By Tom Williams, Holtzman Inc
Much has been written on the operational challenges for cable systems caused by upstream noise, especially burst noise, radio frequency ingress, and common path distortion. There is also an upstream problem with linear distortions caused by discrete echoes, amplifier tilt, group delay and micro-reflections. Diplex and lightning filters in amplifiers, missing terminators, damaged components, design problems, and construction mistakes create these linear distortions. The effect of uncorrected linear distortions is to increase the bit error rate in the presence of random noise or other additive impairments. Uncorrected linear distortion also limits an operatorís ability to go to higher-order modulation schemes. In the return band a micro-reflection problem is aggravated by the low loss of the coaxial cable. This paper presents a burst reference signal measurement technique for measuring linear distortions on working plant as well as test data from active usptream cable plant. Simulation plots and curves show increases in intersymbol interference from linear distortions.