User:Spinningspark/Otto Julius Zobel

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Zobel's signature in his draughtsmanlike hand as it appears on a patent application
Zobel's signature in his draughtsmanlike hand as it appears on a patent application

Otto Julius Zobel (1887 - 1970) was a design engineer working for the American Telephone & Telegraph Company (AT&T) in the early part of the 20th century. These were the early days of radio broadcasting and many concepts which are second nature to engineers today were then unknown or at least obscure. Zobel's work on filter design was ground-breaking at the time and led, along with the work of J R Carson, to a very significant commercial advantage for AT&T in the field of frequency division multiplex (FDM) telephone transmissions.[1]

Although much of Zobels work has now been superceded by more modern filter designs, his work remains the basis of filter theory and his papers are still much referenced today. Zobel invented the m-derived filter[2] and the constant resistance filter,[3] which is still in use today.

Zobel and Carson also did much to clarify the nature of noise in electric circuits, establishing that it was not (contrary to much mainstream opinion of the time)[4] even theoretically possible to filter out noise entirely and that noise would always be a limiting factor on what it was possible to transmit. Thus, they anticipated the later work of Claude Shannon who showed how the theoretical information rate of a channel was related to the noise of the channel.

Contents

[edit] Early life, education and career

Otto Julius Zobel was born in 1887. He studied at the University of Wisconsin, graduating from there with a PhD in 1915.[2] He joined AT&T in 1916 where he worked on transmission techniques, moving to Maplewood, New Jersey and later (1926), New York. Later (from 1934) he was with Bell Telephone Laboratories (Bell Labs).[5]

The last of his prolific list of patents[6][7] occured for Bell Labs in the 1950s by which time he was now residing back in New Jersey at Morristown, where he died in January 1970.

[edit] Background to AT&T research

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 and is unambiguous within the field of electronics.

After the work of J R Carson in 1915[8] it became clear that multiplexed telephone transmissions could be greatly improved by the use of single sideband supressed 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 frequency in AM represents the majority of the transmitted power but contains no information whatsover. 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 (ie 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.

G A Campbell and Zobel were working 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.

Initially, the baseband pass range used was 200Hz to 2500Hz but later the International Telecommunication Union set a standard of 300Hz to 3.4kHz with 4kHz spacing. Thus the filtering was required to go from fully pass to fully stop in the space of 900Hz. 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.

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 stop-band rejection to any arbitrary design specification could be achieved merely by increasing the length of the ladder. The filter designs used by Campbell[9] were described by Zobel as constant k filters although this was not a term used by Campbell himself. [2][10]

[edit] Zobel's innovations

After Zobel arrived at the Engineering Department of ATT Co he used his mathematical skills to further improve the design of electric wave filters. Carson and Zobel developed the mathematical method of analysing the behaviour of filters now known as the image method whereaby the impedance and transmission parameters of each section are calcualted as if it is part of an infinite chain of identical sections.[11]

[edit] Wave filters

An original drawing by Zobel of an m-derived series T section band-pass filter
An original drawing by Zobel of an m-derived series T section band-pass filter

Zobel invented the m-derived (or m-type) filter section in 1920[2], 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. To use a well known quaint engineer's expression it "goes off like the side of a house". A fast transition between pass-band and stop-band was, of course, one of the primary requirements for cramming as many telephone channels as possible into one cable.

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 stop-band and then falling again. 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 stop-band rejection of the constant k.

By 1921 Zobel had further perfected his composite filter designs. He was now using, in addition, m-type half sections at the ends of his composite filters to improve the impedance matching of the filter to the source and the load,[2] a technique in which he held a patent.[12] The difficulty that he was trying to overcome was that the image impedance techniques being used to design filter sections only gave the mathematically predicted response if they were terminated in their respective image impedances. Technically, this was easy to do within the filter as it could always be arranged that adjacent filter sections had matching image impedances (one of the characteristics of m-type sections is that one side or the other of the m-type section will have an image impedance identical to the equivalent constant k section). However, the terminating impedances are a different story. These are normally required to be resistive but the image impedance will be complex. Even worse, it is not even mathematically possible to construct a filter image impedance out of discrete components. The result of impedance mismatch is reflections and a degraded filter performance. Zobel found that a value of m=0.6[13][14] for the end half sections, while not mathematically exact, gave a good match to resistive terminations in the pass-band.[15][10]

Around 1923, Zobel's filter designs were reaching the peak of their complexity. He now had a filter section to which he had doubly applied the m-derivation process resulting in filter sections which he called the mm'-type.[16] This had all the advantages of the previous m-type, but more so. An even faster transition into the stop-band and an even more constant characteristic impedance in the pass-band. At the same time one side would match into the old m-type, just as the m-type could match in to the k-type. Because there were now two arbitrary parameters (m and m') that the filter designer could adjust, much better end matching half-sections could be designed. A composite filter using these sections would have been the very best that could have been achieved at that time. However, the mm'-type sections never became as widespread and well known as the m-type sections, possibly because their greater complexity has deterred designers. They would have been inconvenient to implement with microwave technology and the increased count of components, especially wound components, made them more expensive to implement with conventional LC technology. Certainly, it is hard to find a textbook from any period which covers their design.

[edit] Transmission line simulation

A great deal of Zobel's efforts in the 1920s was directed at constructing networks which could simulate transmission lines.[17][18][19] The result of this work was clearly feeding in to his filter designs, which in turn was producing better line simulators. The whole process was self amplifying and had much to do with the rapid progress being made.

One interesting technique used by Zobel was based on his discovery that the impedance looking in to the end of a filter chain was practically the same (as near as made no difference given component tolerances) as the theoretical impedance of an infinite chain after only a small number of sections had been added to the chain. These "image" impedances had a mathematical characterisation impossible to construct simply out of discrete components. Zobel found that using these impedances constructed out of small filter chains as components in a kind of super filter allowed him to build realistic line simulators.[20] Utterly impractical as a filter in the field, but Zobel found he could construct good controllable line simulators from this design without having the inconvenience of miles of cable to contend with.

[edit] Equalisers

Zobel invented several filters whose defining characteristic was a constant resistance as the input impedance. The resistance remained constant through the pass band and the stop band. With these designs Zobel had completely solved the impedance matching problem.[21]

The main application of these sections has been not so much for filtering out unwanted frequencies, the k-type and m-type filters remained best for this, but rather to equalise the response in the pass band to a flat response.

Perhaps one of Zobel's most fascinating inventions is the lattice filter section. This section is both constant resistance and flat response zero attenuation across the band, yet it is constructed out of inductors and capacitors. The only signal parameter it modifies is the phase of the signal at different frequencies.[22]

[edit] Impedance matching

A common theme throughout Zobel's work is the issue of impedance matching. The obvious approach to filter design is to design directly for the attenuation characteristics desired. With modern computing power, a brute force approach is possible and easy, simply incrementally adjusting each component while recalculating in an iterative process until the desired response is achieved. However, Zobel developed a more indirect line of attack. He realised very early on that mismatched impedances inevitably meant reflections, and reflections meant a loss of signal. Improving the impedance match, conversely, would automatically improve a filters pass-band response.[16]

This impedance matching approach not only led to better filters but the techniques developed could be used to construct circuits whose sole purpose was to match together two disparate impedances.[23][24] Zobel continued to invent impedance matching networks throughout his career. During the second world war he moved on to waveguide filters for use in the newly developed radar technology.[25] Little was published during the war for obvious reasons but towards the end with Bell Labs in the 1950s, Zobel designs for sections to match physically different waveguide sizes appear.[6][7]

As noted above, the circuit which still bears Zobel's name today, the constant resistance network, is perhaps Zobel's finest achievement in impedance matching.[3]

[edit] Zobel and loudspeaker equalisation

The name of Zobel is, perhaps, most well known in respect of impedance compensation networks for loudspeakers. Clearly, his designs have applications in this field. However, none of Zobel's patents or articles appear to discuss this topic. It is unclear whether he did actually design anything specifically for loudspeakers. The closest we get to this is where he speaks of impedance matching into a transducer, but here he is discussing a circuit to equalise a submarine cable,[3] or in another instance where clearly he has in mind the hybrid transformer which terminates a line going into a telephone instrument on a phantom circuit.[23]

[edit] Zobel and Noise

With Carson leading the way theoretically, Zobel was involved in the design of filters for the purpose of noise reduction on transmission systems.[25]

[edit] Background

At the beginning of the 1920s and through to the 1930s, the thinking on noise was dominated by the radio engineers concern with external static. In modern terminology, this would include random (thermal and shot) noise but those concepts were relatively unknown and little understood at the time despite an early paper by Schottky in 1918 on shot noise.[26] To the radio engineers of the time, static meant externally generated interference. The line of attack against noise from the radio engineers included developing directional antennae and moving to higher frequencies where the problem was known not to be so severe.

For telephone engineers, what was then called "fluctuating noise", and would now be described as random noise, ie shot and thermal noise, was much more noticeable than with early radio systems. Carson broadened the radio engineers concept of signal-to-static ratio to a more general signal-to-noise ratio and introduced a figure of merit for noise.[27]

[edit] Can noise be cancelled?

The radio engineers preoccupation with static and the techniques being used to reduce it led to the idea that noise could be totally eliminated by, in some way, compensating for it or cancelling it out. The culmination of this viewpoint was expressed in a 1928 paper by Edwin Armstrong.[28] This led to a famous retort by Carson in a subsequent paper, "Noise, like the poor, will always be with us".[29] Although Armstrong was technically in the wrong in this exchange, possibly he had the last word when his invention in 1933 of wide-band FM enormously improved the noise performance of radio.[30]

Carson and Zobel in 1923 had conclusively shown that filtering cannot remove noise to the same degree as, say, interferance from another station could be removed. To do this they had analysed random noise in the frequency domain and postulated that it contains all frequencies in its spectrum. This was the first use of Fourier analysis to describe random noise and hence described it in terms of a spread of frequencies. Also first published in this paper was the concept of what we would now call band-limited white noise. For Zobel this meant that characteristics of the receiving filter completely determine the figure of merit in the presence of white noise and that the filter design was key to achieving the optimum noise performance.[31]

Although this work by Carson and Zobel was very early, it was not universally accepted that noise could be analysed in the frequency domain in this way. Hence the exchange between Carson and Armstrong noted above was still possible years later. The exact mathematical relationship between noise power and bandwidth for random noise was finally nailed by Harry Nyquist in 1928 thus giving a theoretical limit to what could be achieved by filtering.[32]

This work on noise produced the concept, and led Zobel to pursue the design, of matched filters. In some ways, this was the culmination of the theoretical work, the noise performance of the equipment is optimal when the filter is perfectly matched to the signal one is attempting to transmit. This became important in the development of radar during the second world war in which Zobel played a part.

[edit] Use of Zobel's work in fuzzy logic research

Quite oddly, Zobel's work has recently found an application in research into fuzzy logic. The purpose of of this research is to attempt to demonstrate that the results obtained from genetic programming are comparable to human achievements. Two of the measures that are used to determine whether a genetic programming result is human-competitive are;[33]

  • The result is a patented invention.
  • The result is equal to or better than a result that was considered an achievement in its field at the time of discovery.

One such problem set as a task for a genetic program was to design a crossover filter for woofer and tweeter loudspeakers. The output design was identical in topology to a design found in a patent of Zobel's[34] for a filter to separate multiplexed low and high frequencies on a transmission line. This was judged to be human comparable, not only because of the patent, but also because the high-pass and low-pass sections were "decomposed" as in Zobel's design, but not specifically required to be so in the programs parameters.[33] Whether or not Zobel's filter design would be good for a hi-fi system is another question. The design does not actually cross over, but rather, there is a gap between the two pass-bands where the signal is not transmitted to either output. Essential for multiplexing, but not so desirable for sound reproduction.

A further genetic programnming[35] experiment produced a filter design which consisted of a chain of constant k sections terminated in an m-type half section. This also, was determined to have been a design patented by Zobel.[12]

[edit] Miscellaneous

Zobel co-authored a book in 1913 on the subject of geophysical thermodynamics.[36]

[edit] See Also

[edit] Circuits

[edit] People

[edit] Theories and concepts

[edit] Transmission modes

[edit] Filtering terms

[edit] References

  1. ^ Bray, J, Innovation and the Communications Revolution, p62, Institute of Electrical Engineers
  2. ^ a b c d e "The Past", Journal BT Technology, Vol 18, No 1, January 2000, Springer Netherlands.
  3. ^ a b c Zobel, O J, Distortion Compensator, US patent 1 701 552, filed 26 June 1924, issued 12 Feb 1929.
  4. ^ Schwartz, M, "Improving the Noise Performance of Communication Systems: 1920s to early 1930s", Technologies, Technologists & Networks: A Symposium on the History of Communication Technologies, p9, October 17, 2007, Smithsonian National Postal Museum.
  5. ^ Seising, R, The Fuzzification of Systems, 2007, Springer Berlin / Heidelberg.
  6. ^ a b Zobel, O J, Impedance Transformer, US patent 2 767 380, filed 30 Sept 1952, issued 16 Oct 1956.
  7. ^ a b Zobel, O J, Microwave Filter, US patent 2 623 120, filed 20 April 1950, issued 23 Dec 1952.
  8. ^ Carson, J R, Electric Circuit Theory and Operational Calculus, 1926, McGraw-Hill, New York.
  9. ^ Campbell, G A, "Physical Theory of the Electric Wave-Filter", Bell System Tech J, November 1922, vol 1, no 2, pp 1-32.
  10. ^ a b Bray, J, Innovation and the Communications Revolution, Institute of Electrical Engineers
  11. ^ Chu, W, Chung-Kwei Chang, Transients of Resistance-Terminated Dissipative Low-Pass and High-Pass Electric Wave Filters, Proceedings of the IRE, vol 26, iss 10, pp 1266-1277, October 1938
  12. ^ a b Zobel, O J, Terminating network for filters, US patent 1 557 229, filed 30 April 1920, issued 13 Oct 1925.
  13. ^ Mathaei, Young, Jones Microwave Filters, Impedance-Matching Networks, and Coupling Structures, pp 72-74, McGraw-Hill 1964
  14. ^ Redifon Radio Diary, 1970, p47, William Collins Sons & Co, 1969
  15. ^ Shea, T E, Transmission Networks and Wave Filters, 1929, Bell Telephone Laboratories.
  16. ^ a b Zobel, O J, Electrical Wave Filter, US patent 1 850 146, filed 25 Nov 1930, issued 22 March 1932.
  17. ^ Zobel, O J, Electrical network, US patent 1 760 973, filed 27 March 1928, issued 3 June 1930.
  18. ^ Zobel, O J, Electrical Network, US patent 1 720 777, filed 9 Sept 1926, issued 16 July 1929.
  19. ^ Zobel, O J, Electrical Network, US patent 1 591 073, filed 15 Dec 1922, issued 6 July 1926.
  20. ^ Zobel, O J, Selective Constant Resistance Network, US patent 1 724 987, filed 13 April 1928, issued 20 Aug 1929.
  21. ^ Zobel, O J, Electrical Network and Method of Transmitting Electric Currents, US patent 1 603 305, filed 9 Aug 1922, issued 19 Oct 1926.
  22. ^ Zobel, O J, Phase-shifting network, US patent 1 792 523, filed 12 March 1927, issued 17 Feb 1931.
  23. ^ a b Zobel, O J, Electrical Wave Filter, US patent 1 615 252, filed 9 June 1923, issued 25 Jan 1927.
  24. ^ Zobel, O J, Complementary filter, US patent 1 557 230, filed 30 April 1920, issued 13 Oct 1925.
  25. ^ a b Schwartz, M, "Improving the Noise Performance of Communication Systems: 1920s to early 1930s", Technologies, Technologists & Networks: A Symposium on the History of Communication Technologies, delivered at the Smithsonian National Postal Museum, 17 October 2007.
  26. ^ Schottky, W, "Uber spontane Stromschwankungen in verschiedenen Elecktrizitätsleitern", Annalen der Physik, verte folge, Band 57, 1918, pp541-567.
  27. ^ Carson, J R, "Signal-to-Static-Interference Ratio in Radio Telephony", Procedures of the IRE, vol 11, June 1923, pp271-274.
  28. ^ Armstrong, E H, "Methods of Reducing the Effect of Atmospheric Disturbances", Procedures of the IRE, vol 16 no 1, January 1928, pp15-26.
  29. ^ Carson, J R, "Reduction of Atmospherics", Procedures of the IRE, vol 16 no 7, July 1928, pp966-975.
  30. ^ Armstrong, A H, Radiosignaling, US Patent 1 941 069, filed 24 January 1933, issued 26 December 1933
  31. ^ Carson, J R and Zobel, O J, "Transient Oscillation in Electric Wave Filters", Bell Systems Technical Journal, vol 2, July 1923, pp1-29.
  32. ^ Nyquist, H, "Thermal Agitation of Electric Charges in Conductors", Physical Review, vol 32, July 1928, pp110-113.
  33. ^ a b Koza, Bennet, Andre, Keane, Genetic Programming III: Darwinian Invention and Problem Solving, Morgan Kaufmann, San Francisco, 1999. see [1]
  34. ^ Zobel, O J, Wave Filter, US patent 1 538 964, filed 15 Jan 1921, issued 26 May 1925.
  35. ^ Chakrabarti, A, Engineering Design Synthesis: Understanding, Approaches, and Tools, Springer, 2002.
  36. ^ Leonard, R, Zobel, O J, Ingersoll, A C, An Introduction to the Mathematical Theory of Heat Conduction with Engineering and Geological Applications, 1913, Ginn and Co, Boston, New York.

[[Category:Scientists at Bell Labs]] [[Category:American inventors]] [[Category:American electrical engineers]] [[Category:Electronics engineers]] [[Category:University of Wisconsin-Madison alumni]] [[Category:Radio pioneers]] [[Category:1887 births]] [[Category:1970 deaths]]