\



Nationat Bureau of Standards

Library, E-01 Mm\n. BIdg.

MAY 14 1981

3S079G

NBS MONOGRAPH 1 23

DISCARDED B>

U. S. A,

BOLOVAC Systems

for Measuring

Electrical Quantities

From 0.5 MHz Through
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3,

1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the

Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the

United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services

leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions:

Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation^—Nuclear Radiation-—Applied Radiation^—Quantum Electronics^

Electromagnetics^—Time and Frequency^—^Laboratory Astrophysics^—Cryogenics^.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies;

and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions:

Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the

development of technological standards (including mandatory safety standards), codes

and methods of test; and provides technical advice and services to Government agencies

upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation

Technology—Building Research—Electronic Technology—^Technical Analysis

Measurement Engineering.

THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for

automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions:

Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination

and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference

Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the

following organizational units:

Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of Public Information—Office of International

Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D C. 20234.

" Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
UNITED STATES DEPARTMENT OF COMMERCE AU(^ 1 6 W6

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS Lewis M. Branscomb, Director

MSSG

BOLOVAC Systems for Measuring Electrical Quantities

From 0.5 MHz Through Microwaves

/%»^0<?r^pK AO. ^-^S

Myron C. Selby

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

(XS National Bureau of Standards Monograph 123

Nat. Bur. Stand. (U.S.) Monogr. 123, 24 pages (Jan. 1972)

CODEN: NBSMA

Issued January 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price 30 cents Stock Number 0303-0929 (Order by SD Catalog No. C 13.44:123)
Library of Congress Catalog Card Number: 79-183998
Contents

Page

1. Introduction 1

2. Principle of Operation 3

3. Bolovac Applications 5

3.1. The Bolovac as a Source of Known Voltage 5

3.2. The Bolovac for Measuring Power 7

a. Power Measurement with Bolovac Operating in the Voltage Mode 8

(1) Evaluation of Source of RF Power 8

(2) Power-meter Calibration 9

b. Power Measurement Using the Bolovac as a Power Absorbing Mount (Power-absorption Mode) 10

(1) Bolovac as Absorption Mount to Evaluate Quality of RF-Power Source 11

(2) The Bolovac as an Absorption Mount to Calibrate Power Meters 11

(3) Calibration of Low-level Signal Generators Using the Bolovac in the Power-absorption Mode 11

3.3. The Bolovac as a RF-current Standard 12

3.4. Application of the Bolovac for Insertion-loss Measurements 13

3.5. Self-evaluation of the Bolovac 13

3.6. Other Bolovac Applications 14

4. Structural Features of the Bolovac 14

4.1. Mechanical Design 14

4.2. Thin-film Materials for the Bolovac 16

5. Auxiliary Equipment 18

6. Conclusion and Future Prospects 18

7. References 19

8. Appendix 20

List of Figures ''"^^

1. Basic components of the Bolovac

(a) Middle plate of the Bolovac supports the bolometer and the center conductor

(b) Equivalent circuit of the bolometer film and its substrate and of the center-conductor supporting bead 1

2. Schematic diagram illustrating principle of standardizing RF voltmeter in terms of known voltage generated

by Rm located in a transverse plane, A, of a TEM mode 3

3. Schematic diagrams showing the applications of the Bolovac for

(a) Voltage standardization and

(b) Current standardization 3

4. Measurement of voltage amplitude bandwidth of oscilloscope with a Bolovac 5

5. Graphical determination of Vb 6

6. Symbol designations unless otherwise labeled 8

7. Measurement of Pc, Po, and Pn with the Bolovac used as a voltmeter. (See a simpler method for Pc and Po

with the BOLOVAC operated in the power-absorption mode.) 8

8. Calibration of power meter in two steps with Bolovac operating in the voltage mode 9

9. Calibration of power meter in one step with Bolovac operating in the voltage mode 10

10. Application of the Bolovac as an absorption type power mount. The sliding short may be replaced by a shielding tube or may be eliminated, depending on the frequency and requirements. Note that the thin film is now facing the

source. 10

11. Measurement of maximum available power from a source with the Bolovac as an absorption power mount 11

12. Measurement of Po with Bolovac as an absorption mount 11

13. Carrier output calibration of low-level signal generators employing Bolovac in the power-absorption mode of operation 12

iii
List of Figures—Continued

Pag

14. Self checking of Bolovac

15. Exploded view of core for voltage standardization

16. Current core exploded

17. Bolometric film on two-mil plastic substrate 1

18. Core (with voltage plate) and auxiliary capacitor of Bolovac 1

19. Voltage core and auxiliary capacitor

20. Interchangeable bolometer cell (employing a 2-mil substrate) mounted in the center plate of the Bolovac 1

21. Bolovac set up to calibrate attenuator-thermoelement voltmeter (ATVM) ]

iv
BOLOVAC Systems for Measuring Electrical Quantities

From 0.5 MHz Through Microwaves

Myron C. Selby*

A novel device—the BOLOVAC—is employed in measurement systems that furnish known

voltages and currents up to 18 GHz, remove serious difficulties in power measurements, and are applicable in impedance, attenuation and other measurements. Typical of such measurements

are the evaluation of rf power sources; calibration of power meters; measurement of net and

incident power flow in a system; calibration of signal generators, voltmeters, current meters,

picosecond-rise-time pulse-display oscilloscopes, spectrum analyzers; determination of insertion

loss of components; and calibration of directional couplers. The Bolovac also serves as an

accurately known resistive termination for match or mismatch applications from LF through

microwaves.

Key words: BOLOVAC; calibration of current meters through microwaves; calibration of power

meters through microwaves; calibration of voltmeters through microwaves; current measurement;

disk-type bolometer evaluation; frequency-domain pulse evaluation; measurement of micro- wave current; measurement of microwave power; measurement of microwave voltage; mismatch

error; picosecond rise-time pulse evaluation; power measurement; pulse-rise time standardization;

sampling oscilloscope standardization; spectrum analyzer standardization; termination mismatch

error elimination ; thin-film bolometers ; voltage measurement.

1 . Introduction

BOLOVAC is an acronym of "BOLOmetric Voltage

and Current," a novel device for measuring electrical

quantities up through microwave frequencies. Generally connected in series with a coaxial system, it measures voltage, current or power. Its upper fre- quency range is limited to 9 GHz in a 14-mm system

or to 18 GHz in a 7-mm system and to higher fre- quencies in smaller diameter systems. At the National

Bureau of Standards (NBS) so far, its application

only with 7-mm systems has been evaluated. The

Bolovac offers the first means of furnishing or measuring microwave voltages directly and calibrating in- dicators of microwave currents. The relatively simple construction of the Bolovac

consists of a coaxial assembly of three washer-type

disks (fig. 1) that form the outer conductor and a common center conductor [1].^ One disk sandwiched

between the other two, carries a bolometric sensing

element connected across the output of the coaxial

mount between the disk and the center conductor. The

sensor consists of a thin-film resistor having a relatively high temperature coefficient of resistivity. A

0.001- to 0.002-in (one- to two-mil) ^ thick dielectric

film insulates the outside disks from the center disk to form rf-blocking capacitors.

The dynamic range of the Bolovac depends on the

resistance and other characteristics of the bolometerelement (bolometer) and on the accuracies desired;

it is typically 100 mV to 2 V (10 mA to 200 mA)

for a lO-Q bolometer and accuracies of the order of one percent. This voltage range corresponds to 200

/xW to 80 mW in a 50-f2 system. Although one-ohm to 50-n and higher resistance bolometers can be used,

the National Bureau of Standards, so far, has only

fabricated a few units above 10-f2. Voltage, current

and power levels depend on the bolometric element

resistance, but the relative dynamic range of each

Middle plate A

of Bolovoc _ I

I

I

Thin bolometric film on 0 0.002" substrate

ocatecJ on the voltage

output side and current ond power input side of the Bolovac.

(a)

Rb = Rs = 00 at bios frequency

Rb Rb °"d Rj >> Rg at rf

Rj = effective equivolent resistance

of substrate,

Rb = effective equivalent resistance

of beod.

Rb = resistance of film

(b)

* Electromagnetics Division, National Bureau of Standards, Boulder, Colo.

80302.

^ Figures in brackets indicate the literature references at the end of

this paper.

^ One inch is equal to 2.54 centimeters.

Figure 1. Basic components of the Bolovac.

(a) Middle plate of the Bolovac supports the bolometer and the center conductor.

(b) Equivalent circuit of the bolometer film and its substrate and of the center-conductor supporting bead.

1
element remains about the same. Accurate high-frequency and microwave voltages

and currents are important to the following needs : [2] —Calibrating voltmeters, signal generators, fieldstrength meters, voltage spectrum analyzers, thermoelements, etc. —Measuring frequency response, i.e., fidelity of

oscilloscopes displaying modern narrow pulses

having rise-times of several picoseconds (ICh^^

second). To date the CW frequency response is the only practical means of standardizing pulsedisplay instruments and now the Bolovac provides

a unique tool for this purpose at the presently

required frequencies. —Measuring voltage and current transfer characteristics of solid state, integrated, monolithic and

epitaxial components and circuits. —^Measuring voltage-breakdown characteristics of

ordinary—and miniature-diameter transmission

lines, including strip lines. —Measuring fields (and their polarization) as detected by probes and antennas in enclosed or open

spaces. —Measuring microwave characteristics of very thin

conductive, semiconductive and superconductive

films, and in other R&D work. —In general for all purposes for which measure- ment of dc voltages and currents are made.

The Bolovac is particularly attractive for power

measurement for the following reasons briefly stated: —It eliminates the need to measure source and load

reflection coefficients. Traditional methods necessitate measuring the magnitude and phase of these

coefficients required to compute power flow from

complex equations. Failure to measure these coefficients can result in uncertainties of 25 percent or more called "termination mismatch errors". Seldom are these measurements and computations

done even in the best of laboratories. —It eliminates the need for charts giving limits of

mismatch errors. Such charts, can be used to re- duce the above uncertainties, provided that the

reflection-coefficient magnitudes are measured.

Again, limits on the uncertainties may be 15 percent or higher. The Bolovac, on the other hand,

permits measuring definite values of power under

any practicable matching conditions. —It eliminates the need to calibrate power meters

for "effective efficiency" or "calibration factors".

With the Bolovac rf power is measured directly

in terms of dc or audio power without any cor- rections for most present day practical needs.

—It eliminates the need to calibrate power meters !

at dc or audio frequencies as nearly always re- I quired. The Bolovac serves as a power-absorption '

mount or furnishes known rf voltages to calibrate

other power meters. ' —The Bolovac's frequency range extends down to ^ 100 kHz or lower as compared to 10 MHz for

most conventional meters. It can also measure \ considerably higher power levels than those indicated.

Other advantages introduced by the Bolovac were

mentioned in previous papers [3, 4] and are pointed

out here. i. The Bolovac, when terminated in a sliding short, i' serves as an accurately known adjustable termination

(for impedance, attenuation or power measurement) '^i over its entire frequency range. It can be used to measure insertion loss (attenuation) of components m and networks, and to calibrate directional couplers or P

to take the place of such couplers as power-transmis- \ sion monitors. With proper adapters it can be used '|

to measure power in any uniconductor waveguides R operating in any mode. It may even be applied for ^

self-evaluation because there are two ways (modes) -

of operation of the Bolovac for power measurement— ?| the voltage mode and the power-absorption mode; [ checking one against the other will expose irregularly i', high internal losses or other defects within the Bolovac f

itself. " The most attractive and practical features of the

j Bolovac are ( 1 ) the relative simplicity of the prin- i

ciples of its operation and application, reducing the I concepts practically to those at dc, and (2) the use i of the Bolovac could eliminate ruinous inflation of the

j uncertainty that results from the present need of cali- j brating power and other meters in a succession of [ laboratories. These laboratories pass on rf-reference

standards from high echelons or lower. The Bolovac i needs no rf calibration and can be used in any labora- ^

tory having dc standards that are good enough for

| immediate needs.

i This paper describes in some detail how the Bolovac

works, what its structural components are, what are, the methods and steps of procedure in its various

major applications, and reasons for its advantages.

The emphasis is on applications. Present status at the

,

National Bureau of Standards relating to fabricating

the most important component of the Bolovac—thei

bolometric disk—is briefly pointed out. Considerable latitude for further innovation, im-j^

provement and refinement is available to manufac-^

turers expressing interest in marketing the Bolovac. j

2
2. Principle of Operation

Because comprehensive analyses of the Bolovac were presented in previous papers [3,4,5], a brief

discussion of the basic elements and derived equations,

I should suffice for its practical applications.

A thin metallic-film placed in a plane A, (fig. 2),

pejpendicular to the axis of a coaxial line, has an

rf resistance theoretically shown [5] to be equal to

its dc resistance to better than 0.015 percent at all frequencies up to 36 GHz. This is true provided (1)

the thickness of the film is 7 percent or less than the

penetration of the current into the corresponding solid

metal at 40 GHz and (2) the electric-field lines in

the coaxial line are perpendicular to its axis. The latter condition holds in the vast majority of normally operated systems. As the current penetration increases at frequencies below 36 GHz, the film thickness is a

smaller fraction of penetration and the rf-to-dc equivalence becomes closer as the frequency decreases.

Imagine two coaxial lines joined by connectors in

plane A-A (fig. 2). The ends of the lines connect

to an rf power source and a load, respectively. The

load, a voltage indicating instrument, and the coaxial

line form a voltmeter that measures the voltage in

plane A. Opening the connectors, inserting a thinfilm disk in plane A and reconnecting the lines, results in an rf voltage across the film disk and rf

1 power dissipated in it. The power is given by:

BtS VB'/Rsrf VbVRBdc (1)

where

Vb = rf voltage across the film disk,

Rnrf = rf resistance of the disk,

Rsdc — dc resistance of the disk.

The rf power dissipated in the film can be measured

to find Vb by the so-called "power substitution"

method as follows. For maximum sensitivity, this film

should have as high as possible a temperature coeffiCoaxial Connector of Voltmeter Probe \

Coaxial Line

Voltmeter

Detecting

Circuitry

Sourcei

Thermistor' A Plane of Location

of Resistive Element

(Thermistors or Conductive Film Disl<s)

Figure 2. Schematic diagram illustrating principle of stand- ardizing RF voltmeter in terms of known voltage generated

by Rm located in a transverse plane, A, of a TEM mode.

cient of resistivity. Physically splitting the disk in

two along a diameter (not shown) perpendicular to

line A-A enables the two halves to be connected in

series for dc and in parallel for rf. We feed dc or audio power (called "bias power") to the film. For

simplicity we will refer to the thin film disk and its supporting substrate material as the "bolometer". The

bias is isolated from the rf power by blocking capacitors, Ch, built into the assembly (fig. 3). The value

of Ci, and its construction is such that the rf-voltage

drop across it is always negligible.

II (^^)-VWS5fvN—) I THEeMOCLeMEWT

0 COUPLE Output

CONSTA.WT /

Figure 3. Schematic diagrams showing the applications of the Bolovac for (a) Voltage standardization and

(b) Current standardization.

With the rf source turned off, a certain amount of

bias power, Pbo, fixes the bolometer resistance at a

desired value of Rsdv So far National Bureau of

Standards uses dc to bias the bolometers. Turning on

the rf source causes the bolometer resistance to change

up or down and some bias power must be removed

from the bolometer to restore its original resistance

Rarf = RbcLc The dc bridges (fig. 3) indicate when

this resistance is restored. A measurement of the

substituted bias power is given by:

Parf — Pbo ~ Pbi (2)

where: Pbi = bias power measured after the rf is turned on. The bridges also indicate what Rsdc is, provided that we incorporate an important structural

detail of the Bolovac. Bolometers comprising the film

disks and their mounts are split in half along one
diameter. Half the bolometer resistance is -^Rt'-, the

bridge measures Rt', but the resistance from the center conductor to the outer conductor in plane A is ^ Rt = RbcIc- For simplicity we designate Rsdc ^ RbThus, Rb is the effective dc resistance of the bolometer.

Measuring the dc voltage across Rt before and after

the rf is applied results in the substituted bias power

Pnrf = ^ (Fio - Fil) (3)

The rf voltage in plane A is given directly in terms of

the dc voltages as follows:

Vb = i (Fio - Vi,)i (4)

Figure 3a shows that Vrf is the same as V^, the

voltage applied to the voltmeter, because the input

plane to the voltmeter is only 1 or 2 mils away from

the bolometer film. Standing-wave effects over a length of 1 or 2 mils even at 36 GHz were shown to be negligible [5].

Figure 3b illustrates how to calibrate a current

sensor, such as a thermoelement, with the Bolovac.

To do this, turn the Bolovac around end for end.

The bolometer film (normally voltage output end)

is now facing the thermoelement; the distance

between the two is kept small to minimize the effect of standing waves. A sliding short (tuning stub), con- nected to the other end of the Bolovac is adjusted for a maximum value of Vb. This is facilitated by using

a "constant rf current" source, e.g., an rf source in

series with a small capacitor. When the combined

impedance of the tuning stub, bead and substrate of

the Bolovac are equivalent to an integral number of

quarter-wave lengths (ra A/4) at a given frequency,

all the current passing through the thermoelement is forced into the film. The current Im is given simply by

Rb

(5)

With the Bolovac, thermoelements were calibrated to 2 GHz for the first time at the National Bureau of

Standards [5].

The same circuit arrangement and position of the

Bolovac as for current measurement is used to apply

the Bolovac as a power-absorption mount. This will

be discussed further on.

It may be of interest to note that inserting the

bolometer and the blocking capacitors into the coaxial

line (fig. 2) would not have increased the original

eflfective length of this line except by a negligible

amount, were it not necessary to support the bolometer

and the center conductor mechanically. Support in- cluded a ^e-in thick middle plate and a solid

substrate. Other versions used a thinner solid bead to support the center conductor and a separate 0.002-in

substrate (fig. la). This increased the original line

length about From the practical standpoint,

this is not important. However, one may ask what the

effect of the resistance or losses in the bead and

substrate have on the basic assumption that the powersubstitution principle holds. Momentary inspection of

the equivalent circuit (fig. lb) shows that the assumption does indeed hold. The bead and substrate resistances at dc or audio is, for all practical purposes,

infinitely large. Therefore, the value of Vb is based on the substituted bias power in the film alone and

is not affected by the bead and substrate. At micro- wave frequencies Rs, in the case of mica, is many

orders of magnitude higher than Rb; thus the likeli- hood of appreciably heating the film with rf power

dissipated in Rs may be safely ruled out. It follows

that Vb is constant at all rf frequencies irrespective

of the rf resistances (or losses) in the bead and substrate because these rf resistances are effectively in

parallel with the film resistance. Vb appearing across

the film is of course the same across any other resistance in parallel with it. However, the rf losses in

the bead and substrate must be considered when the

Bolovac is used as a power-absorption mount; but,

these losses, as shown later, are of the order of a

fraction of a percent and are considered negligible for

practical purposes. Also note that narrower beads and

other kinds of center-conductor supports (e.g. radial

cylindrical pins) can be used.

Figure la shows the position of plane A referred

to in figure 2 where the thin bolometric film is located.

Keep in mind the critical significance of this plane.

Vb is measured in this plane. To apply this known

Vb to any device, (e.g., a voltmeter, oscilloscope,

receiver, etc.) the latter should have a coaxial input

reference plane mechanically equal to the coaxial

cross section in plane A located as close as possible

(about 2 mils to 18 GHz) to plane A. The input

coaxial diameters of the device may be different

from those in plane A only when it is certain that

the voltage in that resultant coaxial step discontinuity

is a continuous function and that the electrical field

lines remain essentially radial. Because such discon9
tinuities are avoided in best practices for various

reasons (to avoid complex reflection coefficients, additional losses, generation of higher modes, etc.) this case need not be considered further.

Figure 2 also points out that bolometers other

than thin films may be used for certain ranges of

frequency and magnitude of the quantity measured.

For example, the National Bureau of Standards uses

15- to 20-mil diameter thermistor beads for frequencies

to 1 GHz.

Because the physical size of the Bolovac is affected

primarily by the value of Cb, (fig. 3), the dependence

of the frequency range on Ci, is briefly discussed here.

Above 100 MHz the required dimensions and struc- ture of the capacitors, d, are best determined by

treating the structure as radial transmission lines

rather than as lumped capacitors. This avoids res- onance effects at some discrete frequencies. At such

resonances, unwanted or relatively high impedances

might be present in series with a voltmeter under

calibration, for example (fig. 2a). Several steps can

be taken to avoid such resonances. Stray inductances

of the capacitor bolts are minimized by insulating

them or replacing them with plastic bolts. Removable

auxiliary capacitors can be attached for frequencies

below 100 MHz. The metallic output plate of the

Bolovac can be replaced by a considerably smaller

diameter plate for measurements at frequencies above

1 GHz. Capacitor diameters were, in this case, esti- mated using published information [6, 7, 8, 9]. However, the uncertainty associated with the termination

impedances of these radial lines, required that experimental data be used in determining the adequacy of

the assemblies. The characteristic impedance of these

radial lines is much smaller than Rb and for a given

desirable minimum value of Cij the smaller the di- ameter of these capacitors, the better. The objective

is to choose diameters safely below those which are

equivalent to a one-half wavelength at the lowest fre- quency used for a given Bolovac output plate. Thus,

at frequencies above 1 GHz the outer conductor

thickness (~ 2 mm) of a widely used 50-f2 solid

coaxial line was found in our case sufficient. Various

designs can, of course, be used instead of the first generation Bolovacs and combinations of the capacitors Ci described below. The entire assembly can be

reduced radically in size, weight, and cost if the fre- quency range is above 1 GHz. The value of Cj, should

also be correlated with the input impedances of the

meters to be calibrated using the Bolovac.

3. Bolovac Applications

3.1. The Bolovac as a Source of Known

Voltage

Various direct applications of voltages above 1 GHz are listed in the Introduction. The normal

voltage-output side of the Bolovac is connected to the

input plane of the "unknown" device under test or

calibration to apply values of Vb. Changing frequencies only requires adjusting the rf input level to the

Bolovac to restore bridge balance. Thus, the same Vb

is obtained for each frequency. Using this procedure

in our laboratory, we measured voltage-versusfrequency points at a rate of 20 to 30 per hour using

manual adjustment of the frequency and voltage level

up to 18 GHz. This compares with 2 to 3 hours per

point as quoted by competent technical operators employing the conventional "power-impedance" method.

In this latter case, one measures the input power and

the input complex impedance of the unknown; the

voltage in the input plane is then computed from the

two values for each frequency.

To illustrate this application consider measuring the

voltage-amplitude bandwidth of a sampling oscilloscope designed to display trapozoidal pulses having

rise times as short as 25 picoseconds (ps). A rule

of thumb [10] expresses the approximate upperfrequency limit in terms of the rise time;

0.4

(6)

where fc is the upper frequency in Hz at which the

uniform amplitude response drops to 0.707 of its medium-frequency value; tr is the rise time in seconds.

For a 25-ps rise time fc is 16 GHz.

A conceptual setup for this kind of measurement

is described (fig. 4). The Bolovac can have any

bolometer resistance that will furnish a typical input

Vb to the oscilloscope, e.g., 0.1 F. If the Bolovac outr"

I To bridge '\J\j

Infernal or externol oscilloscope load

To bridge -oscilloscope

Figure 4. Measurement of voltage amplitude handwith of oscilloscope with a Bolovac,

5
put voltage is held constant irrespective of the load

impedance, Zl, attached to it (in this case the input

impedance of the oscilloscope), the effective internal

impedance of the Bolovac is zero (by Thevenins

theorem). Sinusoidal-waveform amplitudes on the

oscilloscope CRT are observipd as they vary with fre- quency. This procedure is useful for calibrating other

voltage-dependent instruments in the frequency domain, such as voltage spectrum analyzers, voltmeters,

field-strength meters, etc.

In most cases, instruments operate from rf sources having a given fixed internal impedance, Zo, e.g., 50-O at all frequencies for a 7-mm coaxial system. In

these cases the zero-impedance Bolovac renders an

accurate frequency response only if the input impedance of the instrument is 50-Cl at all frequencies.

One must, of course, divide the indication of the

instrument under test (assumed to be linear) by two

to obtain the correct response. This is so because a source of Vb with two 50-f2 resistors (one inside the source and the other in the instrument input) will

reduce Vb by 50 percent when it is applied to the

instrument.

Unfortunately instrument input impedances at these

frequencies are seldom 50-Q and are even less likely

to remain constant with frequency. The best solution

in these cases is to use a 50-fi coaxial resistor in series

with the Bolovac output. At the present state-of-theart several such resistors will be required, each cover- ing a certain frequency range.

Another way of expressing the frequency response

of the unknown in terms of an ideal source having an

internal impedance of Zq is to measure the complex

Zl at each frequency and to compute the internal

voltage of the ideal generator, eg, in terms of the

measured Vb.

[

(Zo + Rsr + X.

(7)

where Rg and Xg are the series resistance and react- ance respectively of the unknown. One may use a sweep frequency technique to measure the impedance

of the unknown and a desk calculator to compute e^; however, it is apparent that a single coaxial resistor

having constant resistance over the desired frequency

range will eliminate all these measurements and computations.

A third solution is to employ a high-resistance film,

e.g., Rs = 5000 n. In this case the Bolovac connects

into rf and microwave systems as any high resistance

voltmeter would at low frequencies.

An additional illustration may help point out the

wide range of applications where the Bolovac serves

as a source of known microwave voltages. Consider

a hypothetical problem where one needs to generate

a relatively high electric field to determine the breakdown characteristics of a coaxial line or of a lumped

miniature component. Conventionally one might use

a high-power source feeding a coaxial line terminated

in its characteristic impedance. This method requires

considerable amount of power to generate high voltages. On the other hand one may apply a known,

relatively low value of Vb to the Bolovac at the input

to a coaxial line whose output is open circuited. High

voltages are then produced in the line with relatively

low power. The voltage values in the line are given

in terms of Vb and the voltage ratios along the line can be measured in a conventional manner.

It would be feasible to design direct-read-out electronics for Vb and Pn, the rf power dissipated in the

bolometer disk of the Bolovac. However, the graphical

method shown in figure 5 avoids most of the computations [4]. One can draw a quadrant of a circle

VgQ(Vgi) = dc voltoge ocross the bolometer without (with) rf opplied to it

Ouadront of circle

of ony radius

Or use Sine - Cosine trigonometric tobies

Figure 5. Graphical determination of Vb.

having a relatively large diameter on linear squarecross-section graph paper. For each ratio of Vbi / Vbo

or of (1 ~ ^VBo/Vbo) on the X axis there is a corre- sponding value of 2 Vb / Vbo on the Y axis. Vb can

then be determined without squaring and taking roots

of values. High precision requires trigonometric tables

in which one simply finds the cosine of the angle,

9, equal to V /Vbo ox \.o (1 - A Vbo /Vbo). The ^

sine of the same angle 6 yields the ratio oi 2 Vb / Vbo

and ^

Vb = \ Vbo sin 6. (8)

The potential accuracy of Vb can be computed by

any user in terms of his equipment accuracies. As is

i
seen from eq (4) and the preceding discussion, the

accuracy of Vb depends primarily upon the accuracy

of the dc voltages. When estimating accuracies, care should be taken as Vbi approaches that of Vbo or when Vbi is a small fraction of Vbo- The uncertainties

in the values of Vp of the order of a fraction of a

volt to several volts in our experimental work were

estimated at a couple percent to 18 GHz and better

at lower frequencies; e.g., it 1 percent at 1 GHz;

these uncertainties are based on the agreement of the

results with the power-impedance method. Because the

Bolovac results depend primarily on dc measurements,

they are felt to be more accurate than the agreement

sited above.

3.2. The Bolovac for Measuring Power

The advantages of the Bolovac for power measure- ment are indicated in the Introduction. Its major

contributions in this respect are eliminating the

effects of serious termination mismatch errors and

the errors accumulated in a chain of three or more

calibrations by laboratories passing rf power standards from one laboratory to the next. The Bolovac,

as was pointed out, needs no rf calibration. It operates

in two modes; the voltage mode and power-absorption

mode. In measuring power the Bolovac can do one

of three things: (1) measure the rf voltage across a known resistive component of a given load; (2) measure the rf voltage in any plane of a coaxial line

of known characteristic impedance, Zq, and known

voltage distribution along this line; or (3) absorb

the rf power as any other absorption-type power

mount does. In the first two cases load resistance and

voltage-distribution measurements can be made sep- arately without the Bolovac, or these measurements

can be combined with the power-measurement procedure employing the Bolovac. In the third case the rf power is, for all practical purposes, absorbed entirely

by the bolometric film and is equal to the biasing

power substituted in the film. Any appreciable un- accounted-for losses of net power flow from a source

to the Bolovac occur outside the Bolovac and can be

determined by employing conventional techniques as well as the Bolovac itself. This is not the case with

most present-day absorption mounts where the in- ternal losses which go undetected by their sensors

are appreciable and must be calibrated out in terms

of rf-power standards [11]. The Bolovac's superiority

rests in the fact that it measures power in terms of

relatively well controlled and measured parameters,

namely dc (or audio) quantities, characteristic impedance of coaxial lines, Zq, and ratios of rf voltages

[3, 4]. This is in contrast to complex reflection coefficients of sources and loads, efficiency factors and

rf-dc substitution factors with reference to standard

calorimeters. Sources of dc having accuracies an order

of magnitude better than for rf are widely available; so are good quality coaxial lines having an accurately

known Zq. Rf-voltage ratios can be measured with

much higher accuracies than their absolute values as

is true for ratios of most other quantities. It follows

that the potential accuracy of measuring power with

the Bolovac approaches the accuracy of measuring the

substituted bias power. Thus, the Bolovac can be considered a new type power-absorption mount; among

the other unique attributes it also includes an rf output

in addition to the rf input connector not found on con- ventional mounts. This structural feature enables several Bolovac applications described below.

As with rf voltage, the policy was to assign a conservative value of several percent to the uncertainty

of measuring milliwatt levels with the Bolovac at 18

GHz. Such policy stems from the limited agreement

with conventional methods of power measurement.

Though the Bolovac is a superior standard of power

for the best of laboratories, our main objective is to encourage its application in all lower-echelon laboratories as well as field measurements. Its use by lowerechelon personnel can reduce uncertainties from about

25 percent, and higher as frequency increases, to about

two percent. The objective is not to reduce uncertainties to fractions of a percent but to achieve a

significant reduction with dramatically reduced efforts compared with present practice.

One of the objectives of measuring power is to evaluate the quality of a power source, i.e., to find the

maximum net (incident minus reflected) power delivered to a load. In this case the load impedance must

be the "conjugate" of the source impedance; in other

words, the load (L) resistance, R[„ must be equal to the source resistance, and the load reactance, Xi,

must be equal and 180 degrees out of phase with the source reactance, Xg. Analytically stated, it is required

that

Rl + jXl = Rs- yx.. (9)

This condition is generally realized by connecting a three-stub tuner between the source and the load; the

stubs are adjusted for maximum power indication in

the load, designated as Pc into a "conjugately

matched" load.

Another index of source quality is the net power Pq

delivered to a load having a VSWR = 1. In this case

7
standing waves are nonexistent in a coaxial line con- necting the source and the load, provided Zo of the

line equals Z^. In practice Zo is resistive and a threestub tuner connected in front of the load is adjusted

for a "Zo match". A slotted line ahead of the stub

tuner allows one to observe the flatness of the voltage

distribution. A relatively simple procedure that ac- complishes this is described in appendix C of [3].

Still another objective in measuring power is to find

the net power, P„, flow from a source to a load under

any matching conditions. Basic schematic diagrams

and steps of procedure are outlined below to measure

Fc, ^0 and P„ using the Bolovac as a source of known

rf voltages at frequencies up to 18 GHz and potentially

to 36 GHz. In all these cases the losses in slotted lines are assumed negligible. However, these losses as well

as losses in stub tuners and other components can be

measured with the Bolovac as will be discussed later. The special case of measuring incident power is dis- cussed briefly later on.

= Bolovac

Center -conductor

supporting bead

Rf source

Power meter or load

1 1

1

1 = Colibroted sliding short

Three -stub tuner

Slotted line

Attenuotor

Solid horizontal line = Cooxiol line

a. Power Measurement with Bolovac Operating

in the Voltage Mode

This method is particularly attractive with the advancing art of automating VSWR measurements.

Two well-known equations are used in this case [3]

Verticol solid line

Pn = V,'/Rl and

Pn ~ VmsL-x./ SZi 2 = (y^ /SZo

(10)

(11)

where

voltage magnitudes are used.

Pn = net power flow to load

Rl = is the parallel resistive component of the

load impedance

ymax — the maximum voltage in a slotted coaxial

line connected between the Bolovac and

the load.

S = Vmax/Vmin in the slotted line = VSWR

Zo = the characteristic impedance of the

slotted line (Note that once Vb is known

only the ratio Vmax to Vb needs to be

known instead of the actual value of

Vmax- )

(1) Evaluation of Source of RF Power.

For sake of simplicity the symbols shown in figure

6 designate setup components in this and the following

figures.

Contact plone of two cooxiol components

Figure 6. Symbol designations unless otherwise labeled.

The maximum available power, Pc, of a source can

be measured using the Bolovac as a voltmeter by a

substitution method in two steps (fig. 7) [3]. However, attention is called at once to a much simpler

method of measuring Pc using the Bolovac as an

absorption mount as described in the next section. In the first step of the voltage mode adjust the threestub tuner for a maximum indication of any power

meter serving as a load. Let its maximum reading

be Z)i. In the second step, feed a Bolovac from any

other source in series with the slotted line; this source

step 1 : (^Zy 7^! 0.

step 2:

Pc = (VB4f^)'/SZ„

Figure 7. Measurement of Pc, Po, and Pn with the Bolovac

used as a voltmeter.

(See a simpler method for Pc and Po with the BOLOVAC operated in the power-absorption mode.)

8

I
and the Bolovac substitutes for the original source under test; in other words they improvise a source

equivalent to the original. Adjust the frequency and

output of the source to reproduce D,. Pc is then given

by equation (11). To measure the ratio Vmax/VB

' locate the plane in the slotted line which is located an integral number of half-wavelengths, nX/2, away

from the output plane of the Bolovac. It is most important that the output plar^e of the Bolovac be located

at the input plane of the slotted line, also that there

be no appreciable discontinuities between this plane

and any other plane in the slotted line where voltage

j ratios are measured. Good quality commercial lines

meet this requirement; it can be readily checked by

comparing (1) the physical distances from the input

to the line and inside planes with (2) the electrical

wavelengths at several frequencies. These distances can

be predetermined (once) for each frequency in terms

of the graduated scale of the slotted line. Thus, the

position of Vb inside the slotted line is known and

its magnitude is generally equal to that measured by

the Bolovac because the losses in the line are negligible for practical purposes. However to be safe one

should make voltage-ratio measurements at the Bolovac

end of the slotted line. The losses at the other end,

if any, and of the stub tuner need not be considered

in this case. Replacing a power meter or other indicating device with a dummy load allows an auxiliary

CW probe to be employed in the slotted line to re- produce a reading in any fixed plane in the two

steps. H the modulation characteristics of the unknown

and the source in step 2 are the same the VSWR

meter may be used for this purpose; using the CW

probe entirely eliminates modulation needs and

problems.

The relative merits of this method compared to the

Bolovac as an absorption power meter is in that it may eliminate the need to terminate the Bolovac in a

sliding short. This will be discussed later.

The Zo-matched power output of a source is measured employing the same two steps as for Pc, except

the three-stub tuner in the first step is adjusted for a

flat voltage distribution in the slotted line.

To measure the net power flow, P„, from any source

to any load, the first step consists simply of the

original system [3]. The system must not be disturbed.

However, when reproducing the power-flow condition

in the second step some kind of monitor is required

that will not upset the impedances and voltage distributions in the original step 1. K the load is a power

meter, its panel-meter reading is used; if the load is an antenna, a loosely coupled receiver or detector may

be used. In many cases field leakage picked up by a

sensitive receiver will serve as a monitor. Where a transmission line is present between the sources and

the load, the line should be considered part of the

load and its input plane should be connected to the

output end of the slotted line in step 2. (2) Power-meter Calibration. When calibrating a power meter using the Bolovac as a voltmeter

first measure the equivalent parallel resistive component of the power meter, Rp, with any conventional

method. Then connect the power meter to the normal

voltage output of a Bolovac (fig. 8-b) and the net

rf power is given by Vs^/Rp. Using a slotted line as shown in figure 8-a for this purpose, we have = SZo{VJV^.^y (12)

where F,„ is the voltage at a plane in the slotted line

nX/2 away from the plane of contact between the

slotted line and the input to the power meter. Impedance charts to find Rp are unnecessary and the

equivalent reactive component Xp is neglected because

it does not absorb power as is evident from figure

8-b. The accuracy of Rp depends on the accuracy of

Zq and of two rf-voltage ratios. It is also important

here that the coaxial slotted line have no discontinuities between the plane of F,„ and the input plane to the power meter.

Power meter

I 1

nX

2

R„ = SZ„ (V„/V„„J2

(No need of Z -chart)

(a)

Power meter

(b)

Figure 8. Calibration of power meter in two steps with

Bolovac operating in the voltage mode.

I
j

©

Ve Slotted

I line

— /SZo

\^

^ \

\ N

i 1

1 Power

) \^ meter \\ 1

P. • 1

(No need computing Rp)

radiation by shielding, or it may be terminated in

a sliding short to tune out residual internal reactances

of the Bolovac. The sliding short, calibrated once for

positions of the piston versus frequency, is connected

as in figure 10; the piston is then preset to the position corresponding to the appropriate frequency withFigure 9. Calibration of power meter in one step with

Bolovac operating in the voltage mode.

Locating the slotted line between the Bolovac and

the power meter is another way to calibrate the power

meter (fig. 9). In this case the net power flow from

the Bolovac to the power meter is measured and knowing the value of Rp is unnecessary. F„ is given by

eq (11).

It is possible to compute the value of the "effective

efficiency", rje, and the "calibration factor", Kc,

usually furnished by the manufacturer as corrections

to its panel meter readings [3]. This may be desirable

in acceptance tests. These corrections are defined as:

Substituted bias power

Net rf power dissipated in the mount

Substituted bias power

Power incident on the mount

The user must in this case calibrate the commercial

mount for substituted bias power. Then rje is given

directly in terms of the P„ obtained with the Bolovac.

The power incident on the mount is given by [3]

fP = Pn{s + ly/is. (13)

b. Power Measurement Using the Bolovac as a Power Absorbing Mount (Power-absorption

Mode)

Considered a new type of power-absorbing mount

the Bolovac differs from most of the present conventional mounts in several important respects. In this

application the normal voltage-output side of the

Bolovac is connected facing the rf source. The other

side of the Bolovac may be left open permitting some

power to be transmitted beyond the Bolovac film. Therefore, the other side may be left open when this

power loss is negligible or it may be terminated in a tube that extends the outer conductor so to reduce

7 /

-3r TFT =

Figure 10. Application of the Bolovac as an absorption type ; power mount.

The sliding short may be replaced by a shielding tube or may be !l eliminated, depending on the frequency and requirements. Note that the

tliin film is now facing the source.

out tuning for each measurement. The major differ- ences between the Bolovac and present conventional

mounts are as follows: —The bolometer of the Bolovac entails no rf-for-dc

substitution error. It has no lead inductance and

for all practical purposes its rf impedance equals

its dc resistance at all frequencies mentioned

above. ! —It has an input port as well as an output port, i which allows one to tune out internal residual'

reactances when necessary. —The bolometric film can be placed to within aj

couple of mils directly in the plane of the output!

of a source. It therefore has no input "connector"'

in the ordinary sense of this term. Its output [ plane is electrically in the plane of the film;j

Rprf (fig. 10) is the equivalent antiresonance

resistance of the sliding short including the losses}

in the bolometer substrate and whatever support-!

ing beads are used in the Bolovac and sliding

short assembly. Rb for this application is gen-^

erally of the order of Zo (e.g., 50-f2) or less and|

Rprf is much larger than Rb- Sliding shorts were!

constructed at the National Bureau of Standards;

having Rprf effective values of 200 Zo or higher

|

at 18 GHz. Therefore, for practical purposes the§

power absorbed by the Bolovac can be made to equal without any corrections Vb'^/Rs to within

a couple percent or better at all frequencies up

to 18 GHz. —Because the Bolovac has an input and output port

the losses of any other components used in the

power measurement setup are external to the

Bolovac and can be measured independently if

10
accuracies better than a couple percent are re- quired. In addition, these two ports make it

possible to measure the internal losses of the

Bolovac itself, if they are of any consequence. A

way of measuring these losses is discussed later.

Thus, the three major sources of errors, i.e., (1)

dc-to-microwave substitution error (2) appreciable rf

losses and (3) the termination mismatch requiring

individual rf calibration of conventional power mounts

[11, 12, 13], can be eliminated by applying the

Bolovac.

(1) Bolovac as Absorption Mount to Evaluate Quality of RF-Power Source. Figure 11

illustrates the setup to measure Pc- It was pointed out

that this method of measuring Pc is simpler than the one that uses the Bolovac voltage mode. A slotted line or an additional power meter shown in figure 7 is not

required. The procedure here is to adjust the stub

tuner for maximum Vn, i.e., for maximum bias power

to be removed from the Bolovac while maintaining its bridge balance. Rb need not be equal to Zq (e.g.,

50-n) and the residual internal reactances of the

Bolovac need not be tuned out because no power is dissipated in them. The losses in the stub tuner are determined by a method described below in the section on measuring insertions loss of components. Equation (3) is used to compute Pc- An important as- sumption throughout this paper is that stable sources

are used and that their Pc is constant at any given

frequency.

y /

1 1

rz7-

I

Pc = Pdc substituted + stub losses

- (Vb„2 - Vbi^) t stub losses

Figure 11. Measurement of maximum available power from

a source with the Bolovac as an absorption power mount.

The best way to measure Pq of a source using the

Bolovac as an absorption mount is to use a Bolovac

having Rb — Zq, and a sliding short as shown in

figure 10. When Rb does not equal Zq a slotted line

and a three-stub tuner must be used (fig. 12). A

sliding short is not needed. The stub tuner is first adjusted for a flat voltage distribution in the coaxial

line and Pq is measured next as was Pc. One may, of

course, adjust the stub as a separate preliminary step

and remove the slotted line to measure Po, provided

1*1 1 H

1 1

1

f //

Pq = ^^Bo^~^Bi^' 'osses in stub and slotted line

Figure 12. Measurement of Po with Bolovac as an absorption mount.

that the impedance looking into the stub tuner is Zq

when the line is removed.

By improvising an equivalent rf-power source employing a Bolovac operating in the voltage mode one

can measure the net power flow, P„, from any source

to any load (fig. 7). Although it may seem too complicated, it is not impossible to improvise an equivalent load using a Bolovac. To do this one must

measure the equivalent parallel input resistance and

reactance of the load, and then reproduce corresponding values of i?B of a Bolovac and shunt it with a reactance equal to the load reactance. Rb values are varied by changing the bias on the Bolovac bolometer

within certain ranges that are limited by its safe power

dissipation and its bolometric constant, y, the change

in ohms per watt of dissipated power. Once the correct value of Rb is obtained a sliding short at the

output of the Bolovac can be adjusted to reproduce

the value of S observed with the original load. P„

can then be measured using this terminated Bolovac

by connecting it directly to the given source in place

of the original load; the substituted bias power is the

desired answer. (2) The Bolovac as an Absorption Mount to

Calibrate Power Meters. A power meter can be

calibrated by substituting it with a Bolovac in a setup

measuring Pc of a stable rf-power source (fig. 11).

First, measure Pc of the given source using the

Bolovac as described above. Second, use the power

meter in place of the Bolovac and adjust the stub

tuner for a maximum power-meter reading. Power

absorbed by the power meter equal Pc measured by

the Bolovac. If is of the same order of magnitude

as the power-meter input impedance the loss in the

stub tuner will remain the same for practical purposes; otherwise it may be necessary to determine

the difference and introduce a correction.

(3) Calibration of Low-level Signal Generators Using the Bolovac in the Power-absorption

Mode. There are two kinds of rf signal generators.

One furnishes standard voltages at its output plane

provided that the load impedance connected to it is

11
two or more orders of magnitude larger than the

generator's internal impedance. The upper frequency

of these generators is seldom more than 30 MHz.

The others are designed to work into matched loads

and are calibrated in terms of voltage appearing across

the matched load, or power absorbed by such loads.

The upper frequency of thiese generators reaches into

the microwaves. Many or most of the second kind are

calibrated in terms of both voltage and power. This

difference in calibrating present conventional generators seems to introduce unnecessary confusion

without justification; the calibration may be made in

power alone assuming the ideal objective to be source and load impedances that equal Zq.

Calibration procedures for the first kind of generators are extensively covered in the literature. The

case of the second kind is discussed here because the

approach has some attractive aspects.

One should keep in mind, however, the possible

application of the Bolovac as a voltage source (as described above) to evaluate pulse-display oscilloscopes. In that case the Bolovac is treated as a zero-impedance

voltage source (with or without a series matching resistor) similar to low-resistance rf Micropotentiometers [14]. In case of signal generators delivering output levels corresponding to 0.1 V or higher, the signal generator is calibrated at the maximum output, employing

any of the methods described above. Specifications

generally call for a load of Zq; therefore a Bolovac

having Rn = Zq and its output terminated in a sliding

short, is preferable. Other Rb values may be used with

a three-stub tuner to present a Zq load to the signal

generator.

For lower outputs one may take advantage of the

presence of the sliding short as shown in figure 13.

A loosely coupled probe is combined with the sliding

short and its output is fed to a sensitive receiver in

series with an external standard attenuator, A. Start- ing with a high attenuation setting of A and maximum

output of the signal generator (measured by the

Bolovac), the settings of the internal attenuator of

the signal generator are calibrated by substitution in

terms of A, keeping the indication of the receiver,

Dj, constant. The advantage of this setup is that once

assembled, the entire calibration may be performed

without moving any of the components. The presence

of the probe need not reduce the effectiveness of the

sliding short appreciably. A slotted line with its slid- ing probe may be used in series with the sliding short;

its probe may then be set at any plane of the sinusodial distribution of the voltage in the slotted line

Slotted line or loosely coupled probe ossembly ^ Sliding short

Sensitive receiver, or

detector, or S meter

Figure 13. Carrier output calibration of low-level signal gen- i erators employing Bolovac in the power-absorption mode of [| operation.

to do two things: (1) to vary the voltage pickup of

the probe if necessary, and (2) to serve as another

calibration of voltage-ratios and as a double check on

A. One may also make use of the sinusoidal distribution of the voltage to calibrate the internal attenuator

when a standard attenuator is not available.

Throughout this procedure, it is best to keep the

Bolovac bridge balanced ( by increasing the bias power

as the rf-signal power is decreased) to keep the load on the signal generator constant; this may be important at high output levels and negligible at low

levels.

3.3. The Bolovac as a RF-current

Standard

Figure 3-b, a schematic for this application differs

from 3-a in that a constant-voltage bridge is shown in

the latter and a constant-current in the former. The

rf current in the current sensor is given by Vb/Rb^

or is computed in terms of the bridge currents [5].

The basic principle involved here was discussed previously. One may add that any current sensor at the

frequencies in question must be treated as an integral

part of its assembly including the shielding enclosures

located between defined input and output planes. The

Bolovac measures current flow in the center conductor

at the output plane of the current-sensor assembly.

One must keep this in mind when designing the nor- mally voltage-output plate of the Bolovac to provide

proper connection facilities. It is evident that the

higher Rn is, the lower are the rf currents that can be measured and vice versa. Current standardization

is particularly important in evaluating current-pulsedisplay instruments for fast-rise-time pulses having

microwave-current components. P

12 t
3.4. Application of the Bolovac for

Insertion-loss Measurements

A practical way to determine rf losses of components, such as three-stub tuners, slotted lines,

adapters, etc., is to measure the power Pc absorbed

by a Bolovac (or a precalibrated power meter) supplied from a stable rf source, i.e., a sou{;ce having a constant Pc. A three-stub tuner must be used to begin

with, when Pc is measured as in figure 11. Nonadjustable components, such as a slotted line, are simply

inserted into the setup; the stub is retuned for maximum indication of the Bolovac and the loss equals

the difference between the two values of the measured

Pc. An uncertainty is introduced by the limited reproducibility of connector contacts and the difference

and reproducibility in the losses of the different

plunger positions of the three-stub tuners. Our experi- ence indicates that this uncertainty is, for practical

purposes, negligible when reasonably good-quality

commercial components are used. The user may of course perform a number of repeated measurements

and establish a reproducibility figure statistically.

Adjustable components, such as stub tuners, can be

treated the same way. It seems unnecessary to use

exact duplicate components for this purpose. Pci can

be measured with a stub manufactured by X, and

Pe2 with an additional stub manufactured by Y. The

difference [Pa ~ Pa) represents the loss of the stub

inserted last. Because the contact resistances of stubtuner plungers can have poor reproducibility, the

limits of uncertainty for the losses of any one threestub tuner can be measured by setting the plungers

at various positions to obtain a maximum possible

error; the number of possible positions yielding a maximum power reading, as is well known, approaches

infinity. Using a precalibrated commercial power

meter might be more convenient and may prove

equally reliable because the ratio Pci/Pc2 is considerably more accurate than either one of these values.

The significance of the losses in the sliding shorts can be checked conveniently in combination with the

Bolovac in preference to measuring their high input

VSWR without the Bolovac. Figure 8-a and equation

(12) are used in this case. The Rp values of the

Bolovac are measured with and without connecting

the sliding short and yield the answer of interest in

power measurement. Equations (4) and (10) and the

measured value of R,, will yield the correct P„. The rf

level used for this measurement should be kept low

enough not to affect Rb', otherwise, it is necessary to keep the Bolovac bridge balanced at each measurement.

The significance of the internal losses of a Bolovac

can also be checked by treating it as a component.

The only difference is that a "dummy" Bolovac of the

given design should be used in which the film has

been omitted. Thus one measures any possible residual

losses of the structure including those of the substrate,

the supporting element of the center pin, and possible

radiation losses through the blocking capacitors

(fig. 3).

3.5. Self-evaluation of the Bolovac

The Bolovac's two operational modes enable measuring its own internal losses and detecting other internal irregularities within the limits of resolution of

the associated equipment. Its voltage output, Fjj, is not affected by its internal losses no matter how large

they might be. On the other hand the power absorbed

by the Bolovac as an absorption mount is of course

affected by its losses. Therefore, one can measure a

fixed value of Pc by the two operational modes (figs.

7 and 11) and compare the two results after cor- recting for residual losses in the slotted line of figure

7. Figure 14 shows the steps more clearly. Step (a)

establishes a reference reading, D^, of any power meter

corresponding to Pc of the given stable source. Step

(b) calibrates the value of Pc in step (a) operating

Any rf source G>

)

i

Pc

Power

meter

(a)

[1

^

)—

1 * 1

(b)

Some source

OS used in step (a) 7^

+ slot line losses

+ stub tuner losses

or Pf. = VgVRp + slot line

losses + stub tuner losses,

(c)

Figure 14. Self checking of Bolovac.

13

I.
the Bolovac in the voltage mode. Step (c) measures Pc of the same source operating the Bolovac in the

power-absorption mode. The method for determining

the slotted-line and the stub-tuner losses have been

described earlier. The values of Pc obtained in steps

(b) and (c) should of course be the same. Chances

are that no difference will be detectable using well

constructed Bolovacs except perhaps at the very upper

frequencies of the range indicated. On the other hand

large differences point to improper assembly or other

defects in the Bolovac.

Another way to check Bolovac quality is to measure

the resistive component of the Bolovac input impedance when terminated in a shielding tube. Equation

(12) yields the value of the R'p of the Bolovac when

no appreciable losses are present. In case R'p is dif- ferent from /?B it can be used instead of i?B to compute

the power absorbed by the Bolovac, i.e.,

Pn = VbVR'p = Wio - Fil)/i?V (14)

3.6 Other Bolovac Applications

Because is known and remains constant at all frequencies to 18 GHz, the input impedance looking

into the Bolovac voltage-output port for power absorbing connection, equals Rb provided the losses of

sliding-short when connected to the other side of the

Bolovac (fig. 10) are negligible. The residual losses

of the film substrate and center-conductor supporting

bead of the Bolovac itself are assumed negligible. Provided this condition on the losses holds, the Bolovac

serves as a known impedance at all frequencies for

various applications; e.g., as a match or mismatch

termination over an unprecedented frequency range

for impedance, attenuation, residual VSWR of slotted

lines, and other measurements. Moreover, its magnitude can be varied over a range of values depending on the film characteristics and can be kept constant

to a high precision by adjusting its bias.

With proper adapters the Bolovac can be used to measure power in single-conductor transmission sys- tems operating in any mode. Adapter losses are determined as described earlier, using adapter pairs.

High-resistance (/?B> 1000fi) Bolovacs can be

used to monitor net power flow in systems during

operation. Because the bolometer consumes a small

fraction of the net power, these power levels are limited by its power dissipation capabilities. The net power flow can exceed a kilowatt. A notable advantage

in such an application is the high sensitivity of the

Bolovac as a detector of small variations in the load

impedance of the system. Such variations are readily

detected as the voltage in the plane of Vb is changing,

whereas they are not detected when reading, for ex- ample, traditional side-arm power meters of directional

couplers. A high-/?/j Bolovac can be connected into a system permanently or temporarily with a slotted

line and operates at all frequencies up to 18 GHz as compared to the limited frequency ranges of directional couplers.

The Bolovac can be connected to the side arm of a

directional coupler. Two Bolovacs can be employed to calibrate the directivity and coupling factor of directional couplers. In either case, the Bolovac serves as a standard of rf power, while conventional meters or

detectors need rf calibration.

One may reasonably expect additional applications

of the Bolovac either as a voltmeter or as a source of

known voltages, a standard of current, or a powerabsorption mount when it is commercially produced

and receives wide laboratory use.

4. Structural Features of the Bolovac

4.1. Mechanical Design

First generation Bolovacs at the National Bureau

of Standards are illustrated in some detail in reference

[3]. Some figures stressing salient features are re- produced for the reader's convenience. Major up-todate modifications are also pointed out.

Figure 1 illustrates conceptually the bolometer as- sembly in the center plate of the Bolovac. The first National Bureau of Standards bolometers had solid

substrates clamped, soldered or cemented to the center

plate. This is also shown schematically in figure 15

and figure 16 shows the physical location of the

bolometer in the split center plate. More recent units

(fig. 17) used 2-mil-thick plastic substrates.

To date the general approach in the National Bureau of Standards design uses three plates plus the

bolometer to form a core that fits into a nest of a large

three-ring assembly adding more capacity, C^, to the

outer conductors of the core (figs. 18 and 19). The

assembly in figure 19 allows the Bolovac to operate

down to about 0.5 MHz. As was partly discussed in

the principle of operation, the core alone operates in a range from about 0.1 to 1 GHz and the metal output

plate of this core is replaced by a bakelite or plastic one for frequencies above 1 GHz. Two or more capacitive rings can be used that fit into each other successively depending on application goals. The values of

Cb and the residual series and shunt inductances that

14
affect the resonant frequencies of these radial trans- mission lines [4] also can be varied by controlling

the thickness of the dielectric and the number and

size of metallic bolts that hold the assembly together.

A version of an interchangeable bolometer (fig. 20)

constructed as a separate unit and employing a 2-mil

I

Figure 15. Exploded view of core for voltage standardization.

Figure 16. Current core exploded.

substrate is one of a number of possible variations in

the Bolovac design. This interchangeable element is referred to as a "bolometer cell". The only thing

common to the rings and core is that the center plate

in all of them is split along the diameter. Figure 21

shows the application of the Bolovac to calibrate a voltage standard (ATVM).

Figure 17. Bolometric film on two-mil plastic substrate.

Figure 18. Core (with voltage plate) and auxiliary capacitor

of Bolovac,

15
Figure 19. Voltage core and auxiliary capacitor.

Figure 20. Interchangeable bolometer cell ( employing a 2-mil

substrate) mounted in the center plate of the Bolovac.

Figure 21. Bolovac set up to calibrate attenuator- thermoelement voltmeter (ATVM).

4.2. Thin-film Materials for the Bolovac ! Details on fabricating bolometers for the Bolovac

{ are a subject for a separate paper to be published by I

the National Bureau of Standards or by another I laboratory choosing to abandon proprietary rights, if any. The remarks to follow are based on up-to-date

experience at the National Bureau of Standards, on a

cursory search of some of the vast amount of published literature on thin films, on direct contact by

the author with experts outside the National Bureau

of Standards and with private concerns, and on the [ opinions and recommendations of Paul E. Werner 6 who was solely responsible for the film deposition for i

these bolometers at the National Bureau of Standards, i The reader is referred to the recent books and surveys on the subject and to exhaustive references to hun- : dreds of papers in these issues [15-27]. i The most important characteristics of the bolometer , are the sheet resistivity, Rg, measured in ohms per

square, the stability of Rs, and its temperature co-j,

efficient of resistance (TCR) in percent per degree

[ C of the film. Others are mechanical stability, power |, dissipation, reliability of electrical contacts to the con-j|

ductors of its mount, and low residual noise. Only|.

the first two, Rg and TCR will be discussed. A basic

|j requirement, as stated earlier, is that the rf resistance

,

equal the dc resistance of the unit at all frequencies!

in question. . Based on free-space characteristic wave impedance, | Rs should be 377 O/sq for any air-dielectric trans-,

mission system having two or more conductors [28]. [, The TCR should be as high as possible. One must keep!;

in mind that the minimum acceptable TCR value de-t

pends entirely on the signal-to-noise ratio (S/N) of

the bridge and Bolovac setup and of the desired

dynamic range. The only objective of the TCR is to ; enable the bridge galvanometer to tell us when the

original value of Rg is restored. At the National Bureau of Standards, using good quality commercial

bridge components, platinum and nickel films on %2-

to %-inch solid glass or on 2-mil plastic substrates ,f are satisfactory for low Rb bolometers. The resulting jfi sensitivity, y, expressed as the change of bolometer

resistance in ohms per watt was in the range of 1

to 10. The corresponding TCR of the film materials was in the range of 0.1 to 1.0 percent/deg. C. One

should be able to improve the S/N ratio and employ

film materials having TCR values of 0.01 to 0.1 par- , ticularly if willing to sacrifice some dynamic range.

Correlation between the dynamic range of the setup,

its sensitivity and resulting accuracy is given in^

reference [29].

16
No serious film-deposition problems were encountered at the National Bureau of Standards for Rs

values up to 10-ohms. One of the first 25-0 platinum

bolometers fabricated on a glass substrate remained

in stable operation for over two years and failed

because of a lose center-pin assembly. However, other

such 25-0 disks were not reproduced. As said before,

50-Q evaporated carbon-film bolometers on 2-mil

plastic substrates lasted long enough for us to establish rf-dc resistance equation to 18 GHz; additional

repeated verifications are needed. Some very limited

laboratory work is in progress at the National Bureau

of Standards to improve the bolometer assembly and

to select the best film materials for high Rs values.

In scanning the thin-film literature one seems to

find little quantitative information on high Rs — high

TCR materials and on film characteristics at frequencies through microwaves. Films are generally subdivided into "thin" and "uhrathin" [30]. Thin

metallic films seem to imply thicknesses at which their

electrical characteristics approach those of the cor- responding bulk material. Their resistivities are about

45 percent higher and their TCR is 30 to 60 percent

lower than the bulk [31, 32]. Ultrathin films refer to smaller thicknesses starting with the "critical" thickness, do, at which dc conductivity begins. The thin

alkali-metal continuous films generally have thicknesses

exceeding 3 to 4 atomic layers [33], while ultrathin

films consist of "islands" of the material coming in contact with each other at the thickness of do- Unfortunately there is a wide disagreement on the numerical

values of these thicknesses. For example, thin copper

films are quoted in one case as starting at 300

and in another at 3800 A [34]. Since the TCR is a function of the film thickness, as well as of other conditions, its value seems equally uncertain. Ultrathin

films generally refer to thicknesses below 100 A [35] ; their resistances may or may not remain constant with

time, depending on many parameters of the material

and conditions under which they were produced.

Among these factors are the characteristics of the film

material itself, the characteristics and temperature of

the substrate materials and the conditions of its surface, the rate of the film disposition, the deposition

vacuum, the presence of impurities in the deposited

material, the presence of and kind of residual gas in

the vacuum chambers, and the aging of the film. The

mean free path (mfp) of the conduction electrons in

the film and the scattering or reflections of some of

these electrons by both the top and bottom surfaces of

'An angstrom is a unit of length equal to 10-'" meters; its symbol is A.

the film may have an appreciable effect on the result- ing resistivity of the film. A method widely used to determine the structure and characteristics of films is

to measure the electrical impedances of various

samples of known dimensions. One would therefore

have to resort to an enormous amount of experimentation in addition to what has already been done to obtain the missing desired data on high- Rg high

TCR films. It seems more practical for our purpose to employ the trial-and-error approach when choosing the

best combination of film and substrate materials and

film-deposition conditions required for the specific

bolometer Rb values.

Another important justification for the above empirical approach is the advisability of checking the

rf characteristics of each new bolometer type throughout its usable frequency range, even though Rg has

not been found to be frequency dependent at these

frequencies in published literature. Considerable

studies have been made on the so-called "anomalous

skin effect" of metal films i.e., how frequency affects the surface impedance of these films. This effect results

from the expected interdependence between the mfp

of the electrons in the film and the depth of the cur- rent penetration into the film material at various

frequencies. An analytical prediction [36] shows no

appreciable departure from the classical behavior

should be expected at frequencies below the infra-red

at ordinary temperatures (above 0° C) and that the

anomalous skin effect is essentially a low-temperature

phenomenon. The effect was observed for bismuth

films at 23.5 GHz at temperatures of 2° K. [37, 38].

Thin and ultra-thin films having resistivities many

times that of the bulk material may be tried to produce Bolovac disks as long as their resistances are

stable and reproducible. The Bolovac is particularly

suited for measuring Rs values of various films at microwave frequencies in preference to resonant cavity or other methods. The physical shape of the samples

is essentially the same as recommended for this purpose in the literature [39] and the fairly simple procedure for measuring Rs described above in Section

3.2a. (2) can be applied. Direct comparisons of high

and low Rg values can be readily obtained. The

Bolovac thus turns out to be a valuable and efficient

tool for studying properties of materials.

Films having high Rs value can be obtained by employing (1) ultrathin films of single metals, (2) thin

refractory metals, (3) thin metallic alloys, (4) semiconductors and (5) cermets. Anisotropic metals, such

as molybdenum, should be avoided. It is likely that a number of laboratories already have the desired

17
materials and deposition techniques. Unfortunately

and understandably the information is most likely of

a proprietory nature. Some of the possible promising

prospects of materials are indicated below [40].

Radial narrow-strip sensing elements (instead of

half-disks) may be used for Rg values of 50-Q. employing platinum, nickel and other low-resistivity

metals. This approach is practical only if one can

eliminate the residual inductance of the diametrically

placed strips; this is required to avoid a possible

serious frequency effect on R^. We had no opportunity

to try this type of bolometer at the National Bureau

of Standards, but it does have a wide application in

commercial power mounts [41].

High Rg and high negative TCR can also be fabricated with single metals [42]. Ultrathin single-metal

films deposited on a plastic substrate have large positive TRC's when the thermal-expansion coefficient of

the substrate is 10 times larger than that of typical

metal [42]. It may be worth while to explore the

frequency effect on such films.

Among the high-resistivity and refractory metals,

bismuth, tungsten and zirconium can be considered.

Bismuth has a dg of 35 A and a high resistivity and

high negative TCR at a thickness of several hundred

angstroms [43, 44]. Resistances of up to 500f2/sq

with fair negative TCR's can be obtained with tungsten and other refractory metals [45].

Of the various alloys referred to in the literature

chromium-nickel, Cr-Ni(20:80) , is applicable for Rs

values of 400 Q/sq and positive TCR's of 0.01 to

0.02 percent per degree C. Others are listed in the same reference [45].

As previously indicated, encouraging results were

obtained at the National Bureau of Standards with

evaporated carbon using an evaporation technique

similar to the one described in [46, 47]. Tin oxide

having a sufficient TCR is another semiconductor material to be considered [47]. Semiconductor films,

e.g., indium-antimonide, should be tried, though little work has been done to date on these materials [48].

Cermets are mixtures of a metal and a dielectric.

Chromium and silicon oxide, C, — SjO, are often

mentioned for /?« values up to 600n/sq with fair negative temperature coefficients [45]. These types of films seem to open up a wide range of prospects for the

present needs [49]. Relatively high Rg bolometric

films fabricated outside the National Bureau of Standards are briefly referred to in the appendix.

5. Auxiliary Equipment

i Designing direct-readout circuits for various ap-j

plications of the Bolovac seems feasible, but is outsidelj

the scope of this paper. Therefore, the immediate ap-i

plications above assume familiarity by the user with,

various components and rudimentary precautions re-j quired in making rf measurements at the frequencieS|

in question. These components include bias-power

substitution bridges, sources of rf and biasing power,]

high-impedance dc or audio voltmeters (DVM's),;^

galvanometers, frequency meters, isolators, coaxialj

switches, etc. The quality and the inclusion of specificj

components into a measurement setup will affect thai accuracy of the results. In all cases commercial equip-'

ment is adequate to realize the advantages indicated]

above in measuring power. Voltage- and current-l^ measurement applications are a completely new ex-j

tension into a range of frequencies where accurate,

measurements were not previously possible. , Power-substitution bridges of various kinds havei

been described extensively in published literature [5,^

50, 51]. For high precision measurements the readerj

is referred to a recently developed National Bureau

of Standards dc biasing bridge [52]. Because the

Bolovac has a much wider dynamic range and range

of Rs values than typical present-day commercial

mounts, the range of resistors, currents, voltages, and

power of a bridge will have to be either broad enoughi

to cover an entire class of Bolovacs (e.g., Rb values

of 0.1 to 50 n) or limited to a narrow range (e.g.,

45 to 55 n).

6. Conclusion and Future Prospects

Bolovac applications open up new approaches to investigations that employ direct measurement of

microwave voltages and currents. They introduce

superior power-measurement methods based on more

familiar concepts and procedures and yield higher ac- curacies than present test equipment provides.

The physical version of the National Bureau of

Standards-constructed Bolovac can be reproduced and

used with solid- or flexible-substrate bolometers.

Bolometers having Rb values of 10 fi (surface re^

sistivities of about 80 fl/sq) or lower, using platinum

or nickel films were used succesfully at the National

Bureau of Standards. Carbon films show good promise

for higher Rn values but were not yet tried on solid

substrates. Searching the up-to-date literature on thin

film deposition indicates promising prospects for high

resistance metals and other materials for high-i?^

bolometers.

I?

iiitl

(lilt

ispec

rape y

A'

to I

p

18
j One can logically expect further improvements and

industrial refinements on any first-generation devices

Ls they are placed on the market. However, manufac-

'jturing the Bolovac affords selections of a very wide

range in structural features far exceeding relatively

'{simple modifications. A manufacturer caij readily exploit these as proprietory-right elements. For example,

[there is a wide range of tbin-film rnaterials and struc-

'Itural details that secure high stability and sensitivity

'lof the bolometer plus a wide range of frequencies and

jdynamic ranges to choose from. The sensitivity and

'iaccuracy of the Bolovac and associated bridge offers

'another aspect to be considered. Size and weight of

'the Bolovac, size of the transmission lines, type of

connectors, including a universal design to accommodate many types of connectors (e.g., type N, LPC,

etc.) for a given system, ease of assembly and modifying the Bolovac to add or remove capacity rings, de-

]sign to eliminate capacity rings, all are factors in

ichoosing a specific design. The economic aspect, life

lexpectancy and reliability are most important in this

respect as with any product on a competitive market.

Readout-circuit development of incorporating the

jBolovac in automated systems of measurement are

Itwo other areas that offer room for individual imagination and initiative; these seem particularly challenging for wide-band calibration and evaluation of

power meters, signal generators, sources of rf, pulsedisplay oscilloscopes, spectrum analyzers and the like. An entirely different type device closely similar to

the Bolovac and nearly identical to its application is

the "Thermovac" [1]. Employing a thermoelectric

detector in place of the bolometer it may prove attractive for marketing primarily because of two features.

(1) The bridge required for the Bolovac is replaced

with a dc or af voltage output reading instrument. (2)

It may be more economical to fabricate high-7?/j value

thermoelectric disks; a certain type of such disks

has in fact been used recently with commercial power

meters [53]. Thus, the prospects for variability in

marketing and application of the Bolovac and its equivalents seem almost limitless.

j Appreciation is hereby expressed to P. E. Werner

ifor the numerous discussions we had with him and

for his review of the section on thin films; also to A. R. Ondrejka and R. A. Lawton for their careful

review of the manuscript.

7. References

[I] Selby, M. C, Electrothermic instruments for measuring

voltage and current, U.S. Patent 3 487 305 (Dec. 1969).

[2] Selby, M. C, Voltage measurement at high and micro- wave frequencies in coaxial systems, Proc. IEEE 55,

No. 6, 877-882 (June 1967).

[3] Selby, M. C, Bolovac application for H.F. and microwave

power measurement and standardization, J. Res. Nat.

Bur. Stand. (U.S.), 74C (Eng. and Instr.), Nos. 3 and

4, 123-133 (July-Dec. 1970).

[4] Selby, M. C, The Bolovac and its applications, IEEE

Trans. Instr. Meas. IM-19, No. 4, 324-331 (Nov. 1970).

[5] Selby, M. C, Bolometric voltage and current (BOLOVAC) standard for high and microwave frequencies,

J. Res. Nat. Bur Stand. (U.S.), 72C (Eng. and Instr.), No. 1, 61-79 (Jan.-March 1968).

[6] Ragan, G. L., Microwave Transmission Circuits, MIT

Rad. Lab. Series 9, pp. 105-106, McGraw-Hill Book Co.,

Inc., New York, N.Y., (1948).

[7] Ramo, S., and Whimery, J. R., Field and Waves in Modern Radio, 2d ed., 395-401 (John Wiley & Sons,

Inc., New York, N.Y., 1953).

[8] DeLoach, B. C, Jr., Radial-line coaxial filters in the microwave range, IEEE Trans. MTT 11, 50-55 (Jan.

1963).

[9] Varon, D., Radial-line band rejection filters in coaxial

waveguides, IEEE Trans, MTT 15, 680-687 (Dec. 1967).

[10] Selby, M. C, Single-bolometer series-parallel circuit for accurate voltage and power measurement, Proc. 19th Annual ISA Conf. & Exhibit, New York, N.Y., Oct. 12-

15, 1964, pp. 21.6.1.64-1 to 21.6.164-12 (Oct. 1964).

[II] Sucher and Fox (editors). Handbook of Microwave

Measurements, 3d ed.. Vol. 1, p. 186ff (Polytechnic

Press, Polytechnic Institute of Brooklyn, Brooklyn, N.Y.,

1963)

.

[12] Ginzton, E. L., Microwave Measurements (McGraw-Hill

Book Co., Inc., New York, N.Y., 1947).

[13] Microwave power measurements, Hewlett-Packard Company, Appl. Note 64, October 1965.

[14] Selby, M. C, Accurate radio-frequency microvoltages. Trans. AIEE 72 (Commun. and Elect.), Paper 52-33,

158-163 (May 1953).

[15] Chopra, K. L., Thin film phenomena (McGraw-Hill Book

Co., Inc., New York, N.Y., 1969).

[16] Maissel, L. I., and Glang, R., Handbook of thin film technology (McGraw-Hill Book Co., Inc., New York,

N.Y., 1970).

[17] Dummer, G. W. A., Materials for Conductive and Resistive Functions (Hayden Book Company, New York,

N.Y., 1970).

[18] Espe, Werner, Materials of High Vacuum Technology

(Pergamon Press, Elmsford, N.Y., 1966).

[19] Keonjian, Edward, Microelectronics, pp. 181-191, 256-

257 (McGraw-Hill Book Co., Inc., 1963).

[20] Halaby, S. A., Gregor, L. V., and Ruben, S. M., The

materials of thin-film devices, Electro-Technology, No.

57, 95-122d (Sept. 1963).

[21] Hass, Georg, and Thun, R. F., Physics of Thin Films,

3, (Academic Press, New York, N.Y., 1966).

[22] Powell, C. F., Oxley, J. H., and Blocher, J. M., Jr., Vapor Deposition (John Wiley & Sons, Inc., New York,

1966).

[23] Dushman, Saul, Scientific Foundations of Vacuum Technique (John Wiley & Sons, Inc., New York, N.Y., 1949).

[24] Holland, L., Vacuum Deposition of Thin Films (John

Wiley & Sons, Inc., 1956).

[25] Hughes Electronic Properties Information Center [EPIC],

Interim Reports No. IR-26 and IR-47.

[26] Ault, C. M., Triller, D. P., Curry, R. W., and Hague,

J. D., U. S. Naval Avionic Facility, Fabrication and

Evaluation of Vacuum Deposited Cermet (Ci—SiO)

Resistors, Technical Report TR-688, Indianapolis, Indiana

46218, (Jan. 1966).

[27] Reiser, R. B. and Micklin, W. H., Electrical Conductivity Studies of Metallic Films. WADC Technical Report 57-660. ASTIA Document No. AD 1555573 (Dec.

1957).

19
[28] Moreno, Theodore, Microwave transmission design data,

p. 33 ff (Dover Publications, New York, N.Y., 1958).

[29] Selby, M. C, Single-bolometer series-parallel circuit for accurate voltage and power measurement. Op. cit, fig- ure 4. [30] Neugebouer, C. A., Electrical resistivity of ultrathin metallic films. Trans. 9th Nat'l. Vacuum Symposium, p. 45 (The Macmillan Company, New York, N.Y., 1962).

[31] Belser, R. B. and Hicklin, W. H., Temperature coefficients of resistance of metallic films in the temperature range of 25° to 600°C, J. Applied Phys. 30, No. 3, 313-322 (March 1959).

[32] Komnik, Yu F., and Palatnik, L. S., The influence of the structure of thin films on their electrical conductivity,

Soviet Physics-Solid State 7, No. 2, 429-431 (August

1965)

.

[33] Maissel, op. cit., p. 13-11.

[34] Chopra, op. cit., p. 189. [35] Ibid, p. 85. [36] Renter, G. E. H., and Sondheimer, E. H., The theory

of the anomalous skin effect in metals, Proc. Royal Society

(London) A195, 336-364 (1948).

[37] Chopra, op. cit., p. 409. [38] Rohderick, E. H., Electrical properties of thin-films, British J. Appl. Phys. 15, 617 (1964).

[39] Maissel, op. cit., p. 13-26.

[40] Ibid, chapters 13 and 18. [41] Sakurai, K., and Nemoto, T., A thin-film bolometer

unit, IEEE Trans. Instr. Meas. IM-16, No. 3, 206 (Sept.

1967).

[42] Neugebower, op. cit. '

[43] Duggal, V. P., Rup, Raj., and Tripathi, P., Quantum "l size effect in thin bismuth films, Appl. Phys. Letters j 9, No. 8, 293 (Oct. 15, 1966). i [44] Komnik, op. cit. Ii [45] Chopra, op. cit., p. 388, table IIL , [46] Holland, op. cit., p. 398. i; [47] Maissel, op. cit., p. 18-22, 18-23.

[48] Chopra, op. cit.. Chapter VII, p. 434.

j [49] Gregor, op. cit., p. 103, Fig. III-7. I [50] Wind, M., Handbook of Electronic Measurements, I, r 358ff, (1958). I' [51] Sucher and Fox. op. cit., p. 190ff. i [52] Larsen, N. T., and Clague, S. R., The NBS type II

!; power measurement system, ISA 25th Annual Conf. 1 Proc, paper 712-70, (Oct. 26-29, 1970). , [53] Hopfer, S., Riederman, N. H., and Nadler, L. A., The | properties of thermoelectric elements as microwave power

detectors, 1962 IRE International Convention Record, I Part 3, 77-84, (1962).

j [54] Lane, J. A., Transverse film bolometers for the measure- j- ment of power in rectangular waveguides, lEE 105B, | paper No. 2488R, 77-80 (January 1958) . I [55] Lemco, I., and Rogal, B., Resistive film milli-wattmeters

| for the frequency bands 8.2-12.4 GHz, and 26.5^

GHz, Proc. lEE, Pt. B, 427-430 (Sept. 1960). i [56] Schiffman, B. M., Young, L., and Larrick, R. B., Thin-'

film waveguide bolometers for multimode power measurement, IEEE Trans. G-MTT 12, No. 2, 155-163 ^ (March 1%4). ,

!

8. Appendix

i' Lane [54] used thin-film elements in rectangular crown glass having the same sheet resistivities as Lane, f waveguides for measuring power at 10 GHz. Sheet Their limits of uncertainty in measuring power wasF

resistivities of 220 n/sq were deposited by sputtering 2 percent at 8 to 12 GHz for an instrument produced P platinum on 10"--cm mica substrate at a pressure of for the market. They also designed models for 12 tof the order of 10"- mm Hg. The resistance was adjusted 18 and 26 to 40 GHz and indicate promising results! by heating the film on a hot plate. Gold electrodes at these frequencies as well. They investigated sput- were used. The continuous safe-power-dissipation tering platinum on mica but failed to reproduce re

rating was 100 mW and no change in the film re- sistances with this process.

'

sistance was noticed during his 2-month period of Schiffman, Young and Larrick [56] used gold,

observation. He found the rf-dc power substitution silver, gold-germanium mixture, Nichrome or leadj

held to within better than 3 percent and got an agree- tellurium on thin sheets of mica or 2-mil thick corning;

ment of 1 percent with other methods. He concluded glass for bolometers of sheet resistivities up to 400 J

that these elements should be equally reliable for both n/sq. They expressed confidence in the stability of? CW and pulsed power in rectangular waveguides at platinum films on mica and felt that these bolometers,'

any frequency. employing rf-dc substitution measure power accu-, Lemco and Rogal [55] used carbon films deposited rately up to frequencies for which the film thickness is, on 5-mil bakelite and Nichrome evaporated on 4-mil less than the skin depth. !

"iV U.S. GOVERNMENT PRINTING OFFICE: 1972 o 448-203

20
FORM NBS-114A (1-71)

L' " — 1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS MN-123

2, Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

ROT OVA r Sv<?t"PTn'? For Mpa.'iu'rinp Electrical Quantities from

0.5 MHz Through Microwaves

5. Publication Date

January 1972

6. Performing Organization Code

7. AUTHOR(S)

Myron C. Selby

8. Performing Organization

Q DK tr D 17 \/!TMi^ /^P A M T 7 A X TOM M A \XP A\Tr> Ar^T^DTTQQ Jt Kr VJKMliN vj VJKOAIN l^A i HJiN INAMC, Ai\U AUUlVJtloo

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/ Work Unit No.

11. Contract/Grant No.

12, Sponsoring Organization Name and Address

Same as No. 9

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

A novel device - the BOLOVAC - is employed in measurement systems that furnish known

voltages and currents up to 18 GHz, remove serious difficulties in power measurements,

and are applicable in impedance, attenuation and other measurements. Typical of such

measurements are the evaluation of rf power sources; calibration of power meters;

measurement of net and incident power flow in a system; calibration of signal generators, voltmeters, current meters, picosecond-rise-time pulse-display oscilloscopes,

spectrum analyzers; determination of insertion loss of components; and calibration of

directional couplers. The Bolovac also serves as an accurately known resistive termination for match or mismatch applications from LF through microwaves

.

Key words: BOLOVAC; calibration of current meters through microwaves; calibration of

power meters through microwaves; calibration of voltmeters through microwaves; current

measurement; disk-type bolometer evaluation; frequency -domain pulse evaluation; measurement of microwave current; measurement of microwave power; measurement of microwave

voltage; mismatch error; picosecond rise-time pulse evaluation; power measurement;

pulse-rise-time standardization; sampling oscilloscope standardization; spectrum analyzer standardization; termination mismatch error elimination; thin-film bolometers;

voltage measurement.

17. KEY WORDS (Alphabetical order, separated by semicolons)

See above

18. AVAILABILITY STATEMENT

[x] UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLAS^JIED

Tv-rT

''( 20. 'CLASS

(1 PAGE)

UNCLASSIFIED

21. NO. OF PAGES

24

22. Pr

oO cents

USCOMM-DC 66244-P7)
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

OFFICIAL BUSINESS

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

Penalty for Private Use, $300