Electronic apparatus and computer software for pinpointing locations of network impairments such as ingress sources and common path distortion in cable television networks; electronic apparatus and computer software for monitoring cable television networks for the presence and development of network impairments such as ingress sources and common path distortion; electronic apparatus and computer software for creating and updating databases of network devices and locations and relative distances of network devices in a cable television network, and for creating, displaying and updating electronic maps of cable television networks; hand-held field meters for pinpointing locations of network impairments in cable television networks and for calibrating cable lengths and distances to and between network devices and other points in a cable television network, and associated amplifiers and test probes to support the use of the hand-held field meters; electronic probes to be installed at points in a cable television network for encoding ingress signals in the network with relatively unique identification codes, such that the source or sources of the ingress signals can be found; electronic probes to be installed at points in a cable television network that function as remote spectrum analyzers and QAM demodulators, which monitor signals in the network and automatically issue alerts when signal performance deviates from specified values; electronic return path switches for cable television networks; electronic apparatus and computer software for detecting digital and other signals leaking from a cable television network and for locating such leaks; and hand-held field meters for detecting digital and other signals leaking from a cable television network and for locating such leaks