Bandwidth reduction of DXTV video

  
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DX TV enthusiasts often have to deal with relatively crowded VHF bands. In these instances, reduced TV receiver bandwidth results in greater selectivity. The other advantage of bandwidth reduction is improved signal to noise ratio. This results in less noise and distortion of the DX TV picture. Bandwidth reduction is especially useful for weak sig
Bandwidth reduction of DXTV video - schematic

nal modes, such as meteor scatter. Although bandwidth reduction is equally beneficial for TV and FM audio, this article concentrates on TV video. The IF (intermediate frequency amplifiers) strip accepts the ~35 MHz IF output from the VHF (or UHF) tuner and amplifies the required signal to approximately one volt at the detector prior to feeding the video amplifiers(s). It is within this section that the main IF response is determined; the VHF tuner consists of effectively wide band circuits. In passing, some earlier UK dual standard TV receivers utilised the VHF tuner in whole or part to act as an IF preamplifier stage when operating at UHF. If a valve receiver is used then 3 vision IF stages are ideal and if possible 4 stages in the case of a solid state receiver fitted with discrete devises - though with present day IC technology distributed in the mass produced TV receiver an additional IF gain/shaping stage could possibly be fitted prior to the main IF strip input - see later. This number of stages if necessary to obtain the required gain/bandwidth performance; bearing in mind the transmitted signal information may have a bandwidth up to 8 MHz. Although weak signals produce problems undoubtedly an equal problem is interference from high power transmitters, thus effectively limiting the use of high gain aerial amplifiers due to various forms of adjacent and co-channel interference. One way of surmounting such a problem is...



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