282 Philips tech. Rev. 32, 282-291, 1971, No. 9/10/11/12

Solid-state microwave electronics

N. E. Goddard

Introduetion

The history of modern technology includes manyexamples of unrecognized inventions, revived classicaltheories, premature technical developments and somefrustrated industrial applications. These are the conse-quences of vigorous pioneering effort and the recenttechnology of microwave electronics is no exception.Nevertheless steady progress in technique and exploita-tion is maintained. Microwave technology is of in-creasing importance in fields such as communications,broadcasting and domestic electronics, as well as in theloc~iion, control, and navigation of land, sea, air and

'p.~'.- , ' .. . Jspace,,.V~hïcles. •

This'f:issue of Philips Technical Review is devoted toan acco~n't of some topics of recent research on micro-wave devices and associated circuits carried out in re-search laboratories associated with the Philips groupof companies. The present time is opportune becauseradical changes in microwave technique are occurringsimultaneously with the pressures of overcrowding infrequency, space and time in the application and userareas. There are pressures to expand the overcrowdedcommunications and broadcasting frequency spectrumfurther into the microwave bands. There are strongdemands for systems to increase the density and con-trol of traffic flow in the overcrowded space availablefor ships, aircraft and road vehicles. There are demandsfor ever-increasing data processing and transmissionspeeds involving gigabit-per-second logic functions andgigahertz network bandwidths. Extensive research anddevelopment in microwaves has been undertaken in thepast, particularly for demanding and sophisticated mil-itary applications. With recent developments in low-cost devices and circuits a challenging and expandingfield of application can be foreseen with great scope incivil markets.

The early stages of electronic development in themicrowave spectrum, broadly at frequencies from1 GHz to 300 GHz, were beset with problems of radia-tion losses from conductors, dissipation in conductorsand insulators, and the transit time of electron trajec-tories in conventional grid-controlled vacuum tubes.To overcome these problems, large, enclosed trans-mission-line circuits and complex, high-voltage, velo-

N. E. Goddard, M.A., is head of the Systems Division of MullardResearch Laboratories, Redhill, Surrey, England.

city-modulated generators and amplifiers were devel-oped. A very precise and accurate circuit technique wasestablished, backed up by an academically and practi-cally rewarding body of elegant theoretical analysis.But paradoxically microwave electronics was notsynonymous with microelectronics, even at micro-powers. Nor were small wavelengths associated withsmall component and equipment costs. This situationis now undergoing radical change.

As with other branches of electronics, the greatestrecent impact on the design of devices and equipmentsin the microwave spectrum has been made by solid-state physics. The consequent development of novelgenerator, amplifier and control devices was rather likethe opening of Pandora's box. New devices and specu-lation on their applications proliferated; new in-vestigations spread and multiplied with the aid of whatsome may consider the excessively immediate and di-verse means of scientific communication. However,unlike the mythological analogue, more than Hoperemains and many desirable objectives have beenachieved without, as yet, creating too much havocamong mankind.

Some notes on the history of these developments andthe main lines of present investigations will provide abackcloth for the papers which followand references tosome of the better established techniques which are notreported i~this issue.

'"

Solid-state devices';"

"This specious conjecture has no conJincing theor-etical argument in its favor." Thus com~ented H. C.Torrey and C. A. Whitmer [ll [*l in 1946 on the long-accepted assumption that a simple contact rectifier usedas a frequency converter must always have a conversionloss greater than unity. After describing H.Q. North'swelded-contact germanium crystals and showing that anegative intermediate-frequency conductance requiresa barrier capacitance which varies strongly with barriervoltage, they comment further: "No use has yet beenmade of the amplifying properties of the welded recti-fiers used as converters. It is possible that a lightweightreceiver with no tubes, yet capable of detecting audiomodulation of a weak microwave signal close to noiselevel, might be constructed by keeping these crystals on

Philips tech. Rev. 32, No. 9/10/11/12 MICROWAVE ELECTRONICS 283

the verge of oscillation by a suitable feedback arrange-ment. Some work along these lines was started byR. H. Dicke, but inconclusive results had been ob-tained when the war came to an end."A further ten years elapsed and the transistor had

been invented before the growth of solid-state micro-wave electronics, foreshadowed in this prophetic com-ment, gathered pace.In 1948A. van der Ziel [2] underlined the possibility

of low-noise amplification with semiconductor diodes,and in 1956J. M. Manley and H. E. Rowe [3] publishedan analysis of the general properties of non-linearelements and established the energy relationships formulti-frequency converters and amplifiers, The firstexample of a parametrie amplifier wasshortly afterwardsreported by H. Suhl [4] and M. T. Weiss [5]. This ampli-fier was a non-linear inductance amplifier based on thenon-linear properties offerrite materials, but its furtherdevelopment was hindered by the requirement for veryhigh microwave pump powers. This restrietion did notapply to the non-linear-capacitance semiconductordiode or varactor which was also studied at this timefor converter applications and frequency control [6] [7].

The development of degenerate and non-degenerateparametrie amplifiers using varactors quickly fol-lowed [8] [9]. Negative-resistance, single-port, varactorparametrie amplifiers proved capable of extensivedevelopment in low-noise and broad-bandwidth appli-cations [10] [11] [12], despite the difficulties of stabilityand separation of input and output signals. Fortunatelythe solution to the latter problem had already beenfound in the prior extensive work on microwave ferritegyrators, in particular the three- and four-port circu-lators. An extensive range of applications for varactorswas developed, including frequency multipliers, limit-ers, switches and tuners.In 1955, C. H. Townes and co-workers [13] reported

the maser - a new type of microwave amplifier, fre-quency standard and oscillator. Coherent microwaveoscillations were obtained by stimulating transitionsbetween two quantum states of an ammonia system.N. Bloembergen [14] then proposed the three-levelsolid-state maser and this was demonstrated with gado-linium ethyl sulphate by H. E. D. Scovil et al. [15] in1957.With an internal noise temperature of only a fewdegrees Kelvin when cooled to liquid-helium tempera-tures, and with the development of travelling-waveamplifier structures of 1% bandwidth, the solid-statemaser was a vital component in the first Earth-stationreceivers for satellite communications [16]. It has nowbeen largely superseded in this application by paramet-ric amplifiers which have a much better capability andflexibility in bandwidth and operating bath tempera-ture [12].

A new type of semiconductor diode with negative-resistance characteristics was discovered by L. Esaki [17]in 1957. Based on the phenomenon of electron-tun-nelling across a PoN junction, the tunnel diode is cap-able of operating over a very wide frequency rangeand requires only a simple d.c. power supply and asimple signal circuit. Although at first a very wide rangeof applications was predicted for this diode, limitationsof power-handling, stability, fragility and modest noisefactor led to its adoption in only a limited range ofcircuits in which its simplicity could be exploited.The severepower limitations of the small-area, point-

contact and junction diodes required for microwave-frequency operation had long been recognized and thedesirability of a volume mechanism in bulk material, asin the solid-state maser, but capable of supporting highenergy densities, was acknowledged. In 1961B. K. Rid-ley and T. B. Watkins [18] predicted bulk negative re-sistance in III-V semiconductor compounds as a resultof the transferred-electron effect, and in 1963 J. B.Gunn [19] demonstrated microwave oscillations in gal-lium arsenide and indium phosphide which were ex-plained by the Ridley and Watkins mechanism. There-fore the effect described by Ridley and Watkins is oftenreferred to as the Gunn effect. The different modes ofoperation of Gunn diodes and their application forhigh-power generation and wide-range tuning havebeen extensively studied and some of the results are re-ported elsewhere in this issue [20] [21] [22].

Yet another type of semiconductor diode had beensuggested byW. T. Read [23] in the 1950s in which neg-ative resistance could result from avalanche break-down and carrier transit time in a specially designedsemiconductor junction. Practical devices were firstreported in 1965 [24]. As with the Gunn or transferred-electron device, a variety of modes of operation havebeen explored [25] [26] [27]. The relative capabilities andadvantages of Gunn and avalanche devices in differentapplications are not yet clearly resolved.For switching, modulation and limiting functions, a

P-/-N diode was developed in '1958.A similar structurehad been reported earlier by R. N. Hall [28] for powerrectification at low frequencies. At microwave frequen- 'cies there is no rectification and the central region of in-trinsic semiconductor is insulating. When charge car-riers are injected into this region by a forward bias volt-age and the depth of the region is about equal to the re-cornbination-diffusion length, an incident microwavesignal is absorbed with very little reflection. Thusthe device is an electronically controlled attenua-tor [29] [30] [31].

In parallel with all this work on new devices, a steady,

1*1 The references are listed at the end of the article.

284 N. E. GODDARD Philips tech. Rev. 32, No. 9/10/11/12

if less spectacular, application of planar semiconductortechnology has been devoted to the improvement of thewell-established detector and mixer diodes which con-tinue to be used in larger quantity than other diodes.With the metal-semieend uctorj unction of the Schottky-barrier diode, substantial improvements have beenmade in mixer noise figure, conversion loss, burnout,dynamic range and mechanical reliability [321.Thebackward diode, with a highly non-linear I-V charac-teristic due to controlled tunnelling, has also provided asensitive detector [331.

The remaining active device to be mentioned here isthe transistor. Ever since its introduetion in 1948, the

Circuit techniquesTechniques for transmission, filtering and passive

processing of microwave signals have undergone chan-ges as radical as those in active devices. A number ofimportant lines of development can be recognizedwhich include the provision of non-reciprocal circuitswith transmission through ferrite materials, the use oftransmission modes in stripline techniques and thedevelopment of technology for achieving essentiallylumped-constant circuits. There has also been a con-tinuing development of the ubiquitous hybrid couplerwhich is a building brick in so many microwave circuitassemblies.

Fig. La) This photograph illustrates the very considerable difference in size between an earlywaveguide tunnel-diode amplifier, tunable in the 10.7 GHz to 11.7 GHz frequency band, anda lumped-circuit amplifier, shown above it. The diameter ofthe lumped-circuit version is 7 mm.b) A diagram of the lumped-circuit tunnel-diode amplifier, now greatly enlarged. The con-ductors were made by depositing a thin gold-an-nickel chromium film on to a quartz substrate,and the amplifier is coupled to a coaxial circuit by means of concentric rings. At a gain of 10 dBand a noise figure of 7 dB the instantaneous bandwidth is about 3 GHz to 4 GHz.

a

maximum operating frequency of the transistor hasbeen steadily increased and with new constructiontechnologies experimental devices have been operatedat freq uencies beyond lOG Hz. In many microwaveapplications the transistor will be preferred to the two-terminal diode device because it is more readily con-trolled and has an inherently better isolation betweeninput and output.

This great variety of device developments has de-pended on a parallel investigation of material physicsand semiconductor technology which is too extensiveto recount here. Most of the work on germanium andsilicon has been undertaken for lower-frequency de-vices. At microwave frequencies the requirements forhigh carrier mobility, low permittivity and small di-mensions have been met by the development of III- Vcompound semiconductors such as gallium arsenide,and technologies for epi taxiallayers [341and area metal-semiconductor junctions.

b

Stripline transmission circuits in shielded and bal-anced or in open microstrip form have been known for avery long time [35] [36] but a number of developmentshad to be made before technically and industriallyeffective applications could be adopted [37] [38]. Theelectromagnetic field patterns for which simple closedsolutions were not available had to be analysed andtranslated into mode equations suitable for circuitanalysis. Dielectric substrates with low losses and con-trolled permittivity and dimensions had to be developed.But the main impediment to stripline circuit tech-niques was the incompatibility of the essentially planartransmission lines and the available coaxial semi-conductor detectors or vacuum tubes with coaxial andwaveguide oscillatory circuits. Passive planar micro-wave integrated circuits were well developed: corn-plementary active devices were not.

Solid-state amplifier and oscillator devices such asvaractors, avalanche diodes, tunnel diodes and Gunn

Philips tech. Rev. 32, No. 9/10/11/12 MICROWAVE ELECTRONICS 285

devices, especially with encapsulations suitable forstripline mounting, have enormously changed thedesign and application potentialof microwave inte-grated circuits [39].

As a research topic, work on microwave ferrite de-vices is now limited to one or two special areas such asthe propagation of surface waves. Many designs forgyrators and phase-shifters in hollow guide, coaxialline and stripline are available. The problem of corn-patibility between ferrite devices and integrated circuitshas been tackled in several different ways. Thin-filmcirculators on ferrite-substrate inserts have been suc-cessfully designed and are reported elsewhere [371. Twonew approaches are the use of a ferrite substrate for thewhole integrated circuit with magnetized regions forthe non-reciprocal parts, and the design of circulatorswith miniature lumped-constant circuit elements onvery small chips of ferrite [37] [401.

Simple lumped-constant circuits for microwave fre-quencies, such as tu nable wavemeters, have beenknown for some thirty years. With the development ofprecision machining of miniature components, nodoubt such circuits could have been more widely em-ployed. However, as with striplines, there were severecornpatibility problems with the active devices andsuch developments would have had little economic ortechnical value. With semiconductor devices and aplanar circuit technique, the compatibility problem isremoved and lumped circuits in thin-film form seemcertain to become established in many applications [401.

The physical difference, for example, between an earlywaveguide tunnel-diode amplifier and its lumped-circuit equivalent is dramatic (fig. I).

Two of the most important elements in microwavecircuits are the filter and the coupler. For special appli-cations, such as directional filters in a channel-drop-ping network for millimetric HOl transmission systems,it is necessary to use hollow guide for minimum at-tenuation. But for more general purposes stepped-impedance and stubs-and-lines filters in thin-filmtechnique, suitable for integrated circuits, are widelyemployed. With a computer-aided design which op-tirnizes performance within specified physical limi-tations and with a tape-controlled machine, it is prac-tical to program a system for direct production ofmasks from the basic filter performance data (fig. 2,see p. 286). Filters that are tunable have also beendeveloped [371.

At lower frequencies and with lumped elements, thecoupler or hybridjunction was known as a symmetricaltwo-terminal-pair lattice network. Microwave versionswere first developed as waveguide rnagie-T junctionsand ring junctions. One of the first broad-band junc-tions was the phase-reversal coaxial ring which was

developed as a 90° proximity coupler incorporatedwithin a 180° ring [38]. A great diversity of couplers andhybrid junctions has been developed and adapted foruse in thin-film integrated circuits. Following on there-entrant and overlay couplers, one of the mostpromising recent techniques with possible applicationto other circuits is the use in combination of both themicrostrip line and its inverse form known as the slot-line (fig. 3).

25dB -. .....__4--2

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75

70

5

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Fig.3. The "hybrid" coupler in the photograph combines themicrostrip anel slotline techniques. A slotline is formed by a thinslot cut in a metal plane which is deposited on a dielectric sheet.Three pairs of ports are connected by microstrip ; the remainingpair (J ,4) are connected by a slotline. The combination of micro-strip and slotline transmission lines gives a flat coupling and highisolation (4 -+ 2) over a wide band of frequencies.

286 N. E. GODDARD Philips tech. Rev. 32, No. 9/10/11/12

a

4dB

ex

1I

3 IIII\\

2 \

c O~)---------------~~--------------_L~2 3 4G~-f

b

2.0dB

5

1

7.5

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Fig. 2. Compact microstrip microwave components can very conveniently be made by photo-graphic and evaporation techniques. A thin gold strip is deposited on a substrate of aluminawith a ground plane deposited on the other face. With computer-aided design it is practical toprogram a system to make process masks directly from the basic filter-performance data.a) Microstrip band-pass filter for 8-18 GHz, produced in this way.b) As (a), but for the band 2-4 GHz.c) Insertion loss 0: of the filter shown in (b). The solid line shows the measured result; thedashed lines show computed results, with the value of the attenuation constant taken at0.4 dB/wavelength (curve J) and 0.2 dB/wavelength (curve 2).d) Voltage standing-wave ratio (v.s.w.r.) S of the filter shown in (b). The solid line shows themeasured results, the dashed line the computed results.

Devices for existing systems

Having briefly traced the history and variety of solid-state microwave devices and circuits, it is pertinent todiscuss their eligibility for current systems applicationsand their influence on future requirements for equip-ment design. The state-of-the-art performance is con-veniently summarized in terms of detector and receiversensitivity (fig. 4) and power-generating capability(fig. 5). With improving techniques for operation at thehighest freq uencies such summaries rapidly become outof date and it is also difficult to assess the practicalvalidity of the performance of special laboratory de-VIces.

For communication, broadcast, radar and naviga-tion systems operating within the Earth's atmosphere,the well-known radio window between about 1 GHzand 15 GHz is important. For minimum-cost systems,the transistor, the tunnel diode and the mixer diodeprovide noise figures in the range 2 dB to 6 dB depen-ding on the frequency and on the type of device. Thetransistor is expected to find increasing application inthis frequency range as new technologies such as ionimplantation permit improvement in geometry, and asdevices like the field-effect transistor are further devel-oped. The uncooled parametrie amplifier has an inter-nal noise temperature less than atmospheric thermal

Philips tech. Rev. 32, No. 9/10/11/12 MICROW AYE ELECTRONICS

noise at ground level and meets the performance re-quirements of most other Earth systems. The conven-tional amplifier requires a pump source at a frequencymuch higher than the signal frequency but with im-proved solid-state pumps and thin-film circuits a rela-tively inexpensive assembly is already possible. For

À-

frequencies is for very broad-band trunk cornrnunica-tions in low-loss waveguides. Frequencies in the range35 GHz to 110 GHz are proposed and the developmentof suitable devices for repeaters is in hand.

For small-signal receiver applications, with fewexceptions, the semiconductor device is capable of re-

mdB.- 7~0~0----------------~m~----------------~7c~m~----~JOOOK

F

12

2

T

1

JO

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0.7 L_----------------_j_----------------_____J,----------------__jJ07 m m~~

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Fig. 4. In this figure the noise performance ofvarious types ofmicrowave receiver (grey regions)is compared with the noise produced at the aerial by atmospheric absorption (A/In) andextraterrestrial radiation (galactic noise; Ca!). The degree of noisiness is plotted in the verticaldirection and is shown as aerial noise température or excess noise temperature T on the rightand noise figure F on the left; f represents frequency and Je wavelength. Tr transistors. MDmixer diodes. TD tunnel diodes. PARAM parametrie amplifiers. The red curves show thevariation with frequency of noise produced at the aerial for (j) = 00, 87° and 90°, where (j) isthe angle between the direction of the aerial and the zenith. At the lower frequencies the aerialnoise is mainly galactic noise (shaded region); at the higher frequencies the absorption noisedominates. Between about I and 15 GHz there is the "radio window", important for systemsthat operate within the Earth's atmosphere.

systems with large bandwidth operating between theEarth and space stations, the cooled parametrie ampli-fier provides the best combination of sensitivity andfrequency bandwidth. Uncooled parametrie amplifiersare logistically more acceptable for mobile earth sta-tions with narrower bandwidth.

Apart from a few special applications, such as35 GHz radar, the main interest in devices at higher

placing the vacuum tube. As is apparent from fig. 5,semiconductor devices cannot yet compete with vac-uum tubes as generators at the higher power levels butwill find many applications as transmitters in low-powersystems. The devices are also small enough to be usedin matrix arrays and some substantial effective radiatedpower fluxes may be possible by this means.

The advent of cheap microwave generators and

287

288 N. E. GODDARD Philips tech. Rev. 32, No. 9/10/1 IjJ2

\\

" \LSA"" \" \" \

JOo~------~~----~~~ __~~\~ __~

JO~~------------~------------~

5-------f

2 5 10 2 IOOGHz

Fig.5. State-of-the-art power generating capability of varioussolid-state microwave sources, with power P as a function offrequency f Tr transistor. Gunn Gunn oscillator, continuous-wave. LSA Gunn oscillator operating at the LSA-Illocle. Avavalanche diode, continuous-wave (drawn curve) and pulsed(clashed curve).

integrated circuits is also expanding the application ofradar and communication techniques to new marketareas. Simple Doppler radars are already extensivelyused for proteetion of commercial and domestic prem-ises from intruders. Experiments have been made withminiature radars as aids to drivers ofvehicles operatingon airfields in bad visibility. A microminiature radarhas been fitted in a road surface in the space normallyoccupied by a "eat's-eye". It detects vehicles andmeasures their speeds of vehicles for road-traffic con-trol. Many more applications of this kind may be anti-cipated.

Quo vadis?

Excepting a dramatic new discovery, the course ofdevelopment of solid-state microwave devices seemsestablished for the immediate future. For more specu-lative developments in microwave electronics we mustexamine the possibilities for novel systems.

The replacement of vacuum tubes by semiconductorsthroughout the microwave spectrum and in all but thehighest-power applications is a challenging task andshould result in more compact, efficient and economicsystems. This, however, is not the end of the story be-cause an even greater challenge remai ns to be met. Letus look at the situation at lower frequencies for whichsemiconductors are at a more advanced stage of devel-opment. We note the development of portable radio

and television receivers and the rather slower impacton capital equipment such as the telephone network.But the really vigorous and expansive development,made possible by semiconductor technology, is that ofcomputer and data-processing systems. One ofthe mainreasons is that the high component density and lowpower consumption of semiconductor circuits, partic-ularly monolithic integrated circuits, has made it pos-sible to use a very large matrix of circuit functions, inboth parallel and serial modes, which the systemsengineer could barely conceive in vacuum-tube tech-niques. A similar challenge now faces the microwavesystems engineer because microwave integrated circuitsare potentially very small and economic in power con-sumption and cost. It is, perhaps, unlikely that a newindustry on the scale of the computer industry willarise (though it may be noted that logic speeds areheading into the microwave spectrum) but it seemsprobable that new and important applications will befound as systems research and development takes ac-count of the new technology.

Let us pursue this analogy between low-frequencyand microwave solid-state developments and see whereit leads.

Microwave television broadcasting is already beingmade an economic possibility by the development ofthe new tech nology and a practical need by the over-crowding of the lower-frequency bands in densely-populated multilingual areas (fig.6). Microwave-satellite broadcasting is also being seriously studied.

With its large scale of application, broadcasting isclearly an important area for microwave techniques,but, with the solid-state developments at lower frequen-cies already cited, the major novel systems possibilities

Fig.6. An experirnental converter for microwave television re ..ceivers, It converts microwave broadcast signals from Earth orsatellite stations to UHF signals suitable for standard receivers.This microstrip mixer and amplifier circuit receives signals in the12 G Hz band anel provides output signals in the 600-900 GHzUHF band.

Philips tech. Rev. 32, No. 9/ I0/ I 1/12 MICROWAVE ELECTRONICS 289

a .~..5+4

2cmb

Fig. 7. a) Early expertmental phase discriminator in coaxialtechniques for the 2.0-4.0 G Hz band. The circuit includes fivehybrid rings, four detector diodes and three matched loads. b) Amicrostrip discriminator for the sa me band. The phase bridgehere is formed from two overlay couplers, and there are also threema tched-T power dividers, detectors, loads and a long delay line.

may be found in areas other than broadcasting. Untilrecently, the microwave systems engineer has had tothink in terms of high-voltage klystron and magnetrongenerators and expensive, precision circuit "plumbing"in waveguide or coaxial technique. In all but the moresophisticated military systems, one transmitter andreceiver channel was all he could generally afford oraccommodate in a typical installation. It is now econom-ically and technically possible to consider much morecomplex multichannel systems involving signal proces-sing -much as the computer involves data-processing.A well known example is the phased-array aerial systemwith a large matrix of microwave radiators or sensors.By controlling the phase [41l and amplitude of the in-dividual element radiations or by processing the signalsreceived by the sensors, a highly controllable and adapt-

able system may be achieved, with a capability for alarge number of parallel or serial functions. Thus suchan array in a radar system might be used for simul-taneous tracking of a number of targets in combinationwith a volume surveillance mode.

Another significant example which is perhaps apointer to the possibility of new systems is the multiplephase discriminator (fig. 7). Phase measurement inmicrowave systems is important, particularly witharrays of sensors [42l, because a knowledge of thephases and amplitudes of signals in each sensor entirelydefines the information available in the system. Inapplications concerned with the measurement of thelocation and the frequency of transmissions, phasemeasurement is particularly useful.

Phase is a cyclic property of radio signals and by in-creasing the circuit time delay between two signals ofidentical frequency or the path difference between twosensors, a phase measurement can be used to determinefrequency or direction ofarrival ofa transmission witha resolution limited only by the time duration of thesignal. However, the greater the resol ution of the phase-measuring circuit, the greater will be the number ofpossible ambiguities in frequency or direction, corre-sponding to the number of signal-cycle periods in therelative time delay or path difference employed. It canbe demonstrared that all ambiguities may be resolved,with the maximum tolerance to phase perturbationscaused by propagation or instrumental effects, with amultichannel system. In one example of this systemthere are N time or path delays with values in the ratiosI : 2 : 4 : 8 : ... : 2N and N + I receivers. With suit-able processing of the phase measurements, the accur-acy of frequency or direction measurement is deter-mined by the longest time or path delay and the instan-taneous frequency or angular coverage by the circuitwith shortest delay.

One of the first examples of such a system is the in-stantaneous freq uency-measuring receiver (fig. 8). Thisunique receiver accepts signals in an octave frequencyrange and instantaneously measures the frequency withan accuracy deterrnined by the number of delay linesand phase discriminators employed and the durationof the signal. A large number of time-interlaced signalsmay be measured over a wide range of signal ampli-tudes and without the use of any tuned circuits or re-ceiver controls. The phase tolerance of the systempermits the accurate measurement of frequency evenin the presence of simultaneous interfering signalswithin a few decibels of the measured signal. The con-tainment of such a multichannel system within a reason-able equipment size and within practical power supplyrequirements is made possible by the use of hybrid andmonolithic integrated-circuit techniques for all the

290 N. E. GODDARD Philips tech. Rev. 32, No. 9/10/11/12

Fig.8. [nstantaneous frequency-measuring receiver. There areseven hybrid-integrated microwave discriminator channels, eachwith associated integrated-circuit video-frequency amplifiers anddigitizing circuits. Simple logic circuits convert the output signalsfrom the discriminators into an accurate, unambiguous set ofparallel binary digits representing the signal frequency. The re-ceiver measures the frequencies of signals in an octave frequencyrange without the use of tuned circuits or other controls.

Fig. 9. One ofthe frequency discriminator units for the frequency-measuring receiver of fig. 8. The unit is made in stripline tech-nique, with a coaxial delay line, and can be plugged in for rapidinterchangeability. Detectors, loads and a trombone-type phaseadjuster are incorporated within the stripline circuit.

microwave and digital signal processing functions(fig. 9).

A similar example of this system technique, againmade feasible by microwave hybrid-integrated circuits,is the use of microwave interferometers in a system forthe landing guidance of all types of YTOL, STOL andconventional fixed-wing aircraft. In this case a lineararray of receivers, with aerial spacings (path delays)chosen to provide optimum sampling over the aperture,is used to measure the angle of arrival of transmissionsfrom an aircraft beacon. Two arrays, one for azimuthand one for elevation (fig. JO), permit the instan-taneous measurement of the bearing angles of a large

number of aircraft over a very wide range of approachdirections in both elevation (glide slope) and azimuth(localizer). Very high angular accuracies and hightolerance to phase perturbations caused by the environ-ment can be achieved.

These are but two examples of important systemsapplications made possible by the new microwave de-vice and circuit technologies in combination with orig-inal ideas on multichannel measurement techniques.The use of cheap microwave circuits in signal-processing functions, characterized by the preservationof phase and amplitude from individual sensors inmultiple arrays, is perhaps one significant portent forthe future of microwave electronics. The physical prin-ciples are well known and the new microwave technol-ogy is available. With the consequent reorientation ofapproach to equipment design no doubt many moreimportant new systems will be generated.

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diode; this issue, page 370.[21J J. de Groot and A. M ircea, Computer calculations of the

Gunn effect; this issue, page 385.[22J J. Magarshack, Gunn-effect oscillators and amplifiers; this

issue, page 397.[23] W. T. Read, Jr., Bell Syst, tech. J. 37, 401, 1958.[24J R. L. Johnston, B. C. De Loach, Jr., and B. G. Cohen, Bell

Syst, tech. J. 44, 369, 1965.[25J D. de Nobel and M. T. Vlaardingerbroek, IMPATT diodes;

this issue, page 328.[26] K. Mouthaan, I MPATT-diode oscillators; this issue,

page 345.127J P. J. de Waard, Anomalous oscillations with an IMPATT

diode, this issue, page 361.[28] R. N. Hall, Proc. IRE 40,1512, 1952.[29] F. C. de Ronde, H. J. G. Meyer and O. W. Memelink, IRE

Trans. MTT-8, 325, 1960.

Philips tech. Rev. 32, No. 9/ I0/11 /12 MICROW AYE ELECTRONICS 291

Fig. 10. A system that has become feasible through the use of microwave hybrid-integratedcircuits, which are both compact and rugged. Here three microwave interferometers are com-bined to give a high-performance system for guiding helicopters and fixed-wing aircraft to alanding area. The interferometer units incorporate distance-measuring equipment and a datalink. Horizontal horn-aerial arrays (about 2 m long) provide azimuth-approach and overshootguidance over a wide range of approach angles with an r.rn.s. fluctuation of about 0.05°. Thevertical array provides simultaneous elevation guidance for approach angles between 2° and20° with an r.rn.s. fluctuation of about 0.1°.

[301 A. Uhlir, Jr., Proc. IRE 46, 1099, 1958.[31J T. H. B. Baker, Electronic Technology 38,300,1961.[32] H. N. Daglish, J. G. Arrnstrong, J. C. Walling and C. A. P.

FoxelI, Low-noise microwave amplifiers, Cambridge Univ.Press, 1968.

[33] T. Oxley and F. Hilsden, Radio and Electronic Engr. 31, 181,1966.

[34J A. Boucher and B. C. Easton, Epitaxial growth of galliumarsenide; this issue, page 380.

[35] D. D. Grieg and H. F. Engelmann, Proc. IRE 40, 1644, 1952.[36] W. E. Frornrn and E. G. Fubini, Proc. National Electronics

Conf., Chicago 1954, page 58.[37] M. Lemke and W. Schilz, Microwave integrated circuits on a

ferrite substrate; this issue, page 315.P. Röschrnann, YIG filters; this issue, page 322.

[38J S. J. Robinson and P. T. Saaier, Philips tech. Rev. 28, 211,1967.

[39] J. H. C. van Heuven and A. G. van Nie, Microwave integratedcircuits, this issue, page 292.

[40] C. S. Aitchison, Lumped cornponents for microwave fre-quencies; this issue, page 305.

[41J J. H. C. van Heuven, P-I-N switching diodes in phase-shiftersfor electronically scanned aerial arrays; this issue, page 405.

[42J R. N. Alcock, Philips tech. Rev. 28, 226, 1967.

Summary. As an introduetion to this issue on microwave solid-state devices the history of device developments and compatiblecircuit techniques is outlined from the speculations of the 1940sto the present state of the art.

In existing equipment and systems solid-state devices canreplace vacuum tubes often with improved performance andsmaller cost in all but the higher power applications. Microwavetechnology is of increasing importance in systems for communi-cations broadcasting domestic electronics and the control andnavigation of all forms of transportation.

Jt is suggested that the full impact of microwave technology onsystems innovation has not yet been realized. Some novel multi-channel equipments with microwave solid-state circuits integratedinto the signal processing functions may be a portent for thefuture.