Otto Julius Zobel
Otto Julius Zobel was an electrical engineer who worked for the American Telephone & Telegraph Company in the early part of the 20th century. His work on filter design was revolutionary and led, in conjunction with the work of John R. Carson, to significant commercial advances for AT&T. He invented the m-derived filterand the constant-resistance filter, which remains in use.
About Otto Julius Zobel in brief
Otto Julius Zobel was an electrical engineer who worked for the American Telephone & Telegraph Company in the early part of the 20th century. His work on filter design was revolutionary and led, in conjunction with the work of John R. Carson, to significant commercial advances for AT&T in the field of frequency division multiplex telephone transmissions. Although much of his work has been superseded by more modern filter designs, it remains the basis of filter theory and his papers are still referenced today. He invented the m-derived filterand the constant-resistance filter, which remains in use. He died in Morristown, New Jersey, of a heart attack in January 1970. He was born on October 20, 1887 in Ripon, Wisconsin. He first studied at Ripon College, where he received his BA in 1909 with a thesis on Theoretical and experimental treatment of electrical condensers. He then went to the University of Wisconsin and graduated with an MA in physics in 1910. In 1926, still with the company, he moved to New York and in 1934, he transferred to Bell Telephone Laboratories. He retired from Bell Telephone in 1952. The last of his prolific list of patents occurred for Bell Labs in the 1950s, by which time he was residing in Morrristown,. New Jersey. His early work on the transmission line derived the properties of the electric line by analogy with heat conduction. This is based on Fourier’s law and the Fourier conduction equation. It is therefore no surprise that in his paper on the electric wave filter a very similar representation is found for the transmission function of filters.
In many cases the calculation involved makes the solution well-nigh impossible by analytical means. With modern technology such a calculation is trivially easy, but Ingersoll and Zobels recommend the use of harmonic analysers, which are the mechanical counterpart of today’s spectrum analysers. The reverse process is also possible, driving the machine with the function and measuring the Fouriers components as output. The two sidebands in AM represent the majority of the power in the frequency domain but contains no information so only one is required, at least from an information point of view. Up to this time, SSB required a flat response over the sideband of interest. This led to a big effort in research in electric wave filters and it was important that all traces of it were removed to prevent the same crosstalk being retained for the same time. As the idea of SSB was put to use, George A. A. Stromberg came up with a very sharp transition between the two sideband with a maximum rejection of the other sideband. This was to put another signal in the slot vacated by the slot by the side band vacated by another signal. The idea was to make it possible to transmit signals in the same frequency range as the carrier wave in AM but with no distortion at all. In 1915 it became clear that multiplexed telephone transmissions could be greatly improved by theuse of single sideband suppressed carrier transmission.
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