By Tom Williams and Belal Hamzeh, CableLabs and Ron Hranac, Cisco Systems
As cable networks transition from the carriage of analog TV signals to digital signals, the second- and third-order nonlinear distortion products, known as composite second order (CSO) and composite triple beat (CTB), are still present but now appear on conventional spectrum analyzers to be similar to random noise. Thus, to technicians the distortion energy is indistinguishable from random noise. A new digital signal processing (DSP) technique has been developed to process the spectral “noise” in a vacant band to determine if the energy is random, or correlated to a full-band nonlinear distortion signal that could have created it. This paper discusses the processing details of the new test method and presents lab, cascade, and field test results. These data demonstrate the operational benefits of making nonlinear distortion measurements in mostly- or alldigital cable plants.