The ML422B is designed for use with the Bell System FDM hierarchy, while the ML422C is for use with CCITT systems. Both of these instruments cover a wide freguency range from 50 Hz to 30 MHz, providing highly accurate measurement of signal levels, with the stability of freguency needed to manufacture, install, and maintain FDM systems. The ML422B/C can also function as a wideband level meter, psophometer, or voice band analyzer. The ML422B/C has the ability to make both voice channel measurements and carrier frequency measurements. Furthermore, it can measure transmission impairment and search out unknown signals (hot tones). The transmission impairment measuring function allows the user to quickly troubleshoot voice channel problems with weighted noise, noise with tone, phase jitter, and single-level impulse noise measurements. The capacity to make all of the above-mentioned transmission impairment measurements, in combination with both FDM voice channel and carrier frequency measurements, is available with the ML422B/C. The ability to search out unknown signals allows the user to easily identify hot tones which exceed the threshold level. ±0.1 dB level measuring accuracy is achieved by means of an automatic level calibration function, enabling communication systems which are evaluated by means of end-to-end measurement to be maintained at the highest level of operational standards. The high accuracy and resolution is made possible by a synthesized local oscillator, featuring 1 Hz resolution, + - 5 x 10⁻⁷ stability, and an aging rate of 1 x 10⁻⁶ /year, thus providing the ML422B/C with high-resolution tuning characteristics. The automatic requency control (AFC) function makes tuning precise and easy, and can also be used to measure input frequencies. This capacity for the accurate measurement of signal levels, combined with the functions of the steep channel filter (3.1 kHz) and the group filter (48 kHz) with root-mean-square (RMS) detector, permits channel noise and group power measurement without the need for additional devices A General Purpose Interface Bus (GPIB: compatible with IEEE 488 - 1978) control is standard. Automatic operation is possible using an external controller such as the ANRITSU Packet II Hy-personal Computer (Model DDC7706C).The ML422B is designed for use with the Bell System FDM hierarchy, while the ML422C is for use with CCITT systems. Both of these instruments cover a wide freguency range from 50 Hz to 30 MHz, providing highly accurate measurement of signal levels, with the stability of freguency needed to manufacture, install, and maintain FDM systems. The ML422B/C can also function as a wideband level meter, psophometer, or voice band analyzer.