EXPERIMENTAL INVESTIGATION OF PERFORMANCE IMPROVEMENT BY A TV. PREAMPLIFIER

Abstract

Associated with each television transmitter is a belt of area around it, in which the T.V. signal is too weak to be received by T.Y. receivers. To overcome this difficulty the existing methods depend on using a low-noise preamplifier following the antenna. It is found in practice that the improvement in picture. quality which is possible by this method is either nonexisting or very much limited. Detailed experimental work on noise measurement of T.V. frontends and antennas at different T.Y. channels, have been carried out to clearly establish the various conditions in which the application of preamplifier can lead to impro-vements in picture quality. Experimental investigations show thatvarious external and internal limiting factors such as antenna noise temperature, nonlinearity effects of noise and nonlinear distortion effects, severely degrade the signal-to-noise ratio and hence the use of preamplifier may even lead to a deterioration in picture quality. Hence in deep fringe areas, high gain of preamplifier is undesirable. The problem of optimization of preamplifier gain with relation to the overall noise performance, including nonlinearity effects such as crossmodulation, intermodulation etc. is also discussed. Possible improvements in signal-to-noise ratio by r improving the noise performance of T.V. preamplifier is presented in the form of a simple plot.