By Richards. Prodan, Majid Chelehmal, Thomas H. William, Craig M. Chamberlain, Cable Television Laboratories, Inc.
CableLabs is performing a field study oftransient disturbances occurring in cable systemsto evaluate the impact of nonstationaryimpairments on high speed, band-limited transmissionof digital information. This paper describesthe design and operation of a systemwhich has been built to determine the nature ofnon-stationary impairments that interfere withdigital signal transmission.
The system inserts a CW (carrier wave)signal at the headend in the center of a vacantchannel. At the receive site, the carrier isquadrature demodulated to baseband. Dualtriggers are set for the in-phase and quadraturechannels. When triggered by a disturbance, adigital oscilloscope captures data from bothchannels for later time and frequency domainanalysis.
The type of transient disturbance, duration,and inter-arrival statistics are derived from theaccumulation of events over a significant timeperiod (weeks to months). This system can alsobe used for reverse system characterization.
Samples of test data captured and analyzed arepresented.
At this time, the system has been built andsome preliminary testing on a cable system inBoulder, Colorado has been done.