Otto Julius Zobel (October 20, 1887 – January 1970) was an electrical engineer who worked for the American Telephone & Telegraph Company (AT&T) in the early part of the 20th century. Zobel's 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 (FDM) telephone transmissions.Bray, p. 62.
Although much of Zobel's work has been superseded by more modern filter designs, it remains the basis of filter theory and his papers are still referenced today. Zobel invented the m-derived filter and the constant-resistance filter,Zobel, O J, Distortion Compensator, , filed June 26, 1924, issued February 12, 1929. which remain in use.
Zobel and Carson helped to establish the nature of noise in electric circuits, concluding that—contrary to mainstream beliefSchwartz, p. 9.—it is not even theoretically possible to filter out noise entirely and that noise will always be a limiting factor in what is possible to transmit. Thus, they anticipated the later work of Claude Shannon, who showed how the theoretical information rate of a channel is related to the noise of the channel.
Life Otto Julius Zobel was born on October 20, 1887, in Ripon, Wisconsin."Dr. Otto Zobel", Oshkosh Daily Northwestern, January 12, 1970, p. 21 (via Newspapers.com, retrieved November 23, 2016. Poggendorff, J C, Poggendorffs biographisch-literarisches Handwörterbuch für Mathematik, Astronomie, Physik mit Geophysik, Chemie, Kristallographie und verwandte Wissensgebiete, p. 2969, Verlag Chemie, g.m.b.h. 1940 He was the son of Oscar Ewald "Herman" Zobel, who had emigrated to the United States from his native Germany in 1860, and his wife Ernestine, née Kahl."Dr Otto J. Zobel", Fond du Lac (Wisconsin) Commonwealth Reporter, January 12, 1970, p. 26."Zobel Baby Quilt: Quilt Index Record: 15-11-1340", Quilt Index (Michigan State University). Retrieved January 31, 2022. Zobel had seven siblings. After attending Ripon High School, he first studied at Ripon College, where he received his BA in 1909 with a thesis"Student theses ", Ripon College. on Theoretical and experimental treatment of electrical condensers. He later received a Distinguished Alumnus Award from Ripon."Distinguished Alumni Award Recipients ", Ripon College. He then went to the University of Wisconsin and graduated with an MA in physics in 1910. Zobel stayed at the University of Wisconsin as a physics instructor from 1910 to 1915, and graduated with his PhD in 1914; his dissertation concerned "Thermal Conduction and Radiation". This followed his 1913 co-authoring of a book on the subject of geophysical thermodynamics.Leonard et al., passim From 1915 to 1916 he taught physics at the University of Minnesota.American Telephone and Telegraph Company, The Bell System Technical Journal, p. 686, 1922 Having moved to Maplewood, New Jersey, he joined AT&T in 1916, where he worked on transmission techniques. In 1926, still with the company, he moved to New York and in 1934, he transferred to Bell Telephone Laboratories (Bell Labs), the research organisation created jointly by AT&T and Western Electric a few years earlier.Seising, R, The Fuzzification of Systems, 2007, Springer Berlin / Heidelberg He retired from Bell Telephone in 1952.
The last of Zobel's prolific list of patentsZobel, O J, Impedance Transformer, , filed September 30, 1952, issued October 16, 1956.Zobel, O J, Microwave Filter, , filed April 20, 1950, issued December 23, 1952. occurred for Bell Labs in the 1950s, by which time he was residing in Morristown, New Jersey.Record of addresses given in patents He was a fellow of the American Physical Society and of the Acoustical Society of America. He died in Morristown of a heart attack in January 1970.Social Security Death Index database query through WorldVitalRecords.com He had married Irene Staab on May 28, 1949; she was still living when he died but a son predeceased him.
Thermal conduction
Zobel's early work on heat conduction was not pursued in his later career. There are, however, some interesting connections. Lord Kelvin in his early work on the transmission line derived the properties of the electric line by analogy with heat conduction.Hunt, B J, The Maxwellians, p. 63, Cornell University Press, 2005 . This is based on Fourier's law and the Fourier conduction equation. Ingersoll and Zobel describe the work of Kelvin and Fourier in their bookLeonard et al., pp. 9–14. and Kelvin's approach to the representation of transmission functions would consequently have been very familiar to Zobel. It is therefore no surprise that in Zobel's paper on the electric wave filterZobel, pp. 3–4. a very similar representation is found for the transmission function of filters.
Solutions to the Fourier equation can be provided by Fourier series.Leonard et al., pp. 25–26. Ingersoll and Zobel state that 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 Zobel recommend the use of harmonic analysers, which are the mechanical counterpart of today's spectrum analysers. These machines add together mechanical oscillations of various frequencies, phases and amplitudes by combining them through a set of pulleys or springs: one for each oscillator. The reverse process is also possible, driving the machine with the function and measuring the Fourier components as output.Ingersoll and Zobel, pp. 62–64.
Background to AT&T research After the work of John R. Carson in 1915Carson, J R, Electric Circuit Theory and Operational Calculus, 1926, McGraw-Hill, New York. it became clear that multiplexed telephone transmissions could be greatly improved by the use of single sideband suppressed carrier (SSB) transmission. Compared to basic amplitude modulation (AM) SSB has the advantage of half the bandwidth and a fraction of the power (one sideband can have no more than 1/6 of the total power and would typically be a lot less). AM analysed in the frequency domain consists of a carrier and two sidebands. The carrier wave in AM represents the majority of the transmitted power but contains no information whatsoever. The two sidebands both contain identical information so only one is required, at least from an information transmission point of view. Up to this point filtering had been by simple tuned circuits. However, SSB required a flat response over the sideband of interest and maximum rejection of the other sideband with a very sharp transition between the two. As the idea was to put another (completely different) signal in the slot vacated by the unwanted sideband it was important that all traces of it were removed to prevent crosstalk. At the same time minimum distortion (i.e. flat response) is obviously desirable for the sideband being retained. This requirement led to a big research effort in the design of electric wave filters.Bray, pp. 61, 63. Electric wave filters The term electric wave filter was much used around Zobel's time to mean a filter designed to pass or reject waves of specified frequencies across the band. It appears in numerous papers published in the early 20th century. Sometimes used to distinguish these more advanced designs from the simple tuned circuits which preceded them. In modern usage the simpler term filter would be used. This is usually unambiguous within the field of electronics, but frequency filter might be used where it is not. George A. Campbell and Zobel worked on this problem of extracting a single sideband from an amplitude-modulated composite wave for use in multiplexing telephone channels and the related problem of extracting (de-multiplexing) the signal at the far end of the transmission.
In 1921, E. H. Colpitts and O. B. Blackwell described early carrier telephony systems (Type A), which employed electrical band-pass filters to realize single-sideband transmission; such systems had been placed in service by 1918.
Initially, the baseband pass range used was 200 Hz to 2500 Hz but later the International Telecommunication Union set a standard of 300 Hz to 3.4 kHz with 4 kHz spacing. Thus the filtering was required to go from fully pass to fully stop in the space of 900 Hz. This standard in telephony is still in use today and had remained widespread until it began to be supplanted by digital techniques from the 1980s onwards.Bray, pp. 62, 64.
Campbell had previously utilised the condition discovered in the work of Oliver Heaviside for lossless transmission to improve the frequency response of transmission lines using lumped component inductors (loading coils). When Campbell started investigating electric wave filter design from 1910, this previous work naturally led him to filters using ladder network topology using capacitors and inductors. Low-pass, high-pass and band-pass filters were designed. Sharper cut-offs and higher stopband rejection to any arbitrary design specification could be achieved merely by increasing the length of the ladder. The filter designs used by CampbellCampbell, G A, "Physical Theory of the Electric Wave-Filter", Bell System Tech J, November 1922, vol 1, no 2, pp. 1–32. were described by Zobel as constant k filters although this was not a term used by Campbell himself.Bray, p. 53.
Innovations After Zobel arrived at the Engineering Department of AT&T he used his mathematical skills to further improve the design of electric wave filters. Carson and Zobel developed the mathematical method of analyzing the behavior of filters now known as the image method whereby the impedance and transmission parameters of each section are calculated as if it is part of an infinite chain of identical sections.Chu, W, Chung-Kwei Chang, Transients of Resistance-Terminated Dissipative Low-Pass and High-Pass Electric Wave Filters, Proceedings of the IRE, vol 26, no 10, pp. 1266–1277, October 1938
Wave filters
Zobel invented the m-derived (or m-type) filter section in 1920, the distinguishing feature of this design being a pole of attenuation close to the filter cut-off frequency. The result of this design is a filter response which falls very rapidly past the cut-off frequency. A fast transition between passband and stopband was one of the primary requirements for cramming as many telephone channels as possible into one cable.Matthaei et al., p. 65.
One disadvantage of the m-type section was that at frequencies past the pole of attenuation, the response of the filter started to increase again, reaching a peak somewhere in the stopband and then falling again.Ghosh, Smarajit, Network Theory: Analysis and Synthesis, Prentice Hall of India, pp. 564–569. Zobel overcame this problem by designing hybrid filters using a mixture of constant k and m-type sections. This gave Zobel the advantages of both: the fast transition of the m-type and good stopband rejection of the constant k.Zobel, pp. 26–28.