PROCEEDINGS OF THE I.R.E.-Waves and Electrons Section

three-phase power to six-phase power. With the eighthchannel used for synchronizing pulses, seven channelstherefore result. Thirty-five channels would result fromthe breakdown of a cycle into 10-degree samplings.

In another pulse-position system, a subcarrier trans-ducer system is employed in order to make it unneces-sary to introduce reference level markers. The quiescentfrequency of the subcarrier rather than the quiescenttime position of the pulse determines zero level. Thetime interval between the marker pulse and each channelpulse is varied at an audio rate by the incoming signalfrom the transducer. The oscillator-frequency outputis then used to modulate a channel whose pulse outputis transmitted, received, and demodulated to producethe original signal. When it is desired to telemeter avoltage, it can control the frequency of the oscillatordirectly, just as is the case with frequency-modulatedsubcarrier systems employing phase-shift oscillators.Beyond this the similarity between these types ofsystems ends.

Restrictions imposed by security regulations pro-hibit a complete discussion of Navy pulse telemeteringsystems.

CONCLUSIONS

Numerous telemetering systems have been proposed,

and a number have been built which are as different asthe functions to be telemetered.

Telemetering systems may be compared on the basisof several factors:Bandwidth is only a factor when the end instruments

and transducers warrant an equally high time resolu-tion.With a number of island stations, range is not as

important as reliability with various missile attitudes.Reliability is a factor which is based largely upon the

number of links in the chain of circuits.Speed in equipping and calibrating before flight is

important in a test program, and depends largely uponthe number of items which can go wrong.

In small target pilotless aircraft, space is a particu-larly critical factor, and the volume of the equipmentdepends again upon the number of circuits, power, andnumber of functions to be telemetered.The time required to. make an installation, and the

cost of the telemetering installation, is largely a functionof the number of functions to be telemetered; however,this cost is low compared to the missile which is ex-pended in a test flight. A failure of the telemetering re-sults in the waste of a large sum because of the loss ofthe necessary data, necessitating the complete condi-tioning and instrumenting of another missile.

A Waveguide Bridge for Measuring Gain at 4000 Mc*A. L. SAMUELt, FELLOW, IRE, AND C. F. CRANDELL$, MEMBER, IRE

Summary-A bridge has been constructed for measuring thegain and phase delay of amplifiers in the vicinity of 4000 Mc. Theequipment is described, and the methods employed to reduce thepossible errors are discussed. The general method may be adaptedfor use in any desired frequency range.

7fl RIDGE CIRCUITS, although much used atIV) lower frequencies, have not been applied to any

great extent at the high radio frequencies. Thepresent paper describes a waveguide bridge which hasbeen constructed to measure gain (or loss) and phasedelay of devices at frequencies in the vicinity of 4000Mc. The general method consists in comparing the out-put of the device under test to its input by means of anull balance. As in any ac bridge, this balance ipavolvesboth amplitude and phase. The amplitude balance is ob-tained by varying a calibrated attenuator, the calibra-

* Decimal classification: R255.11. Original manuscript received bythe Institute, December 1, 1947. Presented, 1948 IRE National Con-vention, March 24, 1948, New York, N. Y.

t Formerly, Bell Telephone Laboratories, New York, N. Y.; now,University of Illinois, Urbana, Ill.

$ Formerly, Bell Telephone Laboratories, New York, N. Y.; now,Southwestern Bell Telephone Company, Dallas, Texas.

tion giving the gain directly. Phase balance is obtainedby moving the position of a pickup point on a standing-wave detector, the distance by which this point mustbe moved when the amplifier is replaced by a passivecircuit of known characteristics (usually a short sectionof waveguide) being a measure of the phase shift. Bal-ance is indicated by a null observation on a cathode-ray oscilloscope which simultaneously presents the out-put versus frequency characteristic of the amplifier on asecond trace.

GENERAL EXPLANATION

A photograph of the complete bridge is shown in Fig.1. It consists of (1) the high-frequency oscillator anddriving amplifier with their coaxial-line connectionslocated on the extreme upper right corner of the bench,and partly obscured by the waveguide assembly; (2) thewaveguide assembly which constitutes the bridgeproper; and (3) the associated power supplies, monitor-ing circuits and switching circuits, etc., which aremounted on the racks to the right and left and on theshelf above the waveguide unit.

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Samuel and Crandell: Waveguide Bridge for Measuring 4000-Mc Gain

Fig. 1-The complete bridge assembly with all auxiliaryequipment and power supplies.

The waveguide section only is shown in greater de-tail in Fig. 2. Power from the high-frequency oscillator,shown in the upper right with its motor modulating de-vice, is supplied to the driving amplifier (partially hid-den by the waveguide assembly) by means of a flexiblecoaxial cable and from there to the input of the wave-guide which is located just outside the picture to theright. The unseen portion of guide to the right containswave meters and an impedance-matching unit. Enter-ing the visible portion of rectangular waveguide fromthe right, the first unit encountered is an input attenua-tor (with circular dial), then the input directional cou-pler (the three-tier guide section), where samples of in-put power and input reflected power are obtained. Theamplifier under test is in the center foreground betweenthe input directional coupler to its right and a similaroutput directional coupler to its left. The travelingprobe of the standing-wave detector, with its flexiblecoaxial output, is seen behind the amplifier. The cali-brated attenuator with its dial indicator (white faced)can be seen in the waveguide section between the outputdirectional coupler and the traveling probe.

Fig. 2-The waveguide bridge proper showing the arrangementof the parts.

MEASUREMENT OF GAINA simplified schematic of the bridge is shown in Fig.

3. The wave pattern existing in the section of guide con-

taining the traveling probe can be considered as result-

ing from two independent waves. One of these, propor-tional to the input power to the amplifier under test,is traveling to the right, while the other is proportionalto the output power, and is traveling to the left. If theloss introduced by attenuator No. 1 is exactly equal tothe gain introduced by the amplifier (assuming thedirectional couplers to be identical), the amplitudes ofthese two wave components will be equal, producing aresulting standing-wave pattern with a series of nodes.A null balance can therefore be obtained by movingthe pickup probe along the slotted section to one of thesenodal points, and by a critical adjustment of theattenuator.

Fig. 3-A simplified schematic of the bridge illustrating thebasic principle.

It should be noted that this bridge compares the for-ward power in the input line with the forward power inthe output line. The bridge, therefore, measures gaindefined as the ratio of the power impressed on the out-put line to the power impressed on the input to theamplifier. The actual power delivered to a useful loadmay be reduced by an impedance mismatch, and thepower absorbed by the amplifier input may be similarlyreduced by an input mismatch. Measurements made un-der mismatch conditions must be properly interpreted,but they are otherwise perfectly valid. However, it iscustomary to make gain measurements under matchedconditions where the distinctions between availablepower, impressed power, and absorbed power are of noconsequence.

Although the circuit shown in Fig. 3 is perfectly usa-ble for the measurement of gain on an unmodulated cwbasis, the measurement is greatly facilitated by the useof a frequency-modulated source which, in effect, pe-riodically sweeps the frequency of the high-frequencyoscillator through the pass range of the amplifier. Bymeans of a synchronous switch the oscillator is turnedoff for every alternate half cycle of the frequency ex-cursion, so that the frequency sweep is unidirectional.The off period also provides a zero reference line on theoscilloscope. Under these conditions it is possible to ob-serve the band-pass characteristics of the amplifier as afunction of frequency. By means of a channel switchone may view this band-pass characteristic on theoscilloscope at the same time that the oscilloscope isbeing used to observe the null balance of the bridge.Of course this balance will now occur at only a singlefrequency (within the range being swept) for which thegain and phase setting are correct.

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PROCEEDINGS OF THE I.R.E.-Waves and Electrons Section

Fig. 4 is a schematic drawing of the complete bridgecircuit. It includes the frequency-modulated source, to-gether with a number of additional features which have

I

Fig. 4-Schematic drawing of the complete bridge.

not yet been described. Small sketches indicate the ap-

proximate appearance of the signal (or its envelope)existing at various parts of the circuit. These plots areagainst time as an abscissa, but in most cases they can

be thought of as plots against frequency because of thenature of the frequency modulation on the high-fre-quency oscillator. Fig. 4 is a composite drawing andcontains features which are not all used simultaneously.The two-channel display on the oscilloscope makes it

possible to adjust for a null at any desired point in theamplifier pass band, while simultaneously insuring thatthe pass band is of the desired shape. The appearance

of the oscilloscope is indicated in Fig. 5. One trace

Fig. 5-An unretouched photograph of the oscilloscope screen show-ing the simultaneous display of the amplifier pass-band charac-teristic and the bridge balance indication.

shows the bridge balance, while the second shows thepass-band characteristic, which is in the form of theusual resonance curve. Both curves are plotted againsta common abscissa scale proportional to frequency. Thenull point is the center point of the 'W" where it comesinto alignment with the horizontal zero lines. The am-

plitude rises rapidly for frequencies to either side of the

balance. A narrow-band amplifier is used in the bridgeoutput channel to prevent overloading at far off-balance frequencies, thus giving the "W"-shaped pat-tern. This enables one to operate the bridge under condi-tions of maximum sensitivity. The location of the nullpoint along the abscissa scale corresponding to themaximum output on the other trace indicates that thegain is being measured at this point in the pass band.

GAIN MEASUREMENT ERRORSReflections in the balancing arm of the bridge will

introduce an error in the measurement of gain. If thedirectional couplers, their terminations, and the wave-guide components, particularly the elbows, are not allproperly matched in impedance, reflections of one wavecomponent will be added to the other wave componentin random phases, depending upon the exact locationsand magnitudes of the discontinuities which producethem. Furthermore, the relative phases between the out-put wave sample and the input wave sample, and hencethe relative phases between the direct waves and theadditive reflected-wave components, will depend uponthe exact phase delay through the amplifier under testand upon the frequency. The balance obtained underthese conditions is dependent upon these reflections ina complicated way, and an error is introduced whichcannot be allowed for in the calibration. Instead, thiserror must be reduced to a tolerable value. Care wasexercised in the construction of the individual circuitcomponents and, in addition, they were all individuallytrimmed to match the impedance of the standard wave-guide. Adjustment screws opposite the slots of the direc-tional couplers (the top ones being visible in Fig. 2), wereused to compensate for the reflection caused by theseslots. The elbows were designed to be reasonably wellmatched, with the final trimming being provided bytuning plugs. The terminations were of a design whichmatched the waveguide with little or no trimming. Inspite of all possible precautions of this sort it was notfound possible to adjust the individual components tomuch better than 0.1 db standing-wave ratio over thedesired frequency band. Since there are a relativelylarge number of these discontinuities in the system, theover-all error could easily reach 1 db or more. The ef-fect of these discontinuities was further reduced byintroducing 10-db attenuating pads on each side of theslotted section, as shown in Fig. 4. Because of thesepads, the output signal sample on the input side of theslotted section is reduced in level by 20 db, with respectto the level of the input signal sample at this same loca-tion. The input signal sample is similarly reduced by 20db, with respect to the output signal sample on the out-put side of the slotted section. This reduces the effectivesize of the reflected signals by 20 db, and greatly im-proves the accuracy of the bridge. The attenuating padsthemselves must not introduce serious reflections; thepads actually used were somewhat better than 0.1 db

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Samuel and Crandell: Waveguide Bridge for Measuring 4000-Mc Gain

over the necessary band. The maximum error from thissource is probably well under 0.2 db.

Errors in the measurement of phase will occur as aresult of differences in the lengths of the waveguidepaths traversed by the input and output wave samples,and because of the variations in phase delay through thecalibrated attenuator. No serious attempt has yet beenmade to evaluate or reduce this type of error.

MEASUREMENT OF COMPRESSION

An additional power amplifier, as shown in Fig. 4, isof value when one desires to study compression; that is,the variation in gain with power level. It cannot be usedif one wishes to examine the pass-band characteristicof the amplifier tube under test because of the effectof its own relatively narrow pass band on the resultingpicture. This driver amplifier can be arranged so thatits output is switched at a 30-cps rate between twodifferent power levels, which may be adjusted so thatthe power output from the tube under test is in turnalternated between two desired levels, say 1.0 watt and0.1 watt. It is, of course, necessary to remove the fre-quency modulation on the high-frequency oscillator andto operate the amplifier on a cw basis, in order to meas-ure the output power levels by the thermistor watt-meter and to adjust the drive to the desired values.After this has been done, the frequency modulation canthen be restored for the gain measurements. By ap-propriate switching means, it is possible to observe thebridge balance signals corresponding to the two differentlevels, simultaneously, each on a separate oscillographtrace. If there is any difference in the tube gain at thesetwo power levels, only one of the traces can be made tobalance at a time. The amount by which the "Amp.Gain" attenuator must be changed to obtain a balanceon the second trace is a measure of the compression be-tween these two power levels. The ability to measurethe difference in gain between the two power levels whilethe tube is being alternately pulsed between these twolevels eliminates errors due to drifts in adjustment andto long-time variations in gain caused by heating ef-fects. Changes in gain can be measured to an accuracyof the order of 0.1 db by this method, even though theabsolute accuracy of the bridge is probably no betterthan 0.2 db.

MEASUREMENT OF BANDWIDTH

Bandwidth measurements are made by employing thecircuit shown on the bottom of Fig. 4 to provide twofrequency-marker pips which can be switched on onechannel of the 'scope at the same time that the pass-bandcharacteristic is being presented on the other channel.The appearance of these marker pips is illustrated inFig. 6, where they are shown together with the pass-band characteristic of the amplifier. These pips areseparately adjustable as to amplitude, mean frequency,

and frequency spread, so that it is relatively easy tomeasure the difference in frequency between any twodesired points on the pattern, say between the 3-db-downpoints. The calibration of the ordinate scale on thepass-band characteristic may be made in terms of acalibrated attenuator in the output circuit.

Fig. 6-The frequency marker pips as they appear when viewedwith the amplifier pass-band characteristic.

SIGNAL MONITORS

The bridge is arranged to furnish, in addition to theamplifier output, two other monitoring signals whichcome from crystals connected to either end of the topdeck of the input directional coupler. They are sche-matically shown in Fig. 4, where they are labeled 'Re-flected Power" and "Forward Power." The signal fromthe 'Forward Power" crystal represents the power sup-plied to the bridge from the FM oscillator. When theoscillator and bridge are properly adjusted, this signalis a pulse of constant amplitude (with the reference zero

Fig. 7-The input reflected-power pattern of an amplifier, shownwith the amplifier pass band.

line at each end) but of varying frequency. If the inputlevel is not constant, an error will be introduced in thebandwidth measurements. By having the 'ForwardPower" monitor built into the bridge circuit, it is pos-

1948 1417

PROCEEDINGS OF THE I.R.E.-Waves and Electrons Section

sible at any time to check the adjustment of the oscil-lator and the bridge circuits.The appearance of the "Reflected Power" pattern from

an amplifier with the input circuit tuned to match theimpedance of the waveguide at center frequency isshown in Fig. 7 (with the transmission pass-band char-acteristic on the other channel). This pattern provides avery quick method of adjusting the input to the ampli-fier, since the effects of mistuning and mismatching canbe easily seen separated, and without regard to theadjustment of the output circuit. With the input circuitproperly adjusted, it then becomes a simple matter toadjust the output circuit for maximum gain. This meth-od is considerably faster than the method of tuning bothcircuits simultaneously with the output as the only in-dication, since there may be no visible output signal ifthe cavities are far out of tune and it then becomes amatter of trial and error to determine the correct ad-justment.

THE RADIo-FREQUENCY OSCILLATORThe FM oscillator shown in Fig. 8 consists of an ex-

perimental oscillator tube in a wide-range cavity. The

Fig. 8-The FM oscillator.

mean frequency of this oscillator is determined by theposition of a movable piston in one end of the resonant

cavity, while the periodic variations in frequency are

produced by a paddle (shown in detail in Fig. 9) which isdriven by a synchronous motor. This oscillator will tunefrom 6.8 to 8.1 cm.

At 4000 Mc, the frequency where the bridge isnormally used, the oscillator output can be adjusted tobe constant to within 0.2 db over a frequency swing of50 Mc. By introducing an alternating voltage in thepower supply, it is possible to increase the frequencysweep to 600 Mc with not too great a sacrifice in con-

stancy of output. A Western Electric D-168479 relayoperating at a 60-cps rate is used to supply the oscil-lator with a square-wave input voltage.

Fig. 9-The paddle used in the oscillator of Fig. 8 to producethe frequency variation.

THE CHANNEL SWITCH

A relay circuit provides the desired synchronousswitching between any two selected channels. This re-lay is driven by a 30-cps vacuum-tube oscillatorwhich is synchronized by its 60-cps plate supply. Thisarrangement is much superior to an electronic switchwhich was originally used, and which was unsatisfactoryat 30 cps because of its poor low-frequency response.A balance control provides for the injection of asmallvariable dc voltage of either sign into one of the tworelay signal channels to displace one trace on the oscil-loscope with respect to the other. A bank of six inputterminals and input potentiometers, and the associatedswitches, enable one to select the desired pairs of sig-nal channels for simultaneous observation.

ACKNOWLEDGMENT

The construction of this waveguide bridge was orig-iginally undertaken after its feasibility had become ap-parent through discussion held between the writers,J. W. Clark, A. F. Fox, and W. W. Mumford. Theseindividuals, and others, contributed a number of usefulideas which have been incorporated into the bridge inits final form. L. E. Cheesman assisted in the design,construction, and testing of many of the bridge com-ponents.

BIBLIOGRAPHY(1) Allen F. Pomeroy, 'Precision measurement of impedance mis-

matches in wave guide," Bell. Sys. Tech. Jour., vol. 26, pp.446-459; July, 1947.

(2) W. W. Mumford, 'Directional couplers," PROC. I.R.E., vol. 35,pp. 160-166; February, 1947.

(3) W. A. Tyrrell, 'Hybrid circuits for microwaves,' PROC. I.R.E.,vol.35, pp. 1294-1307; November, 1947.

(4) Henry J. Riblet, "Mathematical theory of directional couplers,"PRoc. I.R.E., vol.35, pp. 1307-1313; November, 1947.

(5) William Alter, F. B. Marshall, and L. P. Hunter, 'Probe error instanding-wave detectors,". PROC. I.R.E., vol. 35, pp. 35P-45 P;January, 1946.

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