MIT辐射实验室丛书 V15 CRYSTAL RECTIFIERS

《雷达系统工程》丛书共28本，由美国麻省理工学院辐射实验室集合各方面的专家，总结二战期间参与雷达研制的经验，在1947年由麦克劳希尔出版社出版。Src.1.2]DETECTIONally chosen large compared with the rectifier resistance so that the effecton the output voltage of variation of the forward resistance of the rectifierThe efficiency of rectification is defined as the ratio of the d-c voltageacross the output load resistance to the peak amplitude of the inputsignal. It depends on the ratio of load resistance to the internal resis-ance of the rectifier and the amplitude of the input signal as noted aboveIn the detection of amplitude-modulated waves in radio reception aload consisting of a parallel RC combination is commonly used. Withproper choice of the values of R andc the output voltage will, to a veryclose approximation, vary like the envelope of the amplitude-modulatedwave. Under these conditions, the rectification efficiency of vacuumtube diode rectifiers is normally about 70 to 90 per cent. A detailedanalysis of linear detectors used in radio receivers may be found in stand-ard textbooks on radio engineering and will not be given here. We willreturn to a discussion of the use of one of the crystal rectifier types as ainear detector in Chap. 12.Square-law Detection.The term square-law is applied to a detectorin which the d-C, or rectified, output is proportional to the square of theamplitude of the input signal. It can readily be seen that such a responsedepends on the nonlinearity of the characteristic at the operating pointbe represented by a Taylor expansion terminating in the squared term canOver a limited range the current-voltage characteristic of a rectifieri=f(2)=f(e0)+aed(6e),where eo is the bias voltage determining the operating point, and Se is thesmall input signal voltage. The derivatives are evaluated at the operating point eo. Any rectifier will, therefore, function as a square-lawrectifier when the applied signal is sufficiently small, provided that thesecond derivative of the characteristic does not vanish at the operatingpoint. The linear term is, of course, of no importance as far as rectification is concerned, since it is symmetrical about the operating pointBy means of Eq(1)we can determine analytically the output of therectifier for a given input signal. The analvsis can be summarized briefas follows. Let us consider a signal consisting of a single sinusoidal waveE sin at. In addition to the frequency of the signal, the output willcontain d-c and second-harmonic components with amplitudes proportional to E2. In general, if the signal is composed of a number of sinusoidal components the output will contain, in addition to the frequencycomponents of the signal, the d-c component, second harmonics of each1 For example see F. F. Terman, Radio Engineers Handbook McGraw-Hill, NewYork, 19434INTRODUCTTONSEC. I 3requency component, and sum and difference frequencies formed byevery possible combination of frequencies contained in the input signalThe amplitude of the d-c component will be proportional to the sum of thesquares of the amplitudes of the signal components. The amplitude ofeach second-harmonic component will be proportional to the square ofthe amplitude of the corresponding signal component; the amplitudeof the sum and difference frequencies will be proportional to the productof the amplitude of the input components involved in the combinatioAs an example, let us consider the square-law detection of an amptude-modulated wave given by the expressione= eo(1+ m sin Bt)sin atFor purposes of analysis this wave may be represented by three frequencycomponents, the carrier and two sidebands, with angular frequencies0, and(w+B)tivelyThese are represented graphically inFig. 1 3a. The relative amplitudes ofthe additional components in the out-put of the detector are shown in Fig1 3b for the case where m =0.5The square-la detector is a usefuldevice for the measurement of thepower of an a-c signal because the rectified output is proportional to thesquare of the inpiplAss weshall see later, the crystal rectifier isoften employed as a square-law detectorin monitoring microwave powerIn fact, such a device is serviceable3a outside the square-law region providedFreit is calibrated16)It is clear that the magnitude of9. FIG 1-3. Frequencies involved in the various components arising trominput (modulation percentage = 50): the square term of Eq(1)will be pro(b)additional frequencies in the de- portional to thegnitude of the sec-ond derivative of the characteristic atthe operating point. Maximum sensitivity will then be obtained byadjusting the d-c bias so that the operating point is also the point of max-imum curvature on the characteristic. Other factors of importance in themicrowave region, such as capacitance, noise generation, etc. will be discussed in Chap. 111-3. Frequency Conversion, Heterodyne reception provides a meansof converting the carrier frequency of a signal to a new value. This isSEC. 1-4EARLY USE OF CRYSTAL RECTIFIERSaccomplished by means of a local oscillator and a nonlinear elementThe local oscillator output and the signal are coupled into the nonlineardevice, where they generate-among other frequencies frequencyequal to the difference between the signal and local-oscillator frequenciesUsually, although not always, the local-oscillator power level is largecompared with the signal level. The local oscillation may have either aower or higher frequency than the signal since it is the difference frequency which is usually of interest. Under these conditions the nonlinearelement in so far as it functions in the linear region, generates a differencefrequency called the intermediate frequency, the amplitude of which isproportional to the signal amplitude and independent of the amplitudeof the local-oscillator voltageThe device that contains the nonlinear element and the means forcoupling it to the terminals of the local oscillator and to the input andoutput terminals is called a mirer. The input terminals are used forapplication of signal power and the output terminals are used for deliveryof power at the intermediate frequency. The unit consisting of mixerand local oscillator is called a frequency converter, and the wholeprocess is referred to as“ mixing”or“‘ frequency conversion.”If the signal is an amplitude-modulated wave, the mixer output willconsist of a carrier at intermediate frequency plus sidebands which reproduce the original modulation of the signal. In addition, the nonlinearelement in the mixer will generate harmonics of the local oscillator andhe signal frequencies, sum and difference frequencies of all the appliedsignals, and these in turn will beat with each other to create still morefrequencies and so on, ad infinitum. Fortunately most of these fre-quencies are so weak that they can be ignored. However, some of themare of importance since their existence results in the diversion of powerthat otherwise would appear in the i-f signal. An evaluation of theimportance of these components in microwave receivers will be given inChap 5The nonlinear device used in frequency conversion may be any typeof detector or demodulator. In radio reception, mixer or frequency-converter tubes have been especially designed for the purpose In theheterodyne reception of microwave signals the crystal converter is almostuniversally used at the present timeTHE NATURE OF THE CRYSTAL RECTIFIER1 4. The Discovery and Early Use of Crystal Rectifiers.In the earlydays of the development of radio communication the crystal rectifierwas almost universally used as the detector in radio receivers. a typicaldetector was made by soldering or clamping a small piece of the crystalin a small cup or receptacle. The rectifying contact was made with aINTRODUCTIONISEC. 1-5fexible wire cat whisker which was held in light contact with thecrystal. Good rectification was obtained only from sensitive spots onthe crystal and frequent adjustments of the contact point were necessaryfor good performanceThe development of thermionic tubes made the crystal rectifierbolete in radio receivers. From about 1925 to 1940 the crystal rectifierwas used chiefly as a laboratory device for detecting and monitoring uhfpower. A combination of a silicon crystal and a whisker of tungsten ormoly bdenum was found to be among the most sensitive and was commonly used for this workA typical application of the crystal rectifier in early microwave workdescribed by Southworth and King. 2 A calibrated crystal rectifierwas used by them to measure relative gains in an investigation of metalhorns for directive receivers of microwaves in the region of 10 to 15 cmThe rectifier was made with a silicon crystal and a whisker of 8-miltungsten 2 mm long. The crystal was ground into a cylinder 1 mm indiameter and 1 mm long and pressed into a hole bored into the end of ascrew. The surface of the crystal was carefully polished so that thecontact could slide freely over the surface in seeking a sensitive pointThe rectifying contact was adjusted by advancing the mounting screwand tapping the mount until the ratio of back-to-front resistance was inthe range of 2 to5. It was found that with moderate care an adjustmentcould be maintained fairly constant for several weeks. The d-c outputcurrent of the crystal was used as a measure of the absorbed r-f power15. Recent Developments.Reception of microwave radar echoesrequires a high-gain receiver in which the limit of sensitivity is determined by the masking of the signal by the noise generated in the receivercircuits. The receiver must therefore be designed to introduce a minimum of noise into the input circuitOne approach to the receiver design problem is to employ low-leveldetection of the r-f signal pulse, followed by amplification of the resultantvideo pulse. Because of its relative insensitivity as compared to superheterodyne reception, this method has been used only for beacon receiverswhere sensitivity is not ot prime inportanceAnothele approache amplf the received signalt microwave frequencies. For this purpose amplifier tubes have beendesigned and constructed at the radiation Laboratory for amplificationat a frequency of 3000 Me/sec. The best of these are comparable inperformance to superheterodyne receivers using crvstal mixers. However these tubes are difficult to make and have not been manufactured on1 G. C. Southworth and A. P. King, Metal Horns as Directive Receivers ofUItra-Short Waves, Proc. I.R.E., 27, 95(19392 H. V Neher, RL Report No 61-24, July 10, 1943SEC. 1.5RECENT DEVELOPMENTSa mass-production scale. It is unlikely that an r-f amplifier can competewith crystals at higher frequenciesThe Development of the Mixer Crystal.-The early microwave receivers'used. mixer tubes especially designed for high-frequency applications,such as the Westinghouse 708A and the British Cv58 diode. Howeverthe best of these were noisy, and the noise output increased with frequency. Consequently attention was turned to the crystal rectifier as apossible substituteThe superior performance of the crystal mixer led to further researchand development work, which has continued to this time. The broadgeneral objectives of this work may be put into three general categories,which obviously are mutually dependent:1. The development of manufacturing techniques for quantity pro-duction of high-quality rectifiers for use in the region from 3,000to 30,000 Me/sec.2. Fundamental research on semiconductors, point-contacta rectifica-tion, and the theory of frequency conversion and noise generationat microwave frequencies3. The development of methods and equipment for measuring per-formance and for laboratory and production testingThe extent to which these objectives have been achieved will beindicated in the appropriate following chapters. Discussion here willbe limited to outlining the salient features of current manufacturingtechniques which have produced the crystal rectifier in present useThe research work has been concerned exclusively with silicon, germanium, or boron, but all of the cartridges manufactured commerciallyfor mixers have used silicon crystals. Extensive research on germaniumas resulted, however, in the development of the high-inverse-voltagerectifier and the welded-contact rectifier mentioned laterInvestigation into the possibility of preparing sintered or meltedpellets of boron for use as crystal rectifiers begun in 1943, was successfulellets of pure boron were prepared as well as pellets to which were addedselected impurities in varying amounts. Some of the "doped"pelletsshowed sufficient conductivity to be of interest but exhibited no truerectification. A typical characteristic curve is S-shaped and symmetricalCrystal mixers were used in some early experimental sets. The crystal andcat whisker were independently mounted in the mixer and the contact was adjustableIt should be pointed out here that the copper oxide rectifier and selenium rectifierboth developed commercially, are not point-contact rectifiers. They are, however,contact rectifiers since the rectifying property is obtained by the contact of a thin filmof semiconductor with the metal on which it is deposited We shall be concerned inthis book only with the point-contact rectifier8INTRODUCTIONSEC. 1.5about the origin. All of the pellets were poor rectifiers and the projectwas droppeThe first mixer crystals made by british Thompson-Houston, Ltdused commercial silicon of about 98 per cent purity. These crystalsexhibited the usual sensitive spots, and exhibited considerable variationin sensitivity from lot to lotCrystals of commercial silicon were used in the early rectifiers madeat the radiation Laboratory and the performance of these units in mixerswas comparable to that of the bTh unitMuch of the research was aimed at perfecting a design of cartridgeparts and procedures for assembly and adjustment that would achieveelectrical and mechanical stability, uniformity of r-f and i-f impedancesimproved sensitivity, and decreased noise output. In general it may besaid, however, that the crystals produced at this stage of development leftmuch to be desired; it was common practice then, as a first step in improv-ing performance of radar systems, to replace the crystal rectifier with anew oneAnother important advance in the development was high-burnoutcrystals of which the British red-dot"(so-called because of the cartridgeidentification marking) crystal developed by the General Electric Ccmpany, Ltd. was an example. This early high-burnout crystal dissipatedrelatively large amounts of power without appreciably impairing its per-formance as a mixer. The most significant feature of its manufacturewas the preparation of the silicon crystals. These were obtained frommelts made of highly purified silicon powder to which was added a frac-tion of a per cent of aluminum and beryllium. The crystal surface wasthen prepared by a carefully controlled process of polishing etching andheat treatmentNow it was already well known from the theory of semiconductionthat the conductivity and hence the rectifying properties of silicon areaffected by the presence of small amounts of impurities in the crystals.This fact, together with the success of the red-dot procedure stimulatedthe initiation of research along similar lines at various laboratories. Atthese laboratories adequate manufacturing procedures were then deveoped for large-scale production of high-burnout, high-sensitivity rectifiersThese units have a mechanical stability comparable to vacuum tubesand, under proper operating conditions, a comparable lifeTwo important advances in the art will be mentioned here The firstof these was suggested by Seitz, who initiated a program in connectionwith the Experimental Station of E.I. du pont de Nemours and Com-pany for the development of a method for the production of high-purity1 F. Seitz, " Compounds of Silicon and Germanium, NDRC 14-112, U of PennJune 1942SEC. 15rEcENT DEVELOPMENTSsilicon. They succeeded in producing silicon with a spectroscopic purityf better than 99.9 per cent. Other impurities not detectable by spectro-scopic analysis, however such as carbon, were present in larger amountsIngots made with this silicon to which is added an appropriate impuritare remarkably uniform in conductivity and rectifying property. Recti-fiers using this silicon were high-burnout, low-noise units markedlysuperior in performance to units made using the same techniques, butmade with commercial silicon from other sourcesThe second important advance was the discovery that boron was anunusually effective impurity agent in increasing the conductivity of high-purity silicon. In 1943 it was reported that the addition of boron inquantities of the order of 0.001 per cent resulted in a converter of improvedsensitivity compared with those utilizing other impurity agents. Thiscrystal was also highly resistant to burnout. As a result, " borondoping ' is now widely used in silicon-crystal manufactureA considerable amount of exploratory work has been done on the effectof various impurity agents, but there has as yet been no exhaustivesystematic doping program for silicon. Moreover it is not yet understoodwhy certain impurity agents are better than others in improving burnoutand sensitivity, nor is the process of noise generation in crystal fullunderstood. Finally the etching, polishing, and heat treatment of therectifying surface have been largely empirical developments. The neteffect is that the manufacture of high-quality crystals is still somethingof an art, attained through long experience and careful control of thetechniquesThe Development of Special Types -The term"special types"refersto crystal rectifiers developed for applications other than frequencyconversion. They are special''only in the sense that the principalinterest and effort to date have been on the converter application.Among these special types there are three on which considerable workhas been done: the video crystal for low-level detection, the high-inversevoltage rectifier, and the welded-contact rectifierThe term video crystal commonly means a crystal rectifier thatused as a low-level square-law detector of microwave pulses. The videooutput voltage of the detector is amplified by a video amplifier. Such areceiver is commonly called a crystal-video receiverThe crystal-video receiver was developed somewhat later than thesuperheterodyne receiver to meet the need for a wideband beacon receiverthat would respond to pulses over the range of frequencies encountered1 This statement does not apply to units made using the technique developed atthe Bell Telephone Laboratories.2 H. C. Theuerer, "The Preparation and Rectification Charactcristics of Boro-silicon Alloy, " BTL Report MM-43-120-74, Nov 2, 1943.10INTRODUCTIONin interrogating transmitters. The sensitivity of the crystal-videoreceiver is low compared with that of a superheterodyne receiver, butthis is not prohibitive in the beacon application since the signal level forone-way transmission is high compared with the level of echoes at a com-able range. Wideband superheterodyne receivers have been designedfor beacon reception and have been used in some beacon sets. Howeversuch receivers are diffcult to adjust and are not so light and compact ascrystal-video receivers-considerationg which are of importance in portable beacons. The crystal-video receiver has therefore found extensiveuseThe requirements of the video receiver place limits in particular onthe video resistance of the detector. (See Vol. 3. Video crystals forhe first beacon receivers were selected from the mixer crystal productionfor not all good mixer crystals are good video crystals. It was soon foundthat special procedures were essential to produce good video detectors foruse in the 3-cm region. These special procedures involved special surfacetreatment and adjustment for a very small contact area. Since the latterrequired a small contact force mechanical stability has been diffcult toattain. A cartridge of the coaxial type described in the next chapter, issomewhat more stable mechanically than a cartridge of the ceramic typeIn addition to its use as a low-level detector in the crystal-videoreceiver, the crystal rectifier has also been used extensively in probes andmonitors of microwave power. Special ty pes, however, have not beendeveloped for this purpose. Within the range of square-law response therectified output current of the crystal rectifier may be used directly as arelative power indication. Since, however, the range of square-lawresponse is limited to a few microwatts of r-f power and since the rangevaries from crystal to crystal, it is desirable to calibrate the detectorexcept for very low-level work. To date no special effort has been madeto develop a type having a square-law response over a wide range ofInput power.The high-inverse-voltage rectifier was discovered during the course ofthe research on germanium by the ndrc group at purdue UniversityThey conducted a systematic investigation of the effect of a large numberof impurity agents on the rectifying property of germanium. Theyfound that rectifiers made with tin-doped germanium maintained a veryhigh back resistance for inverse voltages of the order of 100 volts and atthe same time exhibited high forward conductance. Other additionagents that produce these properties are N, Ni, Sr, Cu, and Bi. However,the most consistent production of high-invorse-voltage germanium hasbeen obtained with tin; such rectifiers arc in gencral inferior as mixersThe discovery of the high-inversc-voltage property led to considerablinterest in their use in many types of circuit applications at intermediate