', M.W. Bldg

jBoulder laboratories

PB 161622

92c. 121

PRECISION CALIBRATION OF

RF VACUUM TUBE VOLTMETERS

BY

L. F. BEHRENT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

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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 research are published either in the Bureau's own series of publications or

in the journals of professional and scientific societies. The Bureau itself publishes three periodicals avail- able from the Government Printing Office: The Journal of Research, published in four separate sections,

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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. A complete listing of the Bureau's publications can be found in National Bureau of Standards Circular

460, Publications of the National Bureau of Standards, 1901 to June 1947 ($1.25), and the Supplement to National Bureau of Standards Circular 460, July 1947 to June 1957 ($1.50), and Miscellaneous Publication 240,

July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS

technical <^lote

121

DECEMBER 1961

PRECISION CALIBRATION

OF

RF VACUUM TUBE VOLTMETERS

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

1 CONTENTS

Page

ABSTRACT ii

1. INTRODUCTION 1

2. CALIBRATION 1

2. 1 Accuracy 2

2.2 Working Standards 2

2. 3 Procedure 2

2.4 Precautions 4

3. CONCLUSIONS 6

4. REFERENCES 8

5. FIGURES 9

la RF calibration of vacuum tube voltmeter with an RF

Micropotentiometer

.

lb D-C calibration of the RF Micropotentiometer.

2. Calibrating a vacuum tube voltmeter against a thermal

converter as working standard.

3. Placement of an external shielding braid over the rf

probe of a VTVM and the cable connecting the probe

output to the VTVM.

4. Calibrating a VTVM at low frequencies against an RF

Micropotentiometer at millivolt levels. "A" is an

adapter from type N to terminal posts.

5. Calibrating a VTVM against an AT voltmeter using

commercial type N connectors

11

PRECISION CALIBRATION OF RF VACUUM TUBE VOLTMETERS

by

L. F. Behrent

ABSTRACT

Before any rf vacuum tube voltmeter is to be calibrated, it should be tested for several weeks to determine how well it may retain

its calibration. In this way effort lost in trying to calibrate it to an

accuracy beyond its inherent capabilities can be avoided. AT voltmeters, Thermal Converters and RF Micropotentiometers as working

standards provide simple, accurate and yet rapid means'for calibrating vacuum tube voltmeters.

PRECISION CALIBRATION OF RF VACUUM TUBE VOLTMETERS

by

L. F. Behrent

1. 00 Introduction

In describing calibration procedures for rf vacuum tube voltmeters, the discussion will be limited to voltmeters having an unbalanced, two-terminal input with one side at ground potential. The rf

voltages used in making the calibrations will be confined to cw, essentially sinusoidal, voltages.

Vacuum-tube voltmeters are not inherently stable. From a

survey conducted at NBS some time ago in v/hich the calibrations of

several different types of vacuum-tube voltmeters were checked against

the primary standards of rf voltage, changes in calibration in some

cases as high as 10-20 percent were observed over a period of only a

few months. Therefore, before any vacuum-tube voltmeter is calibrated, it should be checked against a reliable standard for a period of

several weeks to determine its probable stability. The calibration

procedure used should give an accuracy commensurate with the capabilities of the voltmeter under test.

2.00 Calibration

Calibration activities can appropriately be divided into levels

or echelons according to the range of accuracies and the purposes they

serve in the calibrations structure. A practical breakdown has been

proposed!./ in which three echelons are set up. In Echelon I are included the Primary and Secondary standards maintained by the NBS.

Echelon II includes the working standards maintained by standards

laboratories in industry, other branches of government and universities

These standards are traceable to the NBS and are used in calibrating

the working standards of Echelon III. The third level, Echelon III,

contains the standards used in calibrating the instruments used by the

consumer - for example, production line test departments and those

engaged in the repair and calibration of instruments. Vacuum-tube

voltmeters generally should be calibrated at the third echelon because

they lack the stability to warrant the extra effort and expense involved

in higher order accuracy calibrations.
2.10 Accuracy

The accuracies which may be expected depend upon both rf voltage level and frequency. Below 10 Mc, accuracies of 0.5 - 2 percent

can be obtained, while 10-25 percent can be expected between 100 and

1000 Mc. So it is evident that at lower frequencies, even in Echelon III,

accuracies better than needed for VTVM calibrations are possible.

2.20 Working Standards

Suitable working standards which may be used to calibrate

VTVM's are AT voltmeters, Micropotentiometers, and thermal converters _2J_3i_4 /-- the Micropotentiometer for calibrating millivolt rf

voltage levels at all frequencies, thermal converters for higher voltages up to 30 or 100 Mc, and AT voltmeters to complete the coverage

to 1000 Mc.

2. 30 Procedure

In general, to calibrate a VTVM, the working standard and the

voltmeter to be tested are connected in parallel, and with rf voltage

applied to both, the level is adjusted to give the desired indication of

the test instrument. The actual magnitude of the rf voltage applied is

then determined from the calibrated indication of the working standard.

The detailed steps will depend on the working standard being used.

For rf voltage levels of 1-100 mv the RF Micropotentiometer is

the best working standard to use at the present time. Table I shows a

typical selection of such standards to give complete voltage coverage

within this range. The basic circuit illustrating the use of RF Micropotentiometers in VTVM calibration is shown in Figure 1 a.

Table I

Typical Selection of RF Micropoteintiomete rs for MV Levels

Thermoelement Resistance El ement, Standardized mv

rating, ma ohms output

100 1.0 20 - 100

25 1.0 5-25

5 1.0 1 - 5
The rf portion of the calibration procedure is as follows:

1) With the rf input terminals of the VTVM connected to the working

standard's output terminals, energize the voltmeter according to

the manufacturer's recommended procedure. Allow a fifteen minute

warm-up period before beginning the tests, unless the manufacturer

or user specifies a longer one.

2) Make meter zero corrections and/or any other adjustments specified

by the user or manufacturer. (Many VTVM's capable of measuring

millivolts have a suppressed zero, in which case the zero adjustment will be omitted. )

3) Apply a sinusoidal, cw, rf voltage to give the desired test voltmeter

indication, and record the working standard's millivoltmeter indication.

4) Remove the rf signal, and, if the voltmeter does not have a suppressed zero, recheck the zero. If the zero requires resetting the

validity of the rf calibration is doubtful. The data for that measurement should be discarded and the test repeated.

For each point on the meter of the VTVM to be calibrated at

least five independent rf measurements with the working standard

should be made.

There are two ways in which the correction for rf error in the

RF Micropotentiometer may have been expressed in the higher echelon

calibration data. It might have been expressed as a percentage by

which, for a given current in the resistance element, the rf voltage

output was greater or less than the d-c calibration value at each frequency. Or, for a given current level in the resistance, the actual

output at dc and each radio frequency may have been given. In either

case it is necessary to use dc to calibrate the thermoelement monitoring the current into the resistance element as shown in Figure 1 b. This

d-c calibration may be made after each measurement, for best accuracy,

or one complete calibration can be obtained and plotted in graph form so

that d-c data for individual rf measurement points can be read from the

graph. For such a graph, some 20 - 30 points of d-c thermocouple millivolt output versus d-c microvolt output across the resistance element

as measured with a precision d-c measuring potentiometer must be

plotted.
Where a d-c calibration is made immediately following an rf

measurement, enough d-c current is applied to the RF Micropotentiometer to reproduce the millivoltmeter indication obtained during the rf

measurement, and the d-c output in millivolts is measured. The circuit of Figure lb applies to both procudures. Having completed the d-c

calibration and having the rf to d-c correction, the actual rf voltage

output can be accurately calculated.

For rf voltage levels above 0. 1 volt at frequencies of 100 Mc

and less, thermal converters are suitable working standards. Figure 2

presents the rather simple circuit arrangement. The test voltmeter and

the working standard are connected in parallel and to an rf source

through appropriate filters.

The calibration procedure contains very few steps. After the

preliminary steps of allowing an adequate "warm-up" period to stabilize the VT VM, and the checking and adjustment of voltmeter zeros,

only two steps remain. Apply sufficient rf voltage to produce the desired meter indication on the VT VM and record the indication of the

working standard.

Upon completion of the test, remove the rf voltage and recheck

zero settings. As before, any zero error renders the rf test invalid

and the measurement must be repeated.

Above 100 Mc, a satisfactory working standard is the AT voltmeter. The circuit arrangement of Figure 2 applies here, too, except

that where capacitive AT voltmeters are used a frequency meter must

be added to permit checking and adjusting the rf source to the same

frequencies at which the working standard was calibrated. Except for

several precautions which will be discussed in the following text, the

procedure outlined for thermal converters applies here also.

2.40 Precautions

While VTVM's are not stable, as was pointed out before, following the precautions outlined below will help to realize the best performance of which they aire capable.

1) Regulate the line voltage energizing the test instrument. The use of

a good a-c regulator can help reduce voltmeter zero fluctuations

may reduce calibration instability. The line voltage applied during
tests should be recorded and reported so that subsequent users can

adjust the line voltage to the same value at the sites where the

meters are used for measurements. Failure to do this can introduce additional error.

2) Filter the rf voltage applied to the VTVM during calibration. Manyvacuum-tube voltmeters are peak reading instruments. The presence of harmonics in the applied rf voltage can add several percent

error to the measurements in such cases. The rf source should

be filtered sufficiently to place the harmonics 50-60 db below the

fundamental.

3) Use only those rf sources whose rf output voltages have good amplitude stability during the short time it takes to make a measurement.

For example: if an rf calibration measurement accuracy of ± 5

percent is required, the output amplitude of the rf source should

remain stable to within ±0.5 percent during the measurement.

4) Check the zero setting of the test meter both before and after each

measurement. This point was discussed before but it cannot be

overemphasized.

5) Check the entire rf circuit, including rf source, connecting cables,

the working standard and the test VTVM for rf leakage currents.

It sometimes becomes necessary to place the rf source inside a double-screened box and to apply external shielding to connecting

rf cables _/ . Quite frequently vacuum-tube voltmeters with highfrequency probes are not sufficiently shielded and it becomes necessary to place external shielding over the housing of the probe and

the cable connecting the probe output to the voltmeter proper. See

Figure 3.

6) Connect the test voltmeter's rf input terminals to the working standard with the shortest connections possible. Above a few megacycles banana pins and clip leads are not usable because of the

error these lead lengths introduce. Even coaxial connections can

introduce errors at very high frequencies because of the voltage

standing wave in this length of transmission line between the applied

rf voltage and the terminals of the test meter. To illustrate refer

to Figure 4. Here an RF Micropotentiometer is the working
standard. Its standardized rf output voltage is coupled to the input

terminals of the VTVM through an adapter. This arrangement is satisfactory to about 5 Mc. Above this frequency, error will be caused by

the lead lengths and discontinuities introduced by Adapter A. The accurately known rf voltage exists at the voltage reference plane b-b (at

the front face of the type N output of the RF Micropotentiometer).

Another illustration is shown in Figure 5 where a vacuum-tube

voltmeter probe is connected in parallel with an AT voltmeter for highfrequency calibration. To illustrate the importance of short connections, standard type N connectors and adapters are used. The plane

in which the rf voltage is known at the working standard is d-d. With

a type N connection on the test voltmeter probe, known rf voltage should

be applied to the probe at the voltage reference plane f-f . At high radio

frequencies, again because of the approximately 4.5 cm length of transmission line and the discontinuities in the connectors, the voltage at

plane f-f will differ from the voltage at d-d. At 100 Mc a difference of

several percent may occur. This is aggravated, too, because both the

VTVM and the AT voltmeter are high impedances and the transmission

line between,them is not terminated in its characteristic impedance

anywhere - consequently a very high standing wave ratio exists in the

line section between them. Special T connections should be constructed

to reduce the distance between the standard and the test meter as much

as possible and still allow the rf voltage to be fed to them in parallel.

In order that such special T connectors may be usable over the widest

possible frequency range, the length of the T between the voltmeter

under test and the standard should be about one eighth of an inch.

3.00 Conclusions

The importance of knowing the capabilities of a VTVM before

attempting calibration cannot be emphasized too strongly. There have

been many instances where users, without prior investigation of voltmeter stability, have insisted upon 1 or Z percent calibrations of

VTVM's. In a matter of days, the calibration of the test voltmeter

had already changed by a percentage greater than the specified accuracy

and the calibration had to be terminated.

Because one cannot predict when VTVM may lose its calibration,

it is recommended that any facility using VTVM's as voltage standards

calibrate at least three to permit intercomparison. When one of them
disagrees with the other two it should then be removed arid recalibrated,

after making suitable stability tests to determine whether or not it is

still sufficiently stable to warrant recalibration.

Whatever means of connection is used for connecting the test

meter to the working standard, the same rf connection should be employed in making measurements or additional error due to connections

will be introduced.

The working standards recommended here are capable of much

greater accuracy than is required for these tests and may be used in

both Echelon I and Echelon II.
4. REFERENCES

1. IRE recommendation of standard practice for national correlation of measurement accuracy, Draft No. 5, subject to approval

of the IRE Standards Committee.

2. Stable radiofrequency voltmeters, NBS Technical News Bulletin,

40 , No. 2, 29-30 (Feb. 1956).

3. Selby, M. C, Accurate radio frequency microvoltages, AIEE

Paper 53-22 (May 1953).

4. Hermach, F. L. & E. S. Williams, Thermal voltage converters

for accurate voltage measurements to 30 megacycles per second,

AIEE Transactions, Pt. I (Communications and Electronics)

(July I960).

5. Behrent, L. F., Application of rf micropotentiometers for

calibration of signal generators to 1000 Mc, NBS Technical

Note No. 37, PB 151396 (Jan. I960).
5. FIGURES

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RFM =

VTV MILLI THER " X

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O)

i_ZJ

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ist: UJ

a-: ce uj = ^2

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ena

(1) =3

Li_

SOURCE

VX =VTVM UNDER TEST

Vs

= WORKING STANDARD

Figure 2

iirfROBF

A RF) ' EXTERNAL ADDED

SHIELDING BRAID

Figure 3



m3

en

V. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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