A proportional timing generator for measuring intermodulation product distortion on television transposers.

Abstract

Broadcasting authorities presently measure intermodulation distortion by applying a three tone simulation of a composite video and sound signal to the transposer and then measuring the relative amplitude of the major in-band intermodulation product (nominally at vision carrier frequency plus 1,57 MHz in the 625 line I/PAL System) on a spectrum analyser. This method is slow and requires a skilful operator to achieve repeatable results. Furthermore it tests the common RF amplification equipment at one luminance level and one chrominance level and therefore does not subject the transposer equipment to operation over its full range. A new sampling measurement technique has been proposed which overcomes all these problems by selectively mixing, while transmitting a colour bar test pattern, the demodulated output video signal of the frequency transposer with a pulse train coinciding with a particular colour. This thesis describes the design of a very stable proportional timing generator and its application to the measurement of intermodulation distortion on frequency transposers. The timing generator, which locks automatically onto the video signal and produces narrow sampling pulses which coincide accurately with a particular section of each line over a 50°C temperature range, is applicable to all PAL and NTSC TV Systems.