•-. 1 . •as

rtD l b 1:

jSoulder laboratories

PB 151396

^o. 37

APPLICATION OF RF MICROPOTENTIOMETERS

FOR CALIBRATION OF SIGNAL GENERATORS

TO IOOO Mc

BY L. F. BEHRENT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Aclivities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901. as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent with these standards: the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

and structures; advisory services to government agencies on scientific and technical problems; in- vention and development of devices to serve special needs of the Government; and the development

of standard practices, codes, and specifications. The work includes basic and applied research,

development, engineering, instrumentation, testing, evaluation, calibration services, and various

consultation and information services. Research projects are also performed for other government

agencies when the work relates to and supplements the basic program of the Bureau or when the Bureau's unique competence is required. The scope of activities is suggested by the listing of

divisions and sections on the inside of the back cover.

Publications

The results of the Bureau's work take the form of either actual equipment and devices or published papers. These papers appear either in the Bureau's own series of publications or in the journals

of professional and scientific societies. The Bureau itself publishes three periodicals available from

the Government Printing Office: The Journal of Research, published in four separate sections,

presents complete scientific and technical papers; the Technical News Bulletin presents summary

and preliminary reports on work in progress; and Basic Radio Propagation Predictions provides

data for determining the best frequencies to use for radio communications throughout the world.

There are also five series of nonperiodical publications: Monographs, Applied Mathematics Series,

Handbooks, Miscellaneous Publications, and Technical Notes. Information on the Bureau's publications can be found in NBS Circular 460, Publications of the

National Bureau of Standards ($1.25) and its Supplement ($1.50), available from the Superintendent

of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS

technical ^Ylote

37

JANUARY I960

APPLICATION OF RF MICROPOTENTIOMETERS

FOR

CALIBRATION OF SIGNAL GENERATORS TO 1000 Mc

by

L. F. Behrent

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price $ .50

APPLICATION OF RF MICROPOTENTIOMETERS FOR CALIBRATION

OF SIGNAL GENERATORS TO 1000 Mc

L. F. Behrent

ABSTRACT

With the RF Micropotentiometer, signal generator output

voltage can be calibrated to 1000 Mc simply and accurately if

the procedures outlined in this paper are carefully followed.

The sources of error are discussed and methods for minimizing

or eliminating them are described. Topics discussed include:

rf shielding, selection of a suitable rf detector, impedance matching

and the proper selection of voltage reference planes.

CONTENTS

Page

SUMMARY ii

1. INTRODUCTION i 1

2. CALIBRATION PROCEDURE 1

3. PRECAUTIONS 2

4. CONCLUSIONS 6

5. REFERENCES 7

APPENDIX I 8

LIST OF ILLUSTRATIONS

Figure 1. Block diagram of circuitry for calibrating rf

signal generators with RF Micropotentiometers.

(Details of shielding not shown) 9

Figure 2. Photographs of RF Micropotentiometer. (Two

views: Top view to illustrate placement of the

thermoelement in the box and the heavy wall

construction; side view to illustrate d-c

connections. ).. 10

Figure 3. Cross-sectional view of metal box containing the

RF Micropotentiometer components ..... 11

Figure 4. Photograph showing the use of individual screened

boxes for shielding the signal generators and

the detector. Insert shows method of applying

external shielding braid to rf cable. .... 12

Figure 5. Block diagram to illustrate preferred voltage

reference plane for any signal generator with

coaxial output. 13

Figure 6. Block diagram to illustrate location of the matching network for adjusting the input impedance

of the rf detector. 14

ii

APPLICATION OF RF MICROPOTENTIOMETERS

FOR

CALIBRATION OF SIGNAL GENERATORS TO 1000 Mc

SUMMARY

With the RF Micropotentiometer, signal generator output

voltage can be calibrated to 1000 Mc simply and accurately if the

procedures outlined in this paper are carefully followed. The sources

of error are discussed and methods for minimizing or eliminating

them are described. Topics discussed include: rf shielding,

selection of a suitable rf detector, impedance matching and the

proper selection of voltage reference planes.

APPLICATION OF RF MICROPOTENTIOMETERS FOR CALIBRATION

OF SIGNAL GENERATORS TO 1000 Mc

by

L. F. Behrent

1. INTRODUCTION

The RF Micropotentiometer is a simple and accurate standard

for calibrating the rf voltage output of signal generators. However,

from the inquiries received it seems that some precautions basic to

high-frequency measurements are being overlooked or given insufficient consideration. By discussing in detail some difficulties which

have been brought to the attention of the RF Voltage Measurement

Standards Group at the National Bureau of Standards, and describing

the necessary corrective measures, it is hoped that this material will

help improve techniques so that higher accuracies will be obtained.

2. CALIBRATION PROCEDURE

The block diagram of Figure 1 illustrates how the RF Micropotentiometer is used. G is the rf generator to be calibrated (the

unknown); G energizes the reference standard - the RF Micropotentiometer S - and D is the detector, usually a sensitive

receiver, for comparing the rf output voltage of the unknown with the

reference standard.

The usual procedure in calibrating the unknown is to preset its

output level to an arbitrary indicated value. This output is compared

with the standard by switching the detector alternately between them.
The level of the standard is adjusted until the detector shows its

amplitude is equal to the unknown. Since the rf resistance of the

RF Micropotentiometer output is known from previous comparison

with the Primary Standard, and the rf current flowing in it can be

determined from a d-c calibration of the thermoelement, the magnitude of the rf voltage may now be calculated.

One possible combination of thermoelements and RF Micropotentiometer resistance elements which will provide standardized rf

microvoltages at any level between 1 and 100,000 microvolts is presented in Appendix I. This combination of units permits a ready

check of each RF Micropotentiometer against those covering adjacent

ranges.

3. PRECAUTIONS

Stray radiation is the most serious source of error in low

level measurements and the most difficult to eliminate. Even at rf

levels as high as 10 k|j.v errors greater than 100 percent have been

traced to this cause, with even greater errors at lower rf levels. In

the circuit illustrated in Figure 1 there are several possible sources

of radiation. Between the rf input and output of the RF Micropotentiometer there is sometimes as much as 100 db of attenuation. Therefore, the metal box containing the reference standard must be well

constructed to prevent leakage. One very satisfactory and economical

design is illustrated in Figures 2 and 3. Heavy-walled construction

is used to provide large contact surfaces between the box and the

removable cover, the body of the coaxial rf input connector, and the

outer electrode of the RF Micropotentiometer resistance element.

The most effective by-passing of the thermoelement's d-c leads can
be obtained by mounting button type mica capacitors in recesses in the

heavy side wall of the box. (See Figure 3. ) How extensively the signal generators and the detector must

be shielded can best be determined by connecting an antenna to the

detector and probing the system for radiation. Often it is necessary

to place each of these pieces of equipment into individual doublescreened boxes (Figure 4).

Commercial double- screened cabinets can be used to shield

each of the signal generators and the rf detector. The shielding will

be most effective if the inner screened box is grounded to the outer

only where the coaxial rf cable connecting the rf equipment in the

screened cabinet to the outside circuitry passes through the side of

the screened boxes. All other connections into the cabinets required

for energizing and controlling the enclosed equipment must be properly

filtered to prevent rf leakage. There is ample published literature on

2

this phase of the shielding problem . The rf input cable to the detector and the cable connecting signal generator G to the standard S

should be double- shielded coaxial cables, preferably with an additional

external shielding braid. With the equipment arranged as shown in

Figure 4, the most effective shielding against rf leakage was obtained

when external braid was placed over each of the rf cables, with the

braid connected to the internal shield by clamping it tightly to the rf

connectors at the cable ends with adjustable metal hose clamps.

For some calibration work a crystal rectifier and microammeter or an electronic rf millivoltmeter will be an adequate detector,

but in most instances a radio receiver sensitive to 1 microvolt or

less will be required. In any case, there must be enough sensitivity

to resolve incremental changes in the rf input of 1 percent or less.
4

Any receiver so used should be tested to determine the optimum operating conditions and maximum rf input level for which maximum

resolution is obtained. For rf voltages above this, the receiver must

be decoupled to prevent loss of sensitivity due to receiver overloading.

The amplitude of the voltage output from an rf source depends

upon how the external load impedance, whether an rf voltmeter, radio

receiver or other impedance, is connected to it. With a perfect transmission line terminated in its characteristic impedance connected to

the output, the amplitude of the rf voltage would be the same at all

points along the line and it would be immaterial how long a line section

was used. However, because of imperfections in practical rf cables

and connectors, discontinuities will exist which will introduce voltage

variations along the line. At very-high and ultra-high frequencies

large changes in the amplitude of the output voltages may result from

changing the length of the coaxial connection between the output and

the load even by a fraction of an inch. The plane through the rf connection at which the voltage output is measured should be properly

identified. In using the rf source as a reference standard, the magnitude of the voltage is accurately known only at this Voltage Reference Plane. The ideal reference plane for the RF Micropotentiometer

would be the plane in which the resistive annular ring lies, a-a

Figure 3. However, because it was necessary to provide a coaxial

output connection, the best practical reference plane is essentially at

b-b Figure 3. The rf voltage at b-b is compared with the Primary

Voltage Standard at all frequencies when determining dc-rf characteristics of the RF Micropotentiometer.

The output voltage of a signal generator should be measured at

the output connector provided. Where this connector is similar to

that of the RF Micropotentiometer, the recommended voltage reference
plane would pass through the connector essentially at the tip of the

female center pin (b-b Figure 5). Where other types of output connectors are found on the generator output, an adaptor should be used

which henceforth should be considered a part of the generator.

The output of the unknown must be terminated with an accurately known rf impedance connected at the voltage reference plane.

The detector can be used as the termination by adjusting its input

impedance with a suitable matching network. (See Figure 6) However, it is not safe to assume that impedance measurements of the

input to the detector made with an impedance bridge will be valid at

low rf levels. Such bridges impress from 0. 1 to 1 volt or more

across the unknown impedance. For a radio receiver, such a level

would result in complete saturation, causing the input impedance to

change greatly from what it would be at relatively low levels. Moreover, the impedance is a function of the tuning of the receiver. One

method of adjusting a receiver's input impedance to 50 ohms with a

matching network is to connect the input of the matching network to an

rf source through a 20 db, 50 ohm pad of high quality. With only a

few microvolts applied to the receiver input, and the receiver tuned

to the source frequency, adjust the network for maximum receiver

output indication. Once the matching is completed, the receiver

tuning must not be changed. Where the sensitivity of the detector

permits, a high quality attenuator pad may be used in place of the

matching network, thus terminating the unknown in an impedance

essentially that of the attenuator pad.
4. CONCLUSIONS

Observance of the precautions described in the text cannot be

overemphasized. In a recent case brought to the attention of the RF

Voltage Standards Group, insufficient attention to these details resulted

in measurement errors of approximately 14 db at 1000 Mc.
5. REFERENCES

(1) M. C. Selby, Accurate radio-frequency microvoltages, Communications and Electronics, AIEE 6: 158-164 (May 1953)

(2) E. E. Zepler, The technique of radio design, 240-264 (John

Wiley & Sons, Inc. , New York, 1949)
APPENDIX I

Typical Values of RF Micropotentiometer Components for the Range of

1 to 100,000 Microvolts

Resistance Standardized

Thermoelement (RF Micropotentiometer), rf voltage,

rating, ma milliohms microvolts

5 1 1-5

25 1 5 - 25

100 1 20 - 100

50 10 100 - 500

50 50 500 - 2500

50 100 1000 - 5000

25 1000 5000 - 25000

100 1000 20000 - 100000
G is the rf generator to be calibrated.

S is the transfer standard, the RF Micropotentiometer.

G is the signal generator energizing the RF Micropotentiometer.

D is the rf detector.

Figure 1

10

Figure 2

11

output

d-c thermoelement

output connections

rf current by- pas sing capacitors

d-c output connections

rf micropotentiometer

resistance

element

a-a is the plane in which the standardized reference voltage exists.

b-b is the plane in which the rf voltage is compared with the primary

rf voltage standard.

Figure 3

12

Q) U

13

0)tuO

CO 0) I

Pi

0)aao

HH CJ

<u <u u u

<u

in

<du

W)

14

0)u

• 1-1

o

u3i

APARTMENT OF COMMERCE

Frederick H. Mueller, Secretary VTIONAL BUREAU OF STANDARDS

A. V. Afitin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front

cover. WASHINGTON, D.C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Photographic Technology. Length. Engineering

Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Molecular Kinetics. Free

Radicals Research. Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-rays. High

Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic

Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics. Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Feather. Testing and Specifications. Poly- mer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion. Metal

Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating, and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer. Concreting Materials.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital Circuitry.

Digital Systems. Analog Systems. Application Engineering.

• Office of Basic Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction. Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular Propagation

Services. Sun-Earth Relationships. VHF Research. Radio Warning Services. Airglow and Aurora. Radio

Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Research. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation Obstacles Engineering. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards.

Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency and Super High Frequent')

tion Research. Antenna Research. Navigation Systems. Systems Analysis. Field Operation