IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-34, NO. 2, JUNE 1985

1) A "test set" which ideally samples the waves incident onand reflected or transmitted from the device under test.

2) A vector voltmeter or complex ratio detector which mea-sures the waves sampled by the "test set."

In reality "test sets" are nonideal and thus sample bilineartransformation of the wanted quantities. By means of a cali-bration procedure (four-port calibration) the three unknowncomplex constants characterizing the bilinear transformationare determined. The bilinear transformation may be repre-sented by an "error box," i.e., a two-port that is inserted be-tween the device under test, and an ideal network analyzer.As the (l)-(5) are still valid if we include the "error box" inthe redundant circuit, it means that it is the accuracy of thevector voltmeter (or complex ratio detector) that is checkedby the redundant circuit, and not the accuracy of the four-port calibration.

III. EXPERIMENTS

The validity of (5) depends on repeatability of the switch-able components used. No ordinary coaxial switches have yetbeen found that repeat sufficiently well for high-precisionmeasurements. Instead two series-connected "trombone" lineshave been used with positions defined by end stops. In order

to make the test nontrivial two 20-pF capacitors were con-nected across the coaxial lines at the U-bends of each trom-bone. Using a six-port ANA at the National Bureau of Stan-dards (NBS), Boulder, CO, preliminary measurements weremade at 100 MHz of repeatability of the expression on the leftside of (5) caused by repeatability of the six-port ANA itselfand by repeatability of the trombone setting. Finally measure-ments of left and right sides of (5) were compared. The resultsof the measurements were as follows.

Repeatability of six-port ANARepeatability of tromboneLeft side of (5)Right side of (5)Ratio between left and right side of (5)

0.0005 dB0.005 dB0.3 dB0.3 dB0.005 dB

The results are thus in agreement with (5) with an accuracythat is limited by the repeatability of the trombones. Veryrecently it has been found that mercury-wetted reed switchesexist that.are at least as repeatable as the trombones used forthe measurements. It is thus possible to make an electronicallycontrolled, miniature, redundant circuit which can be used forchecking network analyzers with an accuracy of better thana hundredth of a decibel.

Standard Bolometric Mounts on 75-2 Coaxial Lines

FRAN(OIS DELAHAYE AND GERARD GENEVES

Abstract-LCIE has designed and realized standard bolometric mountsin 75-Q2 coasixal line, covering the frequency band 1 MHz to 1 GHZ.Their reflection coefficients are lower than 1 percent in this range andtheir effective efficiencies are higher than 99 percent up to 0.9 GHz.

I. INTRODUCTION

THE development of telephone transmission systems in75-2 characteristic impedance coaxial lines requires the

availability of standards for power measurements between 1MHz and 1 GHz. To satisfy this need The Laboratoire Centraldes Industries Electriques (LCIE) developed bolometric mounts,the effective efficiency of which is determined by a calorimetricmethod. These mounts constitute the French primary stan-dards for power measurements in 7542 line.For the secondary calibration work, power transfer standards

(T splitter or directional coupler + bolometric mount on the

Manuscript received August 20, 1984. This work was supported byPTT-DAII Contract 82 35 046.The authors are with the Laboratoire Central des Industries Elec-

triques, Fontenay-aux-Roses, 92260 France.

side arm) are used. Their calibration factors are measuredusing a standard mount.Consideration of the uncertainty of the power measurement

shows the importance of the reflection coefficient of the stan-dard mount: the maximum value, c, of the uncertainty termdue to mismatch is

e = 2 * Dd * r

where r is the modulus of the reflection coefficient of themount, and Dd the modulus of the equivalent source outputreflection coefficient of the coupler. Consequently r has tobe reduced as much as possible.

II. PRINCIPLEThe bolometric mount configuration is shown on Fig. 1. It

consists in two measurement thermistors polarized by a bolo-metric bridge at a total resistance value of 300 2. The couplingcapacitors Cl and C2 connect these thermistors to the highfrequency circuit. The impedance, as seen by the high fre-quency line, is that of the two thermistors in parallel, i.e.,75 R.

001 8-9456/85/0600-021 5$01 .00 © 1985 IEEE

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215DELAHAYE AND GENEVES: STANDARD BOLOMETRIC MOUNTS

Coaxial C2-tine

R boLometriibridg4

ci )C2 -

Fig. 1. Bolometric mount configuration.

The mount itself is a short length of 75-2 coaxial line of21-mm outer diameter. The coupling capacitors are solderedto the line conductors. The mount can be equipped with inter-changeable input connectors; Dezifix B, GR 900, or N (maleor female).

III. REFLECTION COEFFICIENT CHARACTERISTICSA. Low Frequency RangeNear 1 MHz, the return loss depends mainly on the capaci-

tive part of the impedance of the coupling capacitors.The value of the complex reflection coefficient is given by

Z-RcZ+RC

where Z is the impedance of the bolometric mount and Rc thecharacteristic impedance of the coaxial line.The impedance of the mount is

ZC=R[Cy 2C2] X

and the reflection coefficient modulus, 1':

11 + (2R CCw.2

where,

2C1 C2Cl + 2C2

The value of r remains lower than 1 percent at 1 MHz ifC is greater than '100 nF.

B. High Frequency RangeIn this range, the inductive part of the impedance of the

circuit formed by the capacitors and thermistors exceeds thecapacitive part. If L is the equivalent inductance of the bolo-metric mount circuit

[Lw »TJ-]

the impedance of the mount becomes

Z = RC + jLw.

Assuming L2W2 << 4R 2, the reflection coefficient modulusis

.v_Lx_=

DR

The introduction of an adjustable capacitance y between theinner and the outer conductors of the coaxial line allows thecompensation of the effect produced by the inductance L. Inthis configuration, the admittance of the mount is

1 1Z / RC+ jLw

or

1 1+ jRywc-LyL 2L

Re I + j R X_ c

If

L~= Rn2C

one obtains

1 1 L 2 1 Z=RI Ly2

provided

(L_yc) =[X]

can be neglected compared with 1, which is generally the caseif w/2ir< 1 GHz.The modulus of the reflection coefficient becomes

(2)R Lo,c,2 _1 L2co>2r= RC =_W

2Rc 2 R2The adjustment of y

[y R2does not depend on frequency, thus allowing broadband (upto at least 1 GHz) operation. Comparing (1) and (2) showsthat an uncompensated reflection coefficient of 7 percent

___ = 0.07]

ca be reduced to -1 percent, after tuning.

C. Experimental ResultsResults are in good agreement with the calculations. Fig. 2

shows the reflection coefficient modulus (measured by sweptfrequency reflectometry) of a bolometric mount before andafter compensation.

IV. MEASUREMENT OF EFFECTIVE EFFICIENCYThe effective efficiency i1 of a bolometric mount is the ratio

between the substituted dc power and the RF power dissipatedin the mount. The measurement is made by a calorimetricmethod which enables the increase of temperature resultingfrom the RF power losses in the body of the mount to bedetermined [1I -[3].

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-34, NO. 2, JUNE 1985

Fig. 2. Reflection coefficient of a bolometric mount.

Fig. 4. Effective efficiency of a bolometric mount.

Fig. 3. Head of a symmetrical calorimeter in 75-nl coaxial lineL

A symmetrical microcalorimeter, whose head is shown inFig. 3, has been built for measurements in 75-Q2 coaxial lines.Two mounts are fixed to the head of the calorimeter by meansof two thin-walled, coaxial lines 5 cm long, ensuring the ther-mal insulation of the mounts from ambient. The variations ofternperature AT1 and AT2 between a reference mount andthe mount under test, describing cycles without and with theRF power, are measured with copper-constant thermocouples.Then the uncorrected efficiency is given by

1

1 + P1 AT2 - AT1PI - P2 AT1

where PI , P2: substituted dc powers in the mount without andwith RF power applied.The following correction results from the heating due to the

attenuation (A(dB)) in the thin-walled coaxial line used:

i1=71B (I + 0,1 15.A).The values of the efficiency of a bolometric mount are shown

in Fig. 4: they are higher than 99 percent up to a frequency of=900 MHz.

V. CONCLUSION

We have realized standard bolometric mounts in 75-fl coaxialline whose reflection coefficients are lower than 1 percent inthe range 1 MHz-1 GHz. This enables a reduction in theuncertainty of calibrations with power transfer standards. Theefficiency of these mounts remains higher than 99 percentfrom 0.1 to 900 MHz.

REFERENCES

(1] L. Erard, "Etalons primaire en haute frequence," Rev. G0n. d'Elect.,vol. 85, no. 11, pp. 899-904, Nov. 1976.

[21 -, Bull. BNM, no. 29, pp. 13-15, July 1977.[3] J. Blouet, BulL BNM, no. 36, pp. 87-92, Apr. 1979.

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