Contains reports on three research projects.Office of Scientific Research and Development (OSRD) OEMsr-26

Cerrillo, Manuel V.

Fano, R. M.

Gilbert, Eric

Gray, T. S.

Redheffer, R. M.

Tuller, W. G.

Zabel, C. W.

Zaffarano, F. P.

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IV. Communications and Related ProjectsA. Noise, interference and Distortion in Pulse ModulationStaff: Mr. I. G. TullerprM4~ ental 34ok - A small gas tube ulser, capable of giving pulses with verysteep rise and decay and with very flat top3, has been built. It has been usedto modulate klystron amplifiers, and the resulting spectrum measured. Particularattention has been paid to far-out spectra, in an attempt to check the inter-ference band width of a pulse modulated transmitter. Some small attempt has ,been made to check the effect of cavity resonator filters on pulse shape andspectrum, but this whole project needs much more design time spent on specialpulse viewing equipment, since the presently available envelope viewers areinadequate in one way or another,A pulse generat- or suitable for u3t in pulse ommunicatione researchhas been constructed and is in operation. This unit aeliver, at a repetitionfrequency of IC KC, a triple-pulse coded sychronizing pulse, a phase modulatedpulse, ,and a width modulated pulse. The mean position of either the phase orwidth modulated pulso may be placed in any ne of ten "channels" with respectto the synchronizing pulsa, or one may be continuously varied anywhere in the-cycle while the other ramans fixed. The generator also delivers 100 EC, andI MC siauscidal voltages, locked to the p.r.f., for time measurement purposesor high speed sweeps.. Palse output voltage from this unit is .0 volts; meandurition of palses ar'bibrarily set at one mlcrosecond but may be varied.A high power video amplifier to amplify the output of the pulse generatordescribed above has been bread-boarded anO is awaiting delivery of the two outputtubes for complation. It will deliver pulse voltages adequate for the modulationof cou, magnetrons or other tubes aeeding more than .0 volts for modulation.A pulse decoder, companion unit to the generator diescribed above hasbeen constructed and is now being debugged. This 6evice takes the detectedpulse- output of' a receiver recsiving transmission modulated by the pulse pen-orator and vill, ,hen completed, convert these pulse modulated signals into somesort of replica of the original modulating intelligence. Two methods of decodingpulse lie p mccs uacalated aignal will be tested. The one, on which amplitude and- 39 -frequency distortion tests have been made, uses a variant of the radar automaticranging circuit to give an output voltage proportional to the position of thecenter of gravity of the modulated pulse, The other method is the more conven-tional one of converting the phase modulated.pulse into a wide width-modulatedpulse, which is then decoded by integration. It is expected that the C. G,determination method, which is novel, will give,3 db. better signal-to-noiseratio than the- conventional method,Theoretical Work , A complete analysis of the distortion inherent in the Pounddiscriminator has been made. This analysis is also ap~lieable to the Foster.Seeley discriminator, widely used in ordinary F. M. rpcelvpra but Pbout Whichno distortion analysi- has been -ubli hed. A refort ii in trocpa on thiana!ysi .Preliminary investipation o? theq "prtlevy Lyw ,elntin" IntAl1IP ne,band width, and time of transmission have .shown this l w to be a valuable toolin -he analysis and synthesis of comnrmication systems, and a few analyticalconclusions have been drawn. The effect of' noise on this law has been consideredin a qualitative manner,Some work has been done on an ana.lysis of the automatic ranging circuitused as a pulse phase modulation detector. More needs to be- done, but theresults so far obtained check qualitativel3 with expariment.Specifications have been drawn up for equipment useful in thoroughcommunications research, in the hope that r, staff adenuate to design and constructthis equipment will some day be availablThe effect of pulse length on sigWal tonoise ratio in pulse phasemodulation has, been briefly analyad.Coincidence detectors and integrators for improving signal-to-noiseratio of pulse phase modulated signals have been designed on paper.A. qualitative study of the effect of interference on pulse-phasemodulated signals has been made,IV. B. Stabilized Oscillator Problems1. Modulation and StabilizationStaff: Mr. F. P. ZaffaranoWith minor exceptions, little in addition to repeating the experi-mental work done by R. V, Pound on stabilized oscillators has been accomplishedduring this period, Since' the modulation of stabilized oscillators and not theactual stabilization is the eventual objective, more emphasis has been centeredon the system video frequency characteristics than in PoFinds e xperiments.STwo d.c. stabilizing units have been located, repaired, and put intooperation using the production type double magic tee assemblies available. Avariation of this plumbing assembly to, give btter stabilizing characteristicsfor larM changes in oscillator power was used successfully. Although severaldesirable changes in these units were eonsidered, it was decided to go directlyto the i.fo type stabilizer which had the advantage of having several additionalparameters which could be varied for modulation purposes while simultaneouslyproviding better- stabilizationRapid, alignment of the 30 and 10,000 mc phase adjustments of the twoif, type stabilizers built has been facilitated by using a sawtooth sweep andoscilloscope to plot the discriminator characteristic continuously,Both the d.c. and iof. stabilizers in the form described by R. V, Poundin Reports 815 and 837 have upper half power frequency limits at approximately20 kilocycles. This is due to the capacity intentionally placed across thereflector circuit to prevent oscillation of the system which occurs as a resultof phase reversal of the feedback (the oscillation takes place at approximately1,2 me). Increasing the usable frequency response of the stabilizing elements- tothe order of 2 - 10 megacycles is the present problem at hand*Data on the following items have been obtained in widely varyingdegrees of completeness:a. D.C. versus I.F, stabilizer(L) Overall pain at d.c(2) Frequency-amplitude characteristic(3) Phase-frequency characteristio(4) Stabilization - frequency characteristic(5) Optimum adjustment of variables.b. frequency Modulation of Systems(1) Crystal bias modulation- 41 -(2) Modulation of seandard cavity(a) Mechanically(b) With electron beam(3) Direct reflector voltage modulation(a) With low frequency sabilization only(b) With linearizing stabilization(4) Modulation of source, of IF., frequency(a) I-P phase(b) I-r amplitude(c) I-F frequency(5) Modulation with separate crystal in waveguide circuitThe phase detector (lock-in mixer) of the if, type stabilizeris the present limiting fantor in video response, The "Brute Force" attackusing highzer rf, levels and small plate load resistances, would seem to bethe most promising. It may be possible, however, to bridge the present phasedetector with another detector using a video amplifier. By arranging thecut-off fr -qencies of each detector the combined phase response may bemade to remain essentially constant up to the desired high freqaency'limit.Both attacks will be tried.2. D0.C Amplifier Problems-Staff' Prof. T. S. GrayStudy of the stability problems in d. . amplifiers for use inmeasurement and control applications, and for improvement of microwave discrimina~tore is proposed. In particular, attention will be given to the series-balancedcircuit proposed by Artzt, and a study made of its suitability and practicallimits of amplification for use in a cathode ray oscillograph, and in thediscriminator work described above.IV. C, Circuit ProblemsI. Study of Waveguides With Dissipative WallsStaff, Mr. R. M, FanoWaveguides with highly dissipative, walls have been used uccess-fully in the design of broad band high-poweT loads, It was found in thisconnection- that, if a rectangular guide with highly dissipative walls isjoined to a non-dissipative guide of the same internal dimensions, there isalways a frequency at which a perfect match is obtained. rxperiments showedthat this frequency satisfie th equation;M. Artet, Electronics 18 (A~g. 1945), 112-118- 42 -f I 2bfI awhere b and a are the narrow and wide dimensions of the guide, respectively.It should be possiblie to justify the above equation theoretically, at least inthe case of relatively small dissipation. FArthermore, it would be usekfl to!now the reflection which takes place at other frequencies as a function ofthe guide attenuation, Such knowledge would permit the design of matched loadsand pads with specified characteristics. This question is presently beinginvestigated.2. The Broadbanding of Microwave NetworksStaff: Mr. R. M. PanoThe first question to be investigated in this connection is theexistence of theoretical limitations on the bandwidth when the maximum VSWR isspecified. A solution of this problem will facilitate later the developmentof practical broadbanding techniques. Work along these lines will begin verysoon, using as a starting point the work done on microwave filters during thewarZ. The Extension of the Methods and Procedures of the Circuit Synthesisto Active Networkcs Involving Vacuum TubesStaff* Mr, M. V, CerrilloThis problem was suggested by Professor Guillemin, because not muchconstructive and general work has been done in this connection. Most of thesolutions are given for very specific and particular cases. Mathematical dif-ficulties are the causes of' these- restricted solutions.The way in which the investigation will be conducted to obtain moregeneral solutions is as followsgAn active system will be looked upon as an extended passive one inwhich the variables must- satisfy a specific set of constraints. In thisfashion, vacuum tubes will be easily introduced. For a passive system, thecurrents are such'as to keep a minimum the power dissipated in-the network.Then, the Calculus of Variations, and to be more specific, the problem ofLagrange will be used as the fundamental mathematical tool in this investigation.4. Transients in WaveguidezStaff: Mr. M. V. CerrilloPreliminary work has been started to investigate the transientphenomena in waveguides. Because of the variable, phase constant as a functionof frequency, the wavegiides act like a dispersive medium. The formulation ofthe problem is straightforward, consisting of setting up the proper- 43 -Fourier Integral. It is- epeczd to obtain thae oluion in spries form, Thisproblem was suggested by Dr, Chu,5. Miscellaneous Waveguilde ProblemsStaff: Mr. R. M0 RedhefferThe following problems have been studied.a. Brief invostigation of the air-guide eqcivalences, withregard to the fact that phase is measured perpendicular to the interface inone case, usally, but perrendicular to the wave front in the other.(b) Brief study of the limiting error from probe reflectionas the- load reflection approaches unity,(c) Investigation of the effect of impurity of the frequencyon apparent reflection of a short circuit as measured by two methods,(d) Derivation of a simple but exact free-space relation, andof approximate waveguide relations, for dielectric constant and loss in termsof interface reflection.(e) Note on the possibility of obtaining a probe with highpick-up but zero probe-error by using a tuner which moves with the probe.(f) Investigation of the effect of temperature change on thebridge method of dielectric measur ment.(g) Development of a new method of cancelling error due todirect pick-up between antennas when reflection is measured at arbitrary incidencein free space.(h) Invest'gation of means for finding loss from interfacereflection or from transmission of a sheet at Brewster's angle in free space,6., Impedance of Magic TeesStaff: Mr. . W, ZabelMr. E. GilbertThere are two general types of ring circuit magic tees (colloquially referred to as "rat races!'): (1) The type employing a ring whose cir-cumference is an even number of wavelengths long, and (2) the type whose circum-ference is a wavelength plus an odd number of half wavelengths long, The rat raceused in the balanced dnplexers, is of the second type and has been successful at3.3 cm. A similar design at 10 cm, was not entirely satisfactory.A program was started to find out which of the two types of magictees would be least frequency-sensitive and what design considerations mist bemade to- be able to construct- a satisfactory rat race for various frequencies andguide sizes..Mr. E. Gilbert calcalated the frequency sensitivity of both con-structions and found the first type to be much more frequency sensitive thanthe second. For a 12% band at 3.3 cm, the SW R for the first type was<l(.5,while for the second type the corresponding figure was 1.16, The entire VSWRvs wavelength cuLrve calculated for the second type agreed only roughly withthe measured curves for the duplexar rat race, An unsucceessful attempt toexplain this disagreement was made by assuming the X-plano T in the rat race -to have an equivalent circuit of a simple shunt capacitance.It was found that no theoretical data existed for the equivalentcircuits of the B-plane T of the dimensions used in rat race, An X-bandbench was set up and the equivalent circuit measured in two different ways.Due to an unfortunate choice in dimensions of the apparatus the precision of the,experiments was not good enough to determine all the parameters of the equivalentcircuit,Mr, N, Mwrcuvitz was consulted as to the possibility of extending therange of dimensions in the theoretical solution of theE-plane tee, Thisextension will be available within the next few weeks. Assuming this informa-tion available, and by using the eigenvalue method of Dr. R. H. Dicke, simplerelations were found to exist between the various parameters of the rat racein order to result in a magic tee, These have been determined. only for the ratrace of Type 1.- 45 -

Communications and Related Projects

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Communications and Related Projects

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