The CE-21 Frequency Selective Level meter is used to measure power levels of signals at up to 1800 voice-channel frequencies in telephone communications systems between 4 kHz and 9.1 MHz. (See figure 1-1. ) To tune the instrument, the frequency of the desired channel is dialled in on a lighted display with five frequency selectors, which have a resolution of 100 Hz. Depending on whether the instrument is to be connected to a balanced or unbalanced line, the appropriate condition is selected with the BAL/UNBAL switch. The input to the CE-21 can be obtained by bridging the line or by providing termination appropriate to the line impedance. When the TERM/BRDG switch is placed in TERM, the input impedance can be selected with the push-button INPUT selector (75, 124, 135, or 150fi). When the TERM/ BRDG switch is in BRDG, the input impedance is 20kΩ for balanced lines and 10kΩ for unbalanced lines. Calibration of the CE-21 is semiautomatic and is performed by pushing a single button which automatically switches the input circuit to 75Ω, provides an accurate-level (0 dBm) internally-generated calibration signal, presets the frequency of the 1st LO, and adjusts the attenuators for a 1 MHz, 0 dBm input. The meter is then manually set to zero which calibrates the instrument for all available input impedances at all signal frequencies within CE-21 range. After calibration and with an input signal connected to the instrument, two front-panel level selectors are used to attain a zero reading on the meter. There are two displays so that one can be used to set the level of a signal with a certain frequency (for instance to the transmission level point of a pilot tone), while the other is used to establish the level of a second signal. The second level display will thus give a value that is relative to the level indicated on the first display. Either level display can be used to set the reference, andbothhave two knobs which permit level selection in 10-dB and 1-dB steps. The left display is labeled "dBl" and can be adjusted to read from -79 dB to +19 dB; the one on the right is labeled "dB2" and can be adjusted from -99 dB to -0 dB. As is indicated on the front panel between the two level displays, the measured level in dBm at meter zero (0) is the algebraic sum of the dBl and dB2 readings. When this sum exceeds -99 dBm, a -99 dBm EXCEEDED light comes on to warn the operator that the range of the instrument is being exceeded. Since it is inherently impossible to provide both lowest distortion and lowest noise level at the same time in any instrument, provision has been made for selection of either low-distortion or low-noise operation of the CE-21. Meter readings can be taken from -20 dBm to +2 dBm, or, on an expanded scale, from -2 dBm to +2 dBm. When the expanded scale has been selected, the light adjacent to the meter switch is lit. When the meter -on either scale- indicates a reading other than zero, this reading must be added to the algebraic sum of dBl and dB2, but the quantity indicated on the meter is not included in the -99 dBm that should not be exceeded. Thus, level measurements are possible from - 119dBm (meter reading = -20 dBm, dBl + dB2 = -99 dBm) to +21 dBm (meter reading = +2 dBm, dBl + dB2 = +19 dBm). The BANDWIDTH (NOISE) selector provides a choice of narrow (200 Hz) bandwidth for separation and level readings of suppressed carriers and pilot tones, or wide (2.3 kHz) bandwidth for noise readings. When the broad bandwidth is selected, audio signals can be monitored over the built-in speaker. The audio selector lets the operator select AM, lower sideband (LSB), or upper sideband (USB). A volume control is also provided. On the front panel, under BANDWIDTH, the word NOISE appears in blue, and, under "2.3 kHz", "1.74 kHz" is shown, also in blue. The two numbers relate the bandpass of the CE-21 to the concept of Equivalent Noise Bandwidth and the C-message-weighted curve. As is common knowledge in the telephone industry, the C-message-weighted (Cmw) curve represents the response curve of all telephone handsets used in the United States. The concept "Equivalent Noise Bandwidth" refers to a perfectly rectangular curve which bounds an area equal to the area bounded by a Cmw curve. It has been shown that when the area bounded by a Cmw curve is found by graphic integration, the result is approximately equal to an area bounded by a curve with a uniform width of 1. 74 kHz. It is now almost universally accepted that any bandpass represented by a curve which bounds an area equal to that bound by a 1. 74 kHz rectangular curve permits noise measurements that are, for all practical purposes, the same as would be obtained with a bandpass curve identical to the Cmw curve. Thus, when, on the front panel of the CE-21, the broad bandwidth is specified as "2.3kHz, 1.74 kHz", the CE-21 bandpass curve is 2.3 kHz wide at the -3 dB points, and the area bounded by this curve approximately equals the area bounded by a 1. 74-kHz-wide rectangular curve. A commonly used measurement unit for noise is dBrnC (dB referred to noise, C-message weighted), where 0 dBrnC = -90 dBm. When measuring noise levels with the CE-21, a reading in dBrnC measurement units can be obtained by-adding 90 to the dBm reading. By using the CE-22 Tracking Signal Generator in conjunction with the CE-21, frequency tests can be made on transmission lines or circuits of telecommunications systems. The CE-22 provides a stable signal source the frequency of which is controlled by the CE-21. Thus, the CE-22 tracks exactly; it contains an automatic level control, and it provides the capability of presetting its signal level from -70 dBm to +10 dBm, with a resolution of . 01 dB. Another companion unit for the CE-21 is the CE-23 Spectrum Display, which provides simultaneous high-resolution displays of signals that are being measured with the CE-21. In addition, there is also available the CE-26 Signal Generator, which has all the features of the CE-22 Tracking Signal Generator, with the additional capability of generating its own signal. The Frequency Synthesizer used in the CE-26 is virtually identical to the one used in the CE-21. In contrast to the CE-22 and the CE-23, which must be used with the CE-21, the CE-26 can be used independently.