DESCRIPTION 1-15 The Hewlett-Packard 3746A Selective Level Measuring Set (SLMS) is a high quality tunable power meter dedicated for use by operators and manufacturers of high density Frequency Division Multiplex (FDM) systems. 1-16 The SLMS has been specifically designed for manufacturers and operators who use either CCITT recommendations or the North American Standards. 1-17 The SLMS is basically a tunable receiver, employing a synthesized local oscillator, controlled by a processor. A keyboard provides the interface between the operator and the processor. -18 The SLMS measures signal powers in the range 50Hz to 32MHz at levels between +20dBm and -115dBm, depending upon the measurement bandwidth used, 1-19 Either balanced or unbalanced measurements may be made. 1-20 Measured levels are true RMS values. 1-21 The instrument displays power levels either in dBm or in dB relative to a dBm reference level. Automatic correction ensures that whichever input is selected the display will be correct for the chosen terminating impedance. 1-22 The frequencies at which measurements are required may be entered via the keyboard either directly in kHz, or as a description in a chosen FDM plan. 1-23 This ability to enter FDM descriptions directly into the instrument eliminates the need for FDM charts and line frequency tables to determine the frequencies to which the instrument is to be tuned. 1-24 Tuning to the measurement frequency is accomplished by mixing the received signal with a variable frequency signal from an internal synthesized local oscillator to give a fixed intermediate frequency. To achieve the high degree of selectivity and image rejection required, the SLMS uses multiple intermediate frequency (IF) stages. 1-25 The standard instrument is equipped with selective filters to allow a choice of measurement bandwidth. 1-26 The measurement of narrow bandwidth tones in a wideband measuring set requires extreme accuracy in the local oscillator frequencies used. This requirement is met by the use of a frequency synthesizer as the local oscillator in the SLMS. A master oscillator with a 10MHz crystal source, high spectral purity, and an ageing rate of less than 1.5x10s parts per month is available as an option. 1-27 A processor provides overall control within the instrument, implementing routines which govern the measurement functions of the instrument and calculating from the various autoranging and analog to digital settings the levels to be displayed. The processor also determines the frequency and bandwidth setting required when FDM descriptions are entered. 1-28 The SLMS has a built-in control and drive circuitry which enables the SLMS to control up to 111 Access Switches. 1-29 The processor will accept instructions either from the operator via the keyboard or, if remote control of the instrument is required, through the Hewlett-Packard Interface Bus (HP-IB) connector on the rear panel. A degree of in-built intelligence in the processor enables it to detect if the instrument is required to perform an invalid measurement or if incorrect data is entered. In these circumstances a code number will appear in the TEST-POINT display window. 1-30 The processor performs a calibration of the measuring circuits at periodic intervals, substituting for the incoming signal an accurately defined signal derived from an internal high stability source. The processor stores the result of this measurement and uses any deviation from the expected measurement to modify the results of measurements made on external signals. Thus any inaccuracies resulting from thermal drift or ageing in the measurement stages are greatly reduced.