SCTE 58 2003 PDF

This document describes a test procedure for the laboratory and production measurement of Amplitude Modulation Cross Modulation (or AM-XMOD) that is present in Broadband Systems which carry Frequency Division Multiplexed (FDM), amplitude modulated, analog video channels. In general there are two components to the cross modulation generated in such systems; namely, cross modulation of the direct Amplitude Modulation (AM-XMOD) type and cross modulation of the less obvious Phase Modulation (PMXMOD) type. In the past, the PM-XMOD component was not directly considered, for many pertinent reasons. With the advent of larger cable telecommunications systems (e.g. more video channels with carrier placement at higher frequencies), and the implementation of different equipment and technology (such as fiber optics in the distribution path), it has become necessary that the PM-XMOD at least be described. The physical processes by which the two components of cross modulation are generated have some fundamental differences. However, the resultant video impairment to the viewer can be the same if the proper conditions exist. (That is, both of these components have been shown to cause similar visual disturbances in the desired picture when the proper conditions exist and hence are considered to be true impairments.) The issue of how each component of cross modulation is detected at the subscriber's terminal (i.e. "set-top receiver") and the subsequent perceptibility levels that should be specified will NOT be addressed in this document.

The procedure described in this document for measuring the 'AM-XMOD Distortion Ratio' uses an RF Receiver, to detect the modulation, and a Baseband Analyzer, to measure the detected modulation signal levels. The test method, as given, is meant to mitigate the effects that PM-XMOD has upon the accurate detection of AM-XMOD. A common alternative to this measurement technique is to measure the AM-XMOD using a specific instrument, namely the 'Spectrum Analyzer'. The Spectrum Analyzer has the capability to both detect and measure the magnitude of the modulation level (i.e. the levels of the sidebands) in the frequency domain. An AM-XMOD measurement that uses this "frequency domain" technique will be different from a measurement made using the technique described in this procedure, because the Spectrum Analyzer can not discriminate AM and PM sidebands. The type of detector used in the RF Receiver, as well as the method of detection used in the Baseband Analyzer to measure the sideband energy of the modulation, will be specified.

The detectors used in the RF Receiver and the Baseband Analyzer instruments are different than those used in a typical Spectrum Analyzer. Therefore, a Spectrum Analyzer, where used in this document, is understood to be a separate instrument from either an RF Receiver or a Baseband Analyzer. However, it is generally understood that a Spectrum Analyzer can be used as a receiver instrument, whereby either the IF output is taken and RF demodulation is accomplished externally by a separate detector, or the Spectrum Analyzer's (Linear) demodulator is used. In either approach, after demodulation, a Baseband Analyzer then detects the video modulation.

ANSI :	ANSI Approved
File Size :	1 file , 510 KB
Number of Pages :	34
Published :	01/01/2003