An Automatic Selective Level Measuring

AKING RELIABLE MEASUREMENTS ON fTE-quency-division multiplexed (FDM) baseband signals presents a two-part problem. First, the frequency at which the measurement is to be made must be determined. Then second, the measurement must be made with high accuracy in the presence of numerous interfering signals. At first sight the choice of a measurement frequency may seem to be a trivial problem, but finding the center frequency of a particular channel in a communications system that may have as many as 3600 channels is no easy task and could, in fact, occupy more time than making the actual measurement itself. A maintenance engineer may wish, for example, to measure channel 3 in group 2 of supergroup 13 in mastergroup 5 without having any idea of where in the frequency spectrum that channel may lie. The only way to find out has been through the use of large charts that describe the FDM plan, such as the one shown in part in Fig. 1. Usually these only show each channel's carrier frequency, so the engineer must derive the center frequency ofthe sideband occupying the channel in order to proceed with the measurement. Besides the time taken to hunt for a particular channel's frequency, such an arrangement leads to the possibility of frequent errors. The problem of translating the FDM description of a channel into a tuning frequency has now been eliminated by a new selective level measuring set (SLMS), the Hewlett-Packard Model 3745AlB (Fig. z). This instrument has all the details of FDM plans stored in read-only memories. The stored plans are accessed by special keys, so the user merely enters the specifics of a certain channel (channel number, group number, etc.J by way of the keys, and the instrument then automatically tunes itself so the 3.1-kHz filter covers the active region of that channel exactly. It then makes a true-rms measurement of the power in that channel Printed n U.S A. and displays the results, including the frequency to which it is tuned. on numeric indicators. Furthermore , the instrument is completely autoranging over a 140-dB range, so it displays results directly without the need for setting attenuators manually. Cover: Picking one particular channel out of a babble of thousands of frequency-multiplexed communications channels is greatlY simplified by a new selective level measuring set that talks the language of the telephone engineer. Microprocessor control is the key to this new ability, as …