By I. Switzer, P. Eng., Chief Engineer, Maclean-Hunter Cable TV Limited
Spectrum analyzers are bound to find many applications in cable television systems because of their close family relationship to the familiar signal level meter. Signal level meters are heterodyne receivers tunable manually and are usually single conversion receivers, tuned manually, fixed IF bandwidth with a moving coil meter indication of input signal level. Spectrum analyzers are also heterodyne type receivers but have electrically swept local oscillator(s) and usually display the input signals on an oscilloscope type display signal amplitude versus frequency. The more sophisticated versions have logarithmic display over a range of 70 db or more, selectable bandwidths, and very stable sweep and tuning characteristics.
Our company has been using spectrum analyzers for more than four years, principally for observation of signal levels in various parts of the system and for finding and eliminating many sources of spurious signals in cable television systems. Our early spectrum analyzers had 5KHz bandwidth, tuning range of 1 to 300 MHz in a single display and a 50 db dynamic signal display range. About six months ago we acquired a more sophisticated spectrum analyzer system consisting of a Hewlett-Packard 141T display frame with 8553B RF section, 8552B IF section, 8443A tracking generator/counter, and an 8554L RF section. The 8553B tunes 1 KHz to 110 MHz ( in a single sweep if desired) and has a range of IF bandwidths from 300 KHz down to 10Hz. The associated IF section provides 70 db display (10 db/division) or 16 db of display in a 2 db/division mode. Linear display is also available. The 8554L RF section tunes 1 to 200 MHz but is limited to 300 Hz bandwidth by the less stable oscillators in this RF section. The 8443A which operates with the 8553B RF section adds a tracking oscillator which acts as a sweep generator which frequency tracks with the associated receiver. A built in frequency counter operates with a "marker" on the display to permit 8 digit reading of any desired point on the display. The counter provides 10 Hz resolving power and has an internal clock specified to 3 parts in 108 accuracy. Detailed specifications are available from the manufacturer.