ANSI SCTE 58:2017 pdf download.AM CROSS MODULATION MEASUREMENTS.
2.1 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.
2.2 A measurement procedure for AM-XMOD will be given.
2.3 The measurement procedure for PM-XMOD will NOT be given.
2.4 A more in-depth theoretical discussion of cross modulation, including a description of the physical generation and manifestations, is given in Appendix A. An outline of a derivation for an expression of PM-XMOD is given therein.
2.5 The baseband detection of the AM-XMOD component, and the differences between detection methods, is handled in Appendix B.
2.6 The PM-XMOD component’s effect on the measurement of AM-XMOD, including the resulting errors in measurements of the magnitude of the detected amplitude modulation, is explained in Appendix B.
DEFINITIONS
3.1 Amplitude Modulation (AM) is defined as “a low frequency (i.e. basehand) signal, which is multiplied with a high frequency (i.e. carrier) sinusoidal signal, in order to modulate the peak amplitude levels of the carrier signal by 100% or less. For analog video Cable Telecommunications systems, the modulation characteristic is described as ‘downward only modulation’.” The signal has the form:
3.2 Cross Modulation can be broadly defined as “a distortion, resulting from the nonlinearity of a system, which causes a carrier in the system to be modulated by the various desired signals carried on the other channels in the same system.” This general definition will cover both components of Cross Modulation distortion, AM-XMOD and PM-XMOD.
3.3 The “Amplitude Modulation Component” of the cross modulation, or AMXMOD. was traditionally defined in the same manner as the definition of cross modulation given above in Section 3.2. However, in this document, an additional clarification is made. “AM-XMOD is specifically defined as the ratio of the peak to peak amplitude modulation on the test carrier, caused by the modulating signals on the other carrier signals in the system, to the peak to peak amplitude modulation on a 100% modulated test carrier. It is a measure of’ the relative magnitude of the voltage of the undesired modulation, to that of the desired modulation, at the fundamental frequency of the modulating signal.”
3.4 The conditions br the test define the modulation type, as follows:
All test carriers are to be 100%, downward only, amplitude modulated by a synchronous square-wave signal (i.e. the same baseband signal synchronously modulates all carriers in the system), possessing a modulating frequency equal to the desired video format’s horizontal sync pulse, with a duty cycle of 50%. For the NTSC video signal fbi-mat, this frequency is approximately 15.75 kHz. (Note: this will be the frequency of modulation used throughout this procedure.) However, for testing of fiber optic equipment, approximately 5% of the carrier signals may remain unmodulated in order to provide a minimum signal level within the active fiber optic components. The number, channel and frequency location of these unmodulated signals shall be included as part of the test data.ANSI SCTE 58 pdf download.