Some Notes on the History of Parametric Transducers*

848) PROCEEDINGS OF THE IRE Mlay Some Notes on the History of Parametric Transducers*

W. W. MUMFORDt, FELLOW, IRE

Summary-This paper summarizes briefly the chronology of the there were two complete vibrations of the support for development of parametric transducers. The early works of Michael Faraday (1831), F. Melde (1859), and Lord Rayleigh (1883) are cited each complete vibration of the liquid. Crispations (may as mechanical examples and the pioneering work of L. Kiihn, be) observed upon the surface of liquid in a large wine J. Zenneck, E. F. W. Alexanderson and R. V. L. Hartley are cited as glass or finger glass which is caused to vibrate in the electrical examples. A very brief resum6 of selected contributions usual manner by carrying the moistened finger round follows, dating from the work on H. Q. North's diodes in 1945 to the the circumference. All that is essenitial to the produc- present flurry of excitement beginning in 1954, created by the development of the Signal Corps-Bell Laboratories Task 8 varactor tion of crispations is that the body of liquid with a free diodes. A list of 200 selected references is included. surface be constrained to execute a vertical vibration. Faraday's assertion that the waves have a period double T -I HE recent interest in amplifiers which derive their that of the support has beeni disputed, but it may be gain from variable reactance circuit elements verified in various ways." Faraday's work was pub- stems chiefly from the development of low-loss lished in 1831 and Lord Rayleigh verified his conclu- variable-capacitance diodes. There are two reasons for sions sixty years later, also with considerable ingenuity. this interest. One reason is the fact that such amplifiers The double period oscillation of the water is not readily have low noise and the other is that the diodes are ex- proven by casual observation. pected to have extremely long life. Either one of these The following example of the principle, reported by properties is adequate justification for the excitement Melde in 1859, is, however, readily observed and under- currently rampant throughout the world concerning the stood. Quoting again from Lord Rayleigh, "Perhaps the exploitation of this "new" type of amplifier, but, with best known example is that form of ?Melde's experiment two good reasons readily apparent, this excitation is in which a fine string is maintainied in transverse vibra- doubled. tionl by connecting one of its extremities with a vibrat- Mystery seemed to invade the thoughts of people ing prong of a massive tuning fork, the direction of mo- when the scientists announced this new type of anmpli- tion of the point of attachment being parallel to the length fier which was called a variety of names, such as: of the string. Unider these circumstances . .. the string "Parametric Amplifier," "Reactance Amplifier" and may settle down into a perimianent and vigorous vibra- "MAVAR" (Modulator Amplifier by Variable React- tion, whose period is the double of that of the fork." ance).1 Some of this mi-iystery could have been avoided Lord Rayleigh anialyzed and experimented with this had the moderni men known or mentioned that the prin- and other similar mechanical phenomenia in 1887. This ciple underlying the miiechanism whereby electrical am- led to anialogous experimnenits with electrical circuits. plification was effected was an old principle. This prin- The electrical principle is readily understood by the ciple may be broadly stated thus: The energy of an oscil- following simple explaniatioin. Suppose that we have a lating system may be increased by supplying energy at a capacitor formed by two metal plates separated by air. frequency which differs from the fundamental freqbuency of Assume that a charge exists on the capacitor. The plates the oscillator. One mechanical illustration of this prin- will be attracted to each other because of the equal anid ciple is the simple pendulum. The child in the swing opposite charges so that to separate the plates requires learns that he can "pump up" the amplitude of the work. Uponi separating the plates, say to twice the origi- oscillation of the swing by lowering his center of gravity nal distance, the capacitaince will be reduced to half its on the down swing and raising it on the up swing. He original value and, hence, the voltage mlust be twice the thus "pumps" at twice the frequency of the swing. Who original value, since the charge upoIn the plates remains knows when this was invented? Could it have been in the same. The electrostatic energy, however, has been prehistoric times by a monkey swinging by his tail from doubled, since it is proportionial to the square of the the branch of a tree? voltage and directly proportioinal to the capacitanice. Faraday, Melde and Lord Rayleigh have published The eiiergy required to separate the plates Inow appears observations and calculations concerning this principle. as electrostatic energy in the capacitor. Quoting Lord Rayleigh, "Faraday, . . . with great in- Now suppose that the capacitor is combinied with an genuity and success (upon exaim-iiiiing) . . . the crispa- inductor to form ain oscillating circuit. The voltage on tions upon the surface of water which oscillates ver- the capacitor will reach a imaxiniui-ii value twice each tically, arrived at the conclusion experimentally that cycle. Now if, on each half cycle, the capacitance is de- creased when the voltage is ml-aximum aind increased * Original manuscript received by the IRE, November 17, 1959; revised manuscript received, February 1, 1960. when the voltage is zero, net energy will be imiparted to t Bell Telephone Labs., Whippany, N. J. the oscillations since Ino electrical enlergy is used to re- I These three names are considered herein to be synonymous and to apply to any device which derives its gain from the pumping of a store the capacitor to its original value wheni the voltage variable reactance. is zero. 1960 Mumford: Some Notes on the History of Parametric Transducers 849

Similarly, it is apparent that energy could be im- It appears to me that the fundamental principle . . will find its application to other problems in connection with radio frequency parted to the circuit had the inductance been varied in circuits. One that suggests itself immediately is the amplification of the appropriate phase. This electrical principle was ex- incoming signals. panded to include frequencies other than the two-to-one Alfred N. Goldsmith pointed out the advantages of ratio and the resulting device was used successfully in radio telephone communication betweeni Berlin and Alexanderson's magnetic amplifier over the direct-current-controlled frequency doubler employed by Kuhn. Vienna prior to World War I. This was described by L. Kuhn in 1915. Prof. J. Zenneck, E. F. W. Alexander- Lee De Forest commented that the magnetic amplifier was far more practical as a high-power modulator than son and R. V. L. Hartley pioneered with theoretical and the ensemble of over 500 audion amplifiers used to ob- experimental contributions within the next few years. Alexanderson called these devices "Magnetic Ampli- tain 11 kw at Arlington by the Western Electric Com- pany. "No one can say, however, that the situation will fiers," a name which remains with us today. The objec- not be altered very materially in one, two or three years tive then was to modulate a continuous wave arc trans- after we learn how to build oscillions for large power mitter by means of a nonlinear inductance or saturable will create a dif- reactance. Here the voice currents constituted the sig- outputs, say 5 or 10 kw each. That very ferent situation." nal, and the carrier was the pump. The resulting side- art on banids were radiated, together with the pump (or its har- Thus, there appears to be very old prior monic in some cases). MAVAR, both as modulator and amplifier. How- ever, the interest in magnetic amplifiers as radio fre- I quote the following fromn a paper delivered by quency modulators subsided quickly with the advent E. F. W. Alexanderson at an IRE meeting in New York City onl February 2, 1916: of the high-power vacuum tube modulators. The "dif- ferent situation" predicted by Lee De Forest in Febru- The name "Magnetic Amplifier" has been given to a device for ary, 1916, did, indeed, come to pass. controlling the flow of radio frequency currents because this name In the 1920's and '30's, interest developed in sub- seems to describe its function when it is used for radio telephony better than would any other. As the same device can be used for a harmonic oscillations in electrical circuits containing a variety of other purposes, the above name may in some cases not variable reactance. These "parametric" oscillations seem too appropriate. However, the essential part of the theory that exist at any one of where n is the will be given refers to the amount of amplification which is possible could f/n frequencies, of attainment and the methods of securing a higher ratio of amplifi- subharmonic fraction of the fundamental frequency. In cation than would be given by the device in its simplest form. ... 1954 Von Neumann and Goto independently recognized The ratio of amplification is proportional to the ratio between a existed in the subharmonic the frequency of the radio current and that of the controlling current. that phase ambiguity oscillations and that this ambiguity could be utilized in logic circuits. Goto calls this device a "parametron." (This conclusion was verified by R. V. L. Hartley and About thirty years after the pioneering work of subsequently by Manley and Rowe.) Kuhn, Zenneck, Alexanderson, and Hartley on inductive Alexanderson, in the discussion, also suggested am- reactance modulators, interest developed in capacitance plification of incoming signals by cascaded stages of up- reactance modulators at microwave frequencies. The conversion, rectification and up-conversion, etc. The failure of reciprocity in some crystal converters ob- name of Alexanderson's device withstood the rigors of served in the middle 1940's by L. Apker of General time. Currently, however, we recognize its radio fre- Electric Co., Schenectady, N. Y., and R. N. Smith of quency version as a type of parametric amplifier, re- Purdue University, Lafayette, Inc., and the peculiar actance amplifier, or MAVAR. behavior of welded contact germanium diodes made by In Alexanderson's magnetic amplifier, the chief inter- H. Q. North of General Electric Co., Schenectady, est resided in the mode of operation in which the input N. Y., was interpreted to mean that the contact capac- signal was in the voice frequency band and the useful ity varied with bias. H. C. Torrey of the Massachusetts output power was taken in some radio frequency band. Institute of Technology Radiation Laboratory, Cam- Thus it was a modulator or up-converter. bridge, Mass., gave a thorough discussion of the theory Alexanderson presented curves to show that negative of nonlinear capacity converters. resistance effects could exist. Quoting again from his M. C. Waltz and R. V. Pound at the MIT Radiation 1916 paper: Under some conditions "instability and Laboratory observed negative IF conductance when generation of self-excited oscillations" can exist. "This units like North's were used. Pound gave many inter- is a condition that must be avoided for telephone con- esting details about measured power and gain and also trol; whereas it may have useful applications for other measured negative IF conductance. He obtained a 10- purposes." (One useful application, pointed out by db gain and reasoned that such a receiver should have Eugene Peterson in 1930, was the negative resistance a better noise figure than that of a conventional coIn- straight-through amplifier, in which the negative re- verter which has conversion loss. He was unable, how- sistance effect was enhanced by the suppression of fre- ever, to achieve this. quencies higher than the pumping frequency.) In 1948, A. van der Ziel and, in 1949, V. D. Landon Louis Cohen, in a communicated discussion of the also derived the MAVAR gain relationships; the former paper, said: also pointed out the low-noise figure possibilities. 850 PROCEEDINGS OF THE IRE May

In 1952, C. F. Edwards observed nonreciprocal be- plification of microwaves. Bloom and Chang also dis- havior in converters when he used R. S. Ohl's bom- cussed the case of low frequency pumping. barded silicon diodes which exhibited variable capaci- R. S. Engelbrecht at Bell Telephone Laboratories de- tance as well as variable resistance characteristics. This signed a traveling wave UHF parametric amplifier using observation again triggered a sequence reminiscent of varactor diodes and achieved over 200-mc bandwidth the North diode sequence of the 1940's in which Apker, at UHF with 8 to 10 gain. MTIeasurements indicated an Smith, Pound, and Waltz reported the experimenetal re- "astronom-iy" iioise figure of one db, correspondinig to a sults and Torrey, van der Ziel, and Landon derived the "radar" noise figure of about 3.5 db. (This compares theory. Corresponding names for the early 1950 se- favorably with the best commercially available vac- quence are Ohl, Edwards, Manley, and Rowe. uum tube, whose noise figure is about 5 db.) However, ini neither of these sequences was a very In the meantime, Adler of Zenith, Chicago, Ill. low-loss variable capacitance diode available and hence (in June, 1957), suggested a niovel principle of signlal the gain was limnited and the noise figure was not es- amplification using a pumped electroni beam, and pecially good. Bridges (in February, 1958) suggested and constructed In 1954, the United States Signal Corps sponsored a a parametric amplifier using the variable reactance of a project at Bell Telephone Laboratories, Murray Hill, floating drift tube klystron. Louisell and Quate discussed N. J., to develop semiconductor devices. In the second the capabilities of this type of amplifier, and Adler interim report of this now famous "Task 8," A. E. Baka- demonstrated that the conclusions conicerninig the low- niowski published his derivation of the nonlinear capac- noise capabilities were indeed correct. He achieved a itor as a mixer. The work of Bakaniowski, Cranna and noise figure capability of 1.4 db, of which 0.4 db repre- Uhlir led to the discovery of a technique for making sented the loss in the iniput coupler. low-loss units. The developmenit of the vacuumi tube in Alexander- The technique of making low-loss silicon diode varac- sonIs time curtailed the interest in radio frequency par- tors or varicaps advanced rapidly and interest in these ametric transducers. Thirty or so years later, the inveni- new units began to expand. tion of the transistor then dimiinished the interest in In the meantime, H. Suhl discovered that variable vacuum tubes. But the interest in radio frequenicy reactance in the microwave range was obtained in fer- parametric transducers was resurrected by the develop- rite materials when properly excited by a pumping fre- miienit of the low-noise variable capacitance diode, aind quency. He proposed using this effect to obtain para- this resurrection, in turn, has stimulated the interest in metric amplification and discussed suitable materials in vacuum tubes as parametric tranisducers. the paper published in 1957. M. T. Weiss verified Suhl's What is the next cycle in this see-saw pattern? proposal experimentally. M. E. Hines and H. E. Elder succeeded in demon- LIST OF SELECTED REFERENCES strating gain and oscillationis in a reactance amplifier ON which used silicon varactors and suggested several mi- PARAMETRIC AMPLIFIERS crowave circuits for up-converters and negative resist- (To JULY, 1959) ance work in mi- amplifiers. Their stimulated activity 1. M. Faraday, "On a peculiar class of acoustical figures; and oni crowave applications of "varactor" diodes.2 certain forms assumed by a group of particles upon vibrating elastic surfaces," Phil. Trans. Roy. Soc., London, vol. 121, pp. In 1957, Heffner aiid Wade considered theoretically 299-318; May, 1831. the noise, gain and bandwidth of parametric amplifiers. 2. F. Melde, "Uber erregung stehender Welleni eiiies fadenform- in 1958, the low-noise properties predicted by eigen Korpers," Ann. der Physik u Chemnie, Ser. 2, vol. 109, pp. Early 193-215; month, 1859. theory were verified experimentally at the Bell Tele- 3. J. W. Strutt, Lord Rayleigh, "On the crispations of fluid resting phone Laboratories at 6000 mc by Uenohara anid at 380 upon a vibrating support," Phil. Mulag., vol. 16, pp. 50-53; July, 1883. mc by Engelbrecht. Salzberg at Airborne Instrumenits 4. J. W. Strutt, Lord Rayleigh, "OIn the miainiteniance of Laboratory, Mineola, N. Y., and Heffner and Kotzebue vibrations by forces of double frequency, and on the propagation of waves through a medium enidowed with a periodic structure, " at Stanford University, Stanford, Calif., also achieved Phil. Mag., ser. 5, vol. 24, issue 147, pp. 145-159; August, 1887. low-noise performance working at 1 mc and 1200 Illc, 5. J. W. Strutt, Lord Rayleigh, "Theory of Souniid,'"Macmillan anid Co., Ltd., London, England, 2nd edition, vol. 1, 1894; vol. 1, respectively. 1896. Miyakawa in Japan, Cullen in England and Tien and 6. R. Goldschmidt, "Reduction of hysteresis losses by high- frequency ctirrents," Elektrotech. Z., vol. 31, pp. 218-221; Suhl in America considered the amplificationi anid fre- March, 1910. quency conversion in propagating circuits in which the 7. L. Kuhn, "Uber ein neves radiotelephoniische system," Jahr. fiir dracht. Teleg. und Telephone, vol. 9, pp. 502-534; June, 1915. variable reactors were distributed along a tranismission 8. J. 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