were well established VHF TV tuner concepts and hardware. It soon became apparent that the set top converter did not have to be limited to a meager twelve channels. To achieve more channels, the initial attempts were to either add another TV type tuner or to expand the coverage of the existing tuner. In either case, the 44MHz I-F was retained. As a result, the local oscillator signal emanating from the tuner antenna terminals fell into the CA'IV signal band. When only twelve standard channels are used, this is relatively unimportant. When mid and super bands are us~d, it results in converter to converter "cross talk".