304 IRE TRANSACTIONS ON INSTRUMENTATION December

Recent Developments in the Field of MicrowavePower Measurements at t e Nationa

Bureau of StandardsGLENN F. ENGENt

INTRODUCTION these two sources of error, anid an improved inistrumentbased on this method was recently placed in operation.WI HE bolometric technique, by means of which the The basic theory of the microcalorimeter operation4heating effect of an unknown amount of RE or has been presenited in detail in the earlier work. In brief,microwave power is compared with that of a meas- however, the instrumnentation is such as to permit a

ured amount of dc or audio-frequency power through a calorimetric determ.iation of the total power input totemperature-sensitive resistive element, is a well-knlowntemeraur'seisiiv ra bolometer mount while a simultaneous bolometric de-and extenisively employed method in the low-level meas- termination is made. The difference between the twourement of microwave power.

Themeasuofmicrem ven pofwthi bo tcr sued measurements is ascribed to the mount efficiency andbias pwerhasubement thesbolobjetrooref tsuntile th the substitution error. The method, in its present state

measurement can be carried out to anacuracy o0. at least, permits only a determination of the combinedmeasurementcan be carried out to an accuracy of 0 effect of these phenomena, also referred to as "effectiveper cent.i nThe use of the value thus obtained as a measure of the efficie cy.The problem of making a calorimetric determination

microwave power involves, however, two well-recog- of power (or energy) may be subdivided as follows: 1)nized sources of systematic error. First, the microwaveand dc bias currents take different distributions within seleti o coritr body or object in whichtothe bolometer element which means that the functional disiptethnep to b emeasued,u2echoie ofahsu

willinenerl b difere able technique to measure the temperature rise of thisdependence of the resistance will in general be diferentfor the two sources of power, giving rise to a dc-RF sub- b osu- alitv factor, or functional dependence of the thermome-stitution error; and second, the bolometric method at

best measuresonly the microwavepowerdissip ter response upon the power input. In the microcalorim-best measures only the microwave power dissipated etrecnq,th bomtrmutsrvsatewithin the bolometer element, whereas one is usually I . ' . . . .interested in the power dissipated in the entire terminat- calorimetric body or object in which the power is dissi-

pated and whose temperature rise is subsequentlying waveguide structure or bolometer mount. The ratio mased by means of easui e ther ile,uwhileof these quantities is, by definition, the bolometer mount bratseed bym obseingbte thermopile, responeficiency. bration iS effected by observing the thermop'le responseefficiency. ~~~~~~~toa measured amount of dc power dissipated in theThe evaltuation and measurement of these two phe- bolometerelement.

nomena has been the object and goal of a large percent- .Refinements in the technique include: 1) greatly im-

proved ambient temperature control, permitting higherProject of the Boulder Laboratories, National Bureau sensitivity and resolution, 2) improved dc instrumenta-of Standards, resulting recently in several significant tion, 3) improved mechanical coistruction giving bettercontributions to the existing art. Because these develop- r 4 rments will be reported in detail by papers now in prepa- itisn longereattached dc to therboploe munt,ration, the present paper will only attempt to summarize iths povidnge flexibiltlyin the oi oeterminati,

these results. ~~~~~~~thus providiing flexib'lity in the choice of termination,these results. and 5) a more comprehensive error analysis. These fea-

MICROCALORIMETER tures permit a determination of the effective efficiencyof a bolometer mount to an absolute accuracy of better

The microwave microcalorimeter described by Mac- than 0.2 per cent.pherson and Kerns2 gave the details of a calorimetricmethod for the determination of the combined effect of IMPEDANCE 1METHOD

A second contribution relates to the so-called "im-- ~~~~~~~~~pedance"method of determining bolometer mount ef-

* Manuscript received by the PGJ, September 2, 1958. *f*cient Radio Standards Lab., Nat. Bur. of Standards;, Boulder, CO1O. icecy devised by Kerns.3 As originally outlilned, the' G. F. Engen, "A self-balancing dc bridge for accurate bolo-

metric measurements," J. Res. NBS, vol. 59, R.P. 2776, pp. 1O1-105;August, 1957. 3 D. M. Kerns, "4Determination of efficiency of mnicrowave bolomIe-

2 A. C. Macpherson and D. M. Kerns, "A microwave micro- ter mounts fromz impedance data," J. Res. NBS, vol. 42, R.P. 1995,calorimeter,"' Rev. Sci. Instr., vol. 26, pp. 27-33; January, 1955. pp. 579-000; June, 1949.

1958 Engen: Recent Developments in the Field of Microwave Measurements at the NBS 505

accuracy which could be achieved by this technique was where the F,, F8, and rm are the reflection coefficients ofrather severely limited by the state of the impedance the generator, standard power meter, and meter undermeasuring art, and this led in turn to the development calibration at the point or plane of connection, respec-of a number of modifications with the objective of im- tively.proving the over-all accuracy. Beatty,4 for example, pro- It will be noted that a determination of the middleposed a modification based on certain mathematical ap- factor requires, in general, a knowledge of the reflectionproximations and restrictions in generality which pro- coefficients in both magnitude and phase. A considerablevided improved accuracy, but the associated operating simplification in the expression may be effected by ad-procedures were nonetheless time-consuming and exact- justing the generator for an impedance match so thating, and the accessory instrumentation was never de- rg vanishes. Under these conditions the expression re-veloped or refined to the point where a great deal of duces to the form:confidence could be placed in the results. Further refine-ments or modifications of the technique have been pro- PM =

rmP2 (2)posed by Weinschel,5 Ginzton,6 Lane,7 and perhaps 1- P 2others, but it is probably safe to say that none of these requiring a knowledge of only the magnitudes of P.proposals have yet come into widespread use.Another variation of the impedance method has been and Pm

It has been shown9 that a generator match may berecently developed at the Boulder Laboratories, em- siuae ymaso ig ietvt ietoa11. . . ' 1 ~simulated by means of a high directivity directionalploying directional coupler techniques, which provides

improved accuracy and simplified -operational proce-proand suitable auxiliary apparatus. While the equivalentdures, but unlike the earlier modifications, involves generator match was achieved by Engen9 by means ofneither mathematical approximations nor restrictions ingenerality,although still limited to barret type b a servo loop which held the sidearm power at a constant

generality, although still limited to barretter type bo- level, the same result may also be effectively realizedlometers. A particularly attractive feature of the new w ameho is it independence..(asmn.isptol with a stable generator by a manual attenuation ad-

method is itsindependence (a g ds.. justment or by simply noting the change in sidearmconnectors) of an impedance discontinuity at the inputconnector, which has bee a particularly troublesome power and making an approprate correction.

r .. 1 ~~~~ ^ 1 1 ~Viewed in this manner, the directional coupler andsource of error,in coaxial systems. Another advantage s p.. . . ...... . ~~~~~sidearm power detector assembly becomes an outputis its direct applicability to either matched or un- monitor for the signal source in addition to providingmatched mounts. A preliminary implementation of the

method in waveguide has given agreement of one half dthergnerat impedance match. Thus, In practice, oneof one per cent and better with results obtained by the determines a latmicrocalorimetric method. The details of this technique couple suc thatwill be presented in a later paper. P,

K= , 2)COMPARISON OF POWER METERS PC(1 - p 2) (3)

The mismatch or impedance measurement problems where P, is the power indicated by the sidearm detector,involved in intercomparing power meters have been and the other terms have been previously defined.treated in detail by Beatty and Macpherson.8 One of Thenthe simpler intercomparison techniques is that of alter- P -KP8(1 - F 2) (4)nate connection to a stable generator, and it was shownthat the power, Pm, dissipated in the meter to be cali- which gives the power, Pm, dissipated in the load of re-brated is given in terms of the power, P8, absorbed by flection coefficient, Fm, in terms of the sidearm power,the standard meter by the expression: Pc, Fm I, and the measured constant K.

I1-Fr,r, 2 1. FM 12 Eqs. (3) and (4) have been obtained from (2) whichPM = PS ' 1 assumes a matched generator. In general:

-rgFm 1- F82PK1= - pp 2

R. W. Beatty and F. Reggia, "An improved method of measur- = 1 -11 (5)ing efficiencies of ultra-high-frequency and microwave bolometer Pc 1- FI 2smounts,"' J. Res. NBS, vol. 54, R.P. 2594, pp. 321-327; June, 1955.

5B. 0. Weinschel, "Phase Engineering Rep. No. 2 on Standard andSignal Generator Output Power, Voltage, and Attenuation Calibra-tion Assembly," Contract AF33 (600)-25238, E.O. No. C-30047 1 - F 12SR-6J3, pp. 32-43; May, 1956. Pm = KP ImI 6

6 E. L. Ginzton, "Microwave Measurements," McGraw-Hill 1-cl FrF 12JBook Co., Inc., New York, N. Y., pp. 185-186; 1957. I- mI

7J. A. Lane, "Measurements of efficiency of bolometer andthermistor mounts by impedance methods," Proc. IEE, pt. B, no. 17, The procedure for determining the equivalent P8pp. 485-486;* September, 1957.

8R. Wl. Beatty and A. C. Macpherson, "sMismatch errors in mi- 9G. F. Engen, "'Amplitude stabilization of a microwave signalcrowave power measurements," PROC. IRE, vol. 41, pp. 1112-1119; source,"' IRE TRANS. ON MICROWAVE THEORY AND TECHNIOUES,September, 1953. vol. MTT-6, pp. 202-206; April, 1958.

306 IRE TRANSACTIONS ON INSTRUMENTATION December

achieved by this technique is discussed by Engen' agreement achieved in the microcalorimetric and im-who shows that, in practice, a broad-band match of pedance methods of bolometer mount evaluation, inrI in the range of 0.025 to 0.035 may be readily many cases to 0.2 per cent or so, implies that this error

realized. If the meter under calibration and standard is probably no greater than a few tenths of a per cent.are also nearly matched, the additional factors in (5) (The difference between these two determinationsand (6) are small and their simpler counterparts may be should in principle give the substitution error.) Even ifused, while for larger generator or load mismatches the larger value of 1-2 per cent is assumed, the coolingthese more complete expressions should be employed if process is still approximately linear, and thus even if aaccurate results are to be achieved. strong dependence of the substitution error upon this

Provided that the sources of error associated with the cooling were postulated, a few tenths of a per cent wouldbolometric measurement as discussed earlier, and the still appear to be a conservative estimate of the limitproperties of the coupler are independent of power of this error.level, the technique provides a convenient and accurate Confirmation of this estimate is rather strongly im-method of comparing power meters of different as well plied by the results of an experiment in which a bar-as equal power levels. For example, if it were required retter mount was connected to one arm and a thermistorto compare a 100-mw meter against a 10-mw standard, mount to the other arm of a 3-db coupler. The ratio ofa nominal 10-db coupler would be chosen and the meas- power indicated by the barretter to that of the thermis-urement required for (3) carried out at nominal values tor was found to be constant, within the limit (0.1 perof 1 and 10 mw for P, and P8, respectively. The power cent) of experimental error, over a 20-db range. Thisinput to the coupler would then be increased by 10 db, result may be interwpreted as implying that there is noyielding nominal values of 10 and 100 mw for Pc and change in substitution error within this limit and overPm in (4). If calibration of a 1-mw meter were required, this range, or if there is a change, it is at least the samethe procedure is similar except that the connections to for both elements. Because of the marked difference inthe main and sidearms are reversed. the nature of the barretter and thermistor elements,

It should be noted that this technique of extending the first implication is strongly favored. In another ex-the operating range is different from, and potentially periment the effective efficiency of a thermistor mountmore accurate than the usual method where the param- was determined by the microcalorimetric method usingeters of the individual components are measured. The first unmodulated CW microwave energy, and thenpresent method treats the coupler and power detector pulse energy in which the peak power exceeded theas a unit assembly and measures a single parameter for average by approximately 40 db. Again the results werethe entire system. (A second measurement is required equivalent within the limits of experimental error, lend-if the equivalent generator impedance is desired.) The ing further support to the above conclusion.extension of range is thus based upon the linearity of the A directional coupler with appropriate sidearm bolo-system and the ability to make accurate bolometric metric detector thus provides a useful technique fortype measurements over a wide dynamic range. The realizing a calibrated and matched signal source, andbolometric measurement may be easily carried out over permits an extension of the range (in both directions)a 10-db range and can be extended to 20 db with little of a given power standard by 20 db or so. For example,loss in accuracy if suitable precautions are taken. Re- accurate signals in the range 1 microwatt-1 watt maygarding the assumed linearity, it may be noted that the be derived from a calibrated bolometer mount whichparameters of interest in the coupler and associated operates in the 100-microwatt-10-milliwatt range. Inbolometer mount detector depend primarily upon the order to realize these results, however, care must begeometry and surface impedance of the waveguide used in the associated bolometric measurements, andstructure which are substantially, if not entirely, inde- the techniques described by Engen' are suggested.pendent of power level.The other potential source of nonlinearity is in the

dc-RF substitution error, but here it will be noted thata conservative upper limit of 1-2 per cent has been es- The author extends his thanks to the members of thetablished analytically'0 for this error, and the close Microwave Power Standards Project, particularly

10 H. J. Carlin and M. Sucher, "Accuracy of bolometric power Morris Harvey and William Case, for their contribu-measu1rements," PROC. IRE, vol. 40, pp. 1042-1048; September, 1952. tions to the developments discussed in this paper.