High-Frequency Measurements of Communication Lines

Abstract

Synopsis.-This paper describes briefly a testing technique and standardized high-frequency apparatus units. These include which has been developed for making line measurements at high an oscillator, a detector-amplifier, an impedance bridge, a thermofrequencies in connection with the application of carrier telephone milliammeter set, a variable attenuator, a cross-talk set, and a and telegraph systems in the Bell System. The measurement of frequency meter. A brief description of the individual units is line attenuation, impedance, and cross-talk up to frequencies as high given, as well as their coordinated use in complete testing circuits. as 30,000 cycles is described, using a number of specially developed * * * * * * P RIOR to the introduction of multiplex carrier tele-ular measurements. Simple illustrations of the results phone and telegraph systems, the operating attained in the work have been included. telephone plant was concerned only with the APPARATUS UNITS EMPLOYED transmission of frequencies in the voice range. TheThe several individual types of apparatus which considerable application of carrier communication sys-have been made available are as follows: tems to long distance open-wire circuits, whereby a 1. An oscillator providing a source of high-frequency pair of such open wires carries several messages simul-current adjustable to any frequency between 100 and taneously by the employment of higher frequencies, has 50,000 cycles and having an output of a high degree of however greatly raised the upper limit of the frequency constancy and purity of wave form. range used in the telephone plant. Among other 2. A detector-amplifier which provides a visual or, things, this has required the development of a special by heterodyne action, an audible indicator of hightechnique suitable for the testing at these frequencies frequency currents. of the line circuits over which the various types of 3. An impedance bridge suitable for measurements carrier systems are operated.within the above mentioned frequency range.A brief description of this technique was included in 4. A thermomilliammeter set which provides for the a paper delivered before the Institute by Messrs. absolute measurement of relatively weak high-frequency Colpitts and Blackwell.' It is the purpose of the currents and which also incorporates means for calipresent paper to describe these high-frequency measur-brating the thermocouples. ing methods in greater detail and as they are now 5. A variable attenuator or resistance artificial line. applied with recently improved apparatus. A cross-talk set or variable attenuator of specialThe high-frequency line characteristics of chief type used for measuring cross-talk between mutually interest are attenuation, impedance, and cross-talk for interfering circuits. frequencies up to about 50,000 cycles. It is thought 7. A frequency meter capable of measuring frethat the interest of others in the measuring methods quencies between 3000 and 50,000 cycles. employed may, perhaps, reside not so much in their Thes...