m

1 1

1

^H H^

H

* 1

•^ ,V- H

^B

1 1

1
^*t OF C0„ NBS TECHNICAL NOTE 682

*"»eAU o*

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Implementation of the Notch Technique

as an RF Peak Pulse Power Standard
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 measurement 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 Institute 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 the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

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

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics 2 — Cryogenics- — Electromagnetics- — Time and Frequency-.

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, the Office of Air and Water Measurement, 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 Insti- tute_ consists of the following divisions and Centers: Standards Application and Analysis — Electronic Technology — Center for Consumer

Product Technology: Product Systems Analysis: Product Engineering — Center for Building

Technology: Structures, Materials, and Life Safety; Building Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness 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 Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information 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. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations — Office of International Standards.

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

- Located at Boulder. Colorado 80302.
Implementation of the Notch Technique

as an RF Peak Pulse Power Standard

tal Bursa of s>,^3rsls 5T 4 1976 yvs-V «c.C,

| LOO

* Philip A. Simpson

Paul A. Hudson

-L?t!^-^ ^ * CA^ ^\0 \^- -j v\o

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

\

,m

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Dr. Betsy Ancker-Johnson, Assistant secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued July 1976
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 682

Nat. Bur. Stand. (U.S.), Tech Note 682, 30 pages (July 1976)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

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

(Order by SD Catalog No. C13. 46:682). Price 80 cents (Add 25 percent additional for other than U.S. mailing
CONTENTS

Page

1

INTRODUCTION 1

2

THEORY OF OPERATION 1

2 . 1 Basic Principle 1

2 . 2 Attractive Features of the Notch Wattmeter 9

3

DESCRIPTION OF THE STANDARD 9

3 . 1 Coaxial System 9

3 . 2 Waveguide System 11

4ERROR ANALYSIS 13 4.1 CW Power Measurement Errors 15

4 .Coupling Factors of Couplers A and B 16

4 . 3 Attenuation of the Monitor Coupler 18

4 .

Impedance Mismatch Errors 18

4 .

Bandwidth-Risetime Limitations 21 4.6 On/Off Ratio of the Diode Modulator and Pulse Generator 22 4.7 Leakage of the CW Signal into the Device Being Calibrated 22 4.8 Precision of CW-Pulse Intercomparison/Equalization Process i .

23

4 .

Repeatability from Setup to Setup 23

4 . 10 Other Factors Affecting System Stability 23

5. SUMMARY OF ERRORS FOR EXISTING RANGES OF OPERATION 24

5 .Coaxial System 24

5.2 Waveguide System 24

6REFERENCES 25
LIST OF FIGURES

Page

FIGURE la. CONFIGURATION FOR CALIBRATING HIGH LEVEL

PEAK PULSE POWERS 3 FIGURE lb. CONFIGURATION FOR CALIBRATING LOW LEVEL

PEAK PULSE POWERS 4 FIGURE 2 . IDEALIZED PATTERN SEEN ON MONITOR OSCILLOSCOPE 5 FIGURE 3. EQUIVALENT BLOCK DIAGRAM OF SYSTEM

(a) HIGH POWER CONFIGURATION 6

(b) LOW POWER CONFIGURATION 6 FIGURE 4. COAXIAL NOTCH WATTMETER SYSTEM FOR MEASURING RF PEAK PULSE POWER AT FREQUENCIES UP TO 4.4 GHz 10

FIGURE 5. WAVEGUIDE NOTCH WATTMETER FOR MEASURING RF PEAK

PULSE POWER AT X-BAND 12 FIGURE 6. RELATIVE LINEAR AMPLITUDE SPECTRUM OF 950 MHz,

0. 1 ps PULSE 17 FIGURE 7. COMPARISON OF DETECTED 950 MHz PULSE ENVELOPE

AMPLITUDE AFTER ATTENUATION BY A 20 dB COUPLER (a)

AND BY A 20 dB BROADBAND PAD (b

)

19

IV
IMPLEMENTATION OF THE NOTCH TECHNIQUE AS AN

RF PEAK PULSE POWER STANDARD

Philip A. Simpson and Paul A. Hudson

ABSTRACT

The theory and operation of a standard for measuring rf peak

pulse power is described. The standard is based on the "notch"

principle. It is constructed in coax for frequencies up to 4.4 GHz,

and in WR-90 waveguide (8.2-12.4 GHz). The basic range is 1 to 10 mW but is extendable to cover 10 pW to 10 kW using directional

couplers. Risetime of the system is 14 nanoseconds. A comprehensive

error analysis is given. The uncertainty in coax is 3 percent and

in waveguide is 4 to 6 percent depending upon peak power level.

Key words: Notch wattmeter; pulse-CW equalization; pulse modulated

carrier systems; rf peak pulse power.

1. INTRODUCTION

Pulse modulated rf and microwave carrier systems find extensive use in many widespread

and diverse fields. Tacan, IFF, air traffic control, radar, radar altimeters, and digital

communications are among some of the more prominent areas that use this type of signal. In all of these, for the systems to be most effective and efficient, it is necessary to know

that the peak power of the rf signal is within certain prescribed upper and lower limits.

In order to respond to the measurement needs for this segment of the National

Measurement System, the NBS developed a laboratory standard for rf peak pulse power based

on a sampling-comparison method in 1962 [1] , and from this evolved a calibration service

which was announced in 1965 [2]. This method was used until 1970 when, for certain

advantageous reasons mentioned later, a working standard and calibration system based on

the notch wattmeter [3] was developed. This paper describes the system as it is presently

employed at NBS.

2. THEORY OF OPERATION

2.1 Basic Principle

Measurement of rf peak pulse power* by the notch wattmeter technique is based on a comparison of the pulse with an auxiliary rf signal of adjustable amplitude, and whose

frequency equals the pulse carrier frequency. This comparison includes an adjustment of

the amplitude of the CW signal to equal that of the pulse and is carried out in a unique

manner. Both the pulse and CW signals are introduced into the same transmission line but

*In this paper, rf peak pulse power is a contraction of "Peak Pulse Power, Carrier Frequency"

which IEEE defines as the power averaged over the carrier cycle which occurs at the maximum

of the pulse of power.
are not mixed in the ordinary sense. The CW signal is turned off or "notched" for a time

period slightly longer than the pulse duration and the pulse is positioned, timewise,

within the notch. A notch occurs for each pulse generated. The combined signals are sampled, typically by a directional coupler, detected with a fast crystal diode and then

fed to an oscilloscope for convenient comparison. After the CW signal amplitude has been

adjusted to equal that of the peak of the pulse, the power level of the composite signal

in the main arm of the coupler is measured using a CW power meter. Thus, peak pulse power

measurement is reduced to a measurement of CW power and attenuation which can be performed

conveniently and accurately. The rf pulses generated are generally trapezoidal in shape

with negligible anomalous excursions. It should be noted that this method permits using

pulses with shapes other than trapezoidal. In order to make a measurement all that need

be done is align the peak of the pulse (or any other desired point on the displayed

waveform) with the top of the notched CW. The response of the instrument under test (IUT) to non-trapezoidal waveforms is not covered in this paper since it is primarily dependent

upon the type of instrument and the pulse shape.

System configurations for performing high and low level measurements are shown in

figures la and lb. Their operation is as follows. Referring to figure la, the output from

the rf pulse generator is applied to the IUT through the primary line of coupler A. A known fraction of the power is coupled to the sidearm of coupler A and passes on to the sidearm of coupler B. A fractional sample of this signal is then coupled to the

primary line of coupler B, continues on to the monitor coupler M, and finally is absorbed

by the bolometer mount.

A low power "notched" CW signal (zero to 10 mW) is delivered to the primary input port

of coupler B and is thus added to the existing pulsed signal. The "notch" is generated by means of a fast diode switch modulator which is gated to turn off the CW for a short time

interval each time an rf pulse is generated. The duration of the notch is adjusted to be

slightly greater than the pulse duration. A sample of the composite signal, which includes

the pulse and the notched CW, is coupled to the sidearm of coupler M, detected with a crystal

diode, and then viewed on an oscilloscope. By use of adjustable time delays, the pulse can be precisely positioned within the notch of the CW signal as shown in figure 2. The basic principle of the system is, of course, to allow calculation of the pulse power

incident (P_, ) upon the IUT from a measurement of CW power. This principle is fundamentally

mc

the same as that employed in the familiar bolometer-coupler unit. Accordingly, we have

modelled the essential parts of the system as a bolometer-coupler unit as shown in the block

diagram of figure 3. Here P., P„, and P_ represent net powers at the indicated terminating

planes; f.. , T„, and I", represent the complex reflection coefficients at the same places;

and n

?1 , i"U ?J and n are the efficiencies of coupler B, coupler M, and the bolometer mount,

respectively. The circled numbers refer to the appropriate ports of the various couplers

as determined from figures la and lb. This model will aid in giving a better understanding

of the system theory and operation as well as simplifying the error analysis.
r j

caDJ LU 3 U3 O Q

Q. i—i

CC

u_ en

ai

C3:z _i

i-i <C

3: z

i— i-H i I o oo

z:

..

—*

>- 2:2

£s>1 '

COw

3:o

P-1 w

CO

ft<w

p.>w

l-q wuHaHmHuoO3

ISouw«UH

Cm

ONAS
ocUJ LU 3 C3 O Q

Q. i—i Qi

U. CQ

c£

to

a;

pjo

a,w

CO<

pj

pj>

pj

iso

-jzI—I H<«

i—i <u

OSo

P-, <

DiI—I

P-, zou

PJ

Di

ONAS
FIGURE 2. IDEALIZED PATTERN SEEN ON MONITOR OSCILLOSCOPE
GCUJl-H

UlZ 55og_|S

CO

IO

t-. E p"

in 0-

or Ulg

u_

w <

or =^ Q=

OO 3 UJ

0- Z

UJ

oo

>-

GO 2: U.O 2: 0>-i

I— »—t

s:<c 1— <ct <c CCH) a: csto =3

<;•-. Cfl >—1 Ll_ 1—t

OZ Li_ z: ^ <_>

<_)

O

Oct:

_J UJ a:

CO 3 UJ3

I— a. z Q_

uj 3: _1 C£5 2

<:•->

O

CQ

orUJ

or

UJ

©,

UJh-H

UI2 2209_|2 O

GO

10

t-. EP10 0.

O" ro -Q

UJ

en

C3
wh

We begin by writing

P inc

= Vr i

)

' P

l

(1)

ere K (T ) is the output to sidearm coupling ratio of coupler A when its sidearm is

terminated in a load having reflection coefficient F . As implied by the notation K has a functional dependence on V . It is also dependent upon T but this aspect will

be addressed later. For the present it is assumed T = 0. Multiplying and dividing

eq (1) by (P^P^P^ yields

P (P -P -P ) P inc = VV (P

2 -P

3 -V ' U)

Now, P

2 — Vn 21

P

3 = P 2" n 32

Pb = P,-n

3 m

so that

P.inc

VV^b

T1 21* n 32' nm

(3)

Equation (3) establishes the relationship between P. and P, , the power measured by the

bolometric power meter. The terms K and r\ are coupling ratios, r\ is the insertion loss

of coupler M, and r\ is the effective efficiency of the bolometer mount, m

Equation (3) suggests that a wide range of pulse power can be measured with a much

narrower range of CW power by judicious combinations of coupling values for A and B

(K. and 1 91 ). This is done in actual practice. By using the various combinations of

directional couplers listed in table 1, peak powers from 10 pW to 10 kW (90 dB range) can be measured with CW powers in the range 1 to 10 mW. Theoretically, the limits could be extended indefinitely, but certain factors such as signal-to-noise ratio on the lower end and voltage breakdown of components on the high end

place practical restraints on how wide a range can be covered.
Table la. Values of Directional Couplers for

Measuring Peak Powers Greater than

1 Watt (Refer to Figure la)

.

Power

Level

1 kW - 5 kW

0.1 kW - 1 kW

10 W - 100 W

1 W - 10 W

Coupler

A Coupler

B

40 dB 20 dB

30 dB 20 dB

20 dB 20 dB

10 dB 20 dB

Table lb. Values of Directional Couplers for

Measuring Peak Power Less than

1 Watt (Refer to Figure lb).

Power

Levial

0.1 W - 1 w

10 mW - 100 mW

1 mW - 10 mW

100 uw - 1 mW

10 uW - 100 uW

Coupler

A Coupler

B

10 dB 30 dB

10 dB 20 dB

20 dB 20 dB

30 dB 20 dB

40 dB 20 dB

As thus far described, the system calibrates terminating type instruments. By

rearranging the position of several components the calibration of feed-through meters can also be accomplished. When using the high power configuration, the output port of the

IUT is connected to the input port of coupler A. For low power, feed-through IUT's the

device itself replaces coupler A, and its output port is connected to the coupled port of

coupler B. With these arrangements of components, the input to sidearm coupling ratio

of coupler A is required.

It should be noted that the arrangement for measuring low power, feed-through IUT's

limits the minimum, measurable peak power to approximately twice the power of the notched

CW signal (coupler B = 3 dB) . It might be possible to measure smaller peak powers by

interchanging the signal inputs to coupler B, but since feed-through peak power meters in

this range are not commonplace, the matter of their measurement will not be considered.
2.2 Attractive Features of the Notch Wattmeter

The notch wattmeter technique was chosen as the basis for the standard because of

several attractive features which are inherent to the method. These are:

1. the measurement of rf peak pulsed power is directly related to a CW

power measurement which can be performed accurately and conveniently;

2. it is independent of pulse shape;

3. it is capable of measuring short duration pulses;

4. it is possible to make measurements on pulse trains with either

constant or time varying repetition rates;

5. it is relatively easy to extend the technique to higher microwave

frequencies as required.

It is primarily because of the added advantage of the latter three points that this

technique was chosen to replace the earlier sampling-comparison method.

3. DESCRIPTION OF THE STANDARD

3.1 Coaxial System

A coaxial system is used as the standard at lower microwave frequencies*. Figure 4 shows the arrangement of the precision directional couplers which are used to combine the

pulsed and notched CW rf signals. They are located in the structure immediately over the

sloping front console in the figure. These couplers were developed at NBS [4], and possess

excellent characteristics (low VSWR; high directivity; broadband, calculable coupling).

Their use results in the reduction of many uncertainties in the measurement process. A complete discussion of the uncertainties is given later in the error analysis in section 4. Basic theoretical ranges for the various parameters of the system plus some of the

practical limits are given in Table 2. Such things as maximum generator output, oscillator

frequency range, modulator transition times, and pulser repetition rates impose restrictions

which cause a reduction in the actual operating range of the standard.

*For frequencies in the VHF band, the sampling-comparison method is still employed

because of limited demand.
10
Table 2. Limits of Operation of Coaxial Notch

Wattmeter Standard.

Parameter Theoretical Range Practical Range

Frequency 850 to 2350 MHz

2550 to 7000 MHz

950 to 2350 MHz

4000 to 4400 MHz

Peak Power 10 yW to 10 kW 10 yW to 2 to 5 kWc

Pulse Duration carrier frequency to CW 100 nsec to 500 msec

c 100 nsec to 10 ysec

Pulse Repetition

Rate

carrier frequency

lgle shot to —— —

100 to 100 k pps

100 to 3 k pps

C

Duty Factor to 1 1 X 10

5 to 0.5b

10~5 to 0.0033°

a. dependent upon carrier frequency

b. for P < 1 W

P

c. for P > 1 W

P

3.2 Waveguide System

At higher microwave frequencies a standard has been implemented in WR-90 (8.2-12.4 GHz)

The technique, furthermore, is readily extendable to higher or lower frequency waveguide

sizes. Figure 5 is a photograph of the X-band system. The CW source is a Gunn diode

oscillator. For peak powers up to 1 W, a pulsed signal is derived from the CW signal and

amplified by a TWT. Above 1 W a magnetron is used which generates 10 kW peak at certain

discrete pulse durations and repetition rates. Except for frequency, the theoretical

limits for the various parameters of the waveguide notch wattmeter are the same as those

of the coaxial system. Table 3 gives the actual ranges of operation as limited by

practical considerations.

11
FIGURE 5. WAVEGUIDE NOTCH WATTMETER FOR MEASURING RF PEAK

PULSE POWER AT X-BAND.

12
Table 3. Actual Operating Ranges for X-Band

Notch Wattmeter.

Parameter Current Practical Limited Range

Frequency 2 - 12.4 GHz

Peak Power 10 pW - 10 kW

Pulse Duration 100 nsec - 500 msec

250 nsec, 1 psec

Pulse Repetition Rate 100 - 100 K

100 - 3 K

b

Duty Factor 1 X 10

5 - 0.5

a

2.5 X 10~5 - 7.5 X 10

a. for P < 1 W

P

b. for P > 1 W

P

4. ERROR ANALYSIS

For purposes of analyzing systematic errors, eq (3) can be converted to an alternative

form which explicitly exhibits mismatch effects. Also, it is usual practice to measure

coupling attenuation of couplers and insertion loss of devices rather than efficiency.

Since the coupling ratio of the couplers is measured under nominally matched conditions, we express K (T ) in terms of K (r ), (where T represents a matched load) and the

mismatch effect when the sidearm is terminated in a load having a reflection coefficient,

r_. Since, K.(r n ) = P. /P. and K.(T ) = P. /P , K.(r.) = KA (T )P /P..

1 ' A 1 mc 1 A o mc o A 1 A o o 1 The ratio P /P, must now be determined in terms of the r's. This can be done by

o 1 considering the ratio of the mismatch factors for the power delivered by a generator to a matched and a mismatched load. Engen [5] has shown the mismatch factor, M , between

g*- a generator, g, and a load, i, is

(i-|r | 2 )(i-|r„| 2

)

g v

(4)

13
Thus,

(i-|r

s

[ 2 )(i-|r

o

| 2

) M , ,2 p gn l-r r O O g Q

'

and since T = 0, o

|i-r rJ 2

i g

11

p li-r r,

I

2

P i,2

2,

(i-lr.l ) |s n | (i-|r

2

| ) |s 32 | (i-|r

3

| ) h

(5)

(6)

Substituting the above results in eq (3), we have

k. (r ) |i-r r

| 2 p

p. = A ° «

X ^ (7) mc . | ,2. (l- 1 r ) n 01 n QO n

1' 21 32 ra

The efficiency of a 2-port can be expressed in form*

l s 21 ' 2 <Hrtl 2

>

i-s

22 rj 2 d-|rln | 2

)

and the calibration factor, )L , of the bolometer mount is tL = (l-|r | ) n Substituting for the n's in eq (7) yields

Pinc

= P

b „ . ,2 S ,„ ,2„ ,„ ,2, ,„ ,2,, 71 ,2, (8)

*A suggested text for deriving this expression is "Basic Theory of Waveguide Junctions

and Introductory Microwave Network Analysis," by D. M. Kerns and R. W. Beatty, Pergamon

Press, New York, N.Y. 11101, 1967. Sections 2.2 through 2.4 are especially helpful.

14
Noting that \T

.

= T, and |r. I = |r„ and substituting the proper S parameters

as determined from figure 3 (i.e., S„ = S and S~„ = S ) for the S parameters in

21 B 3132 M

12 eq (8) yields

K.(r )-|i-r r.

|

2 -|i-s R r„|2 -|i-sM

r.| 2 Ao gl 1

' B2 1 M 3

1 P. = P, ; ^

(9) inc b k l 2 -k \ 2 .v

Thus, calculation of P. from a measurement of P, requires only the evaluation of the

inc b terms in eq (9) . The overall systematic uncertainty will, of course, depend upon the

uncertainty in evaluating the individual terms. As noted previously, K and (1/ | S |) are power coupling coefficients, i.e., the

A B31

attenuation with the unused ports terminated with matched loads, of the two directional

couplers A and B. The term (1/ | S |) is the attenuation of coupler M, and K is the

12

calibration factor of the bolometer mount. The other terms in the equation take into

account mismatch effects.

4.1 CW Power Measurement Errors

The CW power is measured with a precision dc power bridge and a thermistor mount

whose calibration factor, IC , is known to an uncertainty limit of 1 percent in coax and

0.9 percent in waveguide. As an alternative to the previous definition, calibration factor

for bolometer mounts may be defined as the ratio of the substituted dc power in the mount

to the rf power incident upon the mount. The power bridge measures the substituted dc power

to an uncertainty limit of 0.2 percent. Mismatch effects may also affect the CW power

measurement and this subject will be discussed in section 4.4.

An additional small error in P will occur if the "dead" time, which is defined as the

difference between the average power removed by the notch and the average power of the rf

pulse, becomes significant. Usually, this dead period has an equivalent duration no greater

than 1 ysecond per pulse so that repetition rate becomes the dominant factor. Assuming a maximum duration of 1 ysecond, a PRR of 1000 pps will result in 0.1 percent error. However,

if necessary this error can easily be eliminated by simply removing the pulsed rf and

notching signals, and making a normal CW power measurement immediately after the CW and

pulse signals have been matched in amplitude. This operation is valid since tests have

shown the CW generator has the necessary stability and the pin diode modulator has the same impedance in the low insertion loss state whether it is being switched or is on continuously.

In summary, the total uncertainty in measurement of the CW power is the sum of the

individual uncertainties in IC and the bridge, and is 1.2 percent for coax and 1.1 percent

for waveguide. "Dead" time may or may not be significant, but its effect can be eliminated

if necessary.

15
4.2 Coupling Factors of Couplers A and B

The accuracy with which the coupling factor of the two directional couplers, A and B, is known has a direct bearing on the overall accuracy of the pulsed rf power measurement.

The manner in which the factors are determined is somewhat different for the two systems.

In the coaxial system the couplers are a precision type as mentioned previously. The

VSWR is 1.03 or less and the coupling value is calculable to a high degree of accuracy

from the theoretical equation

K

f =Ko . 2

1 nf < 10 > sin --

o

where

K = the coupling at frequency f expressed as a power ratio;

K = the value of maximum coupling expressed as a power ratio;

f = the frequency at which K is to be determined;

f = the frequency at which K occurs,

o

1 J Q

K and f in this equation are determined by measuring K at 4 or 5 frequencies in

the range of 900 to 2000 MHz and fitting eq (10) to the data. K values can be determined

at NBS to 0.5 percent using the method described by Bramall [6]. For these couplers f o lies in the vicinity of 1.4 GHz, and over the stated frequency range measured values of K are within 0.25 percent of calculated values.

The couplers are also usable in the region of f = 3 f . However, their coupling

values, K , cannot be calculated with the necessary accuracy, but must be determined by

power ratio measurements at the desired frequency. The uncertainty associated with these

measurements is also 0.5 percent.

In contrast to coax waveguide couplers are not calculable but are measured at each

frequency of interest by the Bramall method. The uncertainty of these measurements is

0.02 dB/10 dB for values up to 30 dB and 0.03 dB/10 dB for 40 dB.

Thus the uncertainty for these couplers is as follows

:

10 dB - 0.46%

20 dB - 0.93%

30 dB - 1.39%

40 dB - 2.80%

Since the coupling factors of coupler A and B were measured using CW signals, the

question arises as to whether the coupling will be different for pulsed signals whose

carrier frequency is the same as the CW signal. The pulse signals are, of course, broadband. As shown in figure 6, the spectrum includes components of significant amplitude

out to the 7th side-lobe for a 950 MHz, 0.1 ps pulse having 10 ns risetime. Thus, the

total bandwidth of the signal extends ±70 MHz on each side of the carrier frequency.

16
oID

a.

en

s: uj

1—1 u~>

i—i to I— 3.

LU

17
The power coupling is not linear with frequency, but varies relative to the coupling

2 at f as 1/sin (f/f ). When the carrier frequency, f , is equal to f , then the coupling

O O -i./> c

i Q

> ro

of the side-lobe frequencies will be symmetrical. In the NBS coaxial system, f varies

from 950 to 2000 MHz and the f of the couplers is 1400 MHz. The maximum variation in

o coupling for the 950 MHz pulse described above is -1.7 percent (+8.6% and -10.3%) and some distortion in the detected pulse could be expected. The question is whether the amplitude

of the detected pulse would be affected because we are comparing amplitudes of the detected

CW and the pulse.

In an attempt to answer this question, an experiment was performed wherein the 950 MHz,

0.1 us pulse amplitude was compared after it had been coupled to the sidearm of a 20 dB

coupler and after it had passed through a broadband (dc-18 GHz) 20 dB attenuator pad having

the same loss as the coupler at 950 MHz. From photographs of the two detected pulses on an oscilloscope crt, the two amplitudes appeared identical within the limits of resolution

(1/4 trace width) as shown in figure 7. As a confirmation of this finding, a series of approximate calculations were performed.

The maximum amplitude of the main lobe and the ten upper and lower side lobes of the

s in x envelope of the spectrum of an rf pulse were squared, then multiplied by the value

of coupling at their corresponding frequencies, and summed. This value was compared

against the sum of squared, undistorted amplitudes. The calculations were performed for

pulses of several durations. The results showed a difference of less than 0.01 percent for

durations as short as 100 ns. For a 14 ns rectangular pulse the difference was 0.75 percent.

It is concluded that for pulses having a minimum duration of 100 ns and 10 ns risetime,

the effect of the couplers on the pulse amplitude is negligible.

4.3 Attenuation of the Monitor Coupler

The directional coupler used as the monitor has a nominal coupling factor of 10 dB.

This value is used to provide sufficient sensitivity for the pulse-CW intercomparison while

minimizing the power coupled from the measuring circuit. The monitor couplers used in the

coaxial and waveguide systems are similar to couplers A and B in their respective systems.

The uncertainty in their measured values of attenuation (nominally 1.1) is 0.1 percent.

4.4 Impedance Mismatch Errors

As shown in eq (9), the relationship between P, and P. is modified by impedance

b mc

2 2 2 mismatch effects denoted by the terms 1 1-T T n

I , 1 1-S,, r„ I and 1 1-SW r_ I . Evaluation 1 g l

1

' B n

2' M

22

3

1

of the above terms would require knowledge of both the magnitude and phase of the F's and

S's. In practice, only the magnitudes are known and, thus, upper and lower limits of the

terms are established by using both the plus and minus signs. The terms are then written

2 9 9 as |l±r r | , | 1±S rJ , and | 1±S r, I . Since the value of each term is centered

g u 11 M

22

1 around unity, the maximum diviation from unity is taken as the mismatch uncertainty.

18
f 1

i ' .in'.; . ii . m ii.;.i.... i..»i.|i| » .

Kiitj.>i.,iiiii|l,. |i'i|'>' |.|' .

: '>i.'<!,):i!'>i 1 » > —! ,

:: :

..' 4+4*

LjJ

Q. CC <c >-

a. a

—i <C

z: =c

UJ DC

V) UJ

I I =5 QD- ZDoN O

o oLD CD

en cm Q<m UJ

h- >- O CO Q

uj =a: I— s: d. uj O Q i—< Q

I— Z U_ <C =t ODCQ

S UJ <C O I— O on 1— Di <cr cq

Q_ LU "O

s: i— o u_ o l_> <C C\J

en

CJ3

19
Further, the terms appear as a product in eq (9) and the total mismatch uncertainty is

the product of the maximum deviations from unity.

We can then write an uncertainty factor, y, given by

y = |i±r r | 2 .|i±s r I 2 .|i±s r | 2 (ID

g 11 u

22

An evaluation of the T's and S's follows.

r is the equivalent generator impedance of coupler A and is independent of the

pulsed rf generator and any external components such as isolators, filters, attenuators,

etc. This is a result of the method used whereby changes in the power entering coupler A are compensated for so that a constant reading is maintained on the IUT. Engen [7] has

shown that under this condition V is given by

g

F g " (S 22 " S 12' S^ > (12)

where S are the scattering coefficients of the coupler. Note in this setup the

x,y

subscripts 1, 2, and 3 refer to the input port, the port connected to coupler B, and the

port connected to the IUT, and the actual ports represented by 2 and 3 are reversed when

changing from the high power to the low power configuration. For coax maximum |r | occurs

in the low power configuration. In this case |r | = ±0.0248 (|s__| = 0.0148, |s | ~ 1.0,

I S /S I = 0.01). For waveguide, maximum |r | is in the high power configuration. Here

|r | = ±0.0508 (|S 22 | = 0.0476, |s | = 0.3162, I S 23

/S 13 ' = - 01) -

We will now evaluate the other T's in a direction from the bolometer mount toward the

coupler A-coupler B junction. The reflection coefficient of the bolometer mount (T ) equals ±0.0476 for both coax and waveguide.

The term r is the actual reflection coefficient encountered at the input of coupler M

and is affected by the V of the bolometer mount and to a lesser degree by that of diode

detector. The high directivity (40 dB) of this coupler (and coupler B) renders insignificant

any contribution from the decoupled port. The maximum value of |r | can be calculated by

solving the scattering equations for a three arm junction; thus, using the approximate

relation

I s | 2

l r I I s I 2

l r I

l r J -

|S n

[+ i-|s22

r

2

l + i-|s33

r

3

| <">

where S are again the scattering coefficients of the coupler and V is the reflection

x,y ° ° y coefficient seen by port y. Values to be used in eq (13) to solve for r_ in eq (11) are

for coax |s | = |s | = |s | = 0.0148, |r | = 0.0476 (bolometer), |r | = 0.0476 (detector),

|S 12 | = 0.9, and |s | 2 = 0.1. These give \vj (for eq (11)) = ±0.0624. For waveguide

20
2 |S | = |S | = 0.0244, |S | = 0.0476, |r | - 0.0 (detector is tunable), and |S | = 0.9.

These give (for waveguide) |r | = ±0.0673.

Equation (13) is also used to determine an upper limit for T in eq (11) . In this case the input port is what is usually referred to as the coupled port. For coax,

l S ll' =

I S 22 1 =I S 33 1 = °- 0148 > l r 2

l = °- 0148 > l r 3l = 0.0624, |S 12 | 2 = 0.99, and

2 |s | = 0.01. These yield |r | = ±0.0154. For waveguide the input arm is terminated

internally. The reflection coefficient at the input, with all the other pprts terminated

in matched loads, is 0.0476. This then is the contribution to T. of eq (13) by the first

„ in two terms. For the third term |r | = 0.0673, |s | = 0.01, and |s | = 0.0246. The

maximum value of \T \ = ±0.0483 for waveguide.

A value for y can now be determined from eq (11) . Using the double signed terms we

find for coax 0.9960 < y < 1.0040 and for waveguide 0.9895 < y < 1.0106. Similar treatment

for the other two cases yields 0.9962 < y < 1.0038 for low power coax, and

0.9911 < y < 1.0090 for high power waveguide.

Thus the maximum uncertainty introduced by mismatch effects is

Coax

Low power configuration ±0.38%

High power configuration ±0.40%

Waveguide

Low power configuration ±1.06%

High power configuration ±0.90%

Some discussion of the effect on the system of a mismatched IUT should be made at this

point, since in section 2.1 T TTirT, was assumed to be 0. The first order deviation of P. IUT inc

from the value associated with r

TIIT = is accounted for by defining the response of the

IUT as that instrument's performance when operating in a system of characteristic impedance

Z . The error in P caused by the power reflected from the IUT is insignificant because of

the 40 dB directivity of coupler A. For instance, if \l | =0.2 (VSWR = 1.5) then

4 percent of the power is reflected, which in turn is attenuated 40 dB resulting in

0.0004 percent error. This reasoning on the latter point also requires that the diode

detector on coupler M has a square law response to avoid the possibility of small but

significant errors in the intercomparison of the pulsed and notched CW signals.

4.5 Bandwidth-Risetime Limitations

One factor which limits the bandwidth of the system (and hence sets a minimum risetime

for pulses) is the risetime of the oscilloscope and the diode detector combination. The

risetime of the combination was measured by applying the output of a tunnel diode step

generator having a risetime of 25 ps to the diode and observing the output on the system

oscilloscope. The observed risetime was 8 ns. Since the input step was nearly three orders

21
of magnitude faster than the output from the combination, the above value was taken as the

true risetime. While diodes and oscilloscopes with faster risetimes are available, the ones used here are adequate for present needs.

4.6 On/Off Ratio of the Diode Modulator

and Pulse Generator

If any CW power leaks through the diode notching modulator while it is in the "off"

(high isolation) mode, the unwanted CW will add to the pulse causing an incorrect amplitude.

This, in turn, will cause the P to be in error when it is intercompared with the pulse.

Manufacturer's specifications for the modulators used in both the coax and waveguide

systems quote 80 dB isolation. Tests performed sensitive to 0.01 percent of the power in

the "on" condition showed no detectable leakage of the CW signal during the time period

when it is "notched" off. Thus, the leakage signal is no greater than 0.01 percent of the

CW power and is, therefore, negligible.

Inadequate on/off ratio in the pulse power generator would also cause an error because

any signal in the off condition would add to the CW signal from the low power generator.

The pulse power generator employs a single tube pulsed oscillator and the output is zero

in the interval between pulses.

4.7 Leakage of the CW Signal into the Device Being Calibrated

Even though directional couplers A and B are placed in the system backwards with respect

to the CW signal, it is possible for some of the CW power to find its way into the IUT since

their directivity is not infinite but approximately 40 dB.

An analysis of the arrangement of couplers A and B reveals the CW signal that leaks

into the IUT is attenuated by the same attenuating elements for both configurations. The

CW attenuation is given by the expression

A = h

+

°B + KA +

°A

(14)

where

A = attenuation in dB of the CW signal

Iv. = coupling factor of coupler B in dB

D = directivity of coupler B in dB

K = coupling factor of coupler A in dB

D = directivity of coupler A in dB

Worst case conditions occur for the low power configuration with coupler B equal to 20 dB. Calculations show that with the 40 dB directivity of couplers A and B, the CW is

reduced to a level 100 dB below the peak power of the pulsed signal and is thus negligible.

22
4.8 Precision of CW-Pulse Intercomparison/Equalization Process

The precision with which the CW and pulse amplitudes can be made equal by visual

observation on the oscilloscope crt is a function of the ratio of the trace width to the

signal amplitude in centimeters on the crt. Normally, the scope vertical gain is set at its maximum. When the pulse and CW signals are at 1 mW level (worst case) the deflection

on the crt is 6.5 cm while the trace width is 0.15 cm (1.5 mm). Under these conditions, the

CW amplitude can be adjusted to that of the pulse with a precision of at least a quarter of

a trace width. Thus, the imprecision is —' r—r

' = 0.57 percent. This value agrees

very closely with an experiment repeatability of 0.58 percent (3a). This value was the

maximum that was observed for sets of 10 readings each made at several frequencies in both

coax and waveguide.

In practice, the amplitudes are equalized 6 times and the CW power is measured after

each time. The average of the 6 power measurements is computed and this value is used in

eq (1) and (2). The averaging process reduces the imprecision to 0.16 percent (2a). When

the signals are at the 10 mW level, the imprecision in the equalization process decreases

to approximately 0.10 percent (2a).

4.9 Repeatability from Setup to Setup

Since the directional couplers are frequently changed to cover different levels of

power, there is the possibility of an error being introduced associated with the connection

of different couplers into the setup. This error could result from connector non- repeatability or from misalignment of the directional couplers.

To determine the magnitude of this effect, coupler A was taken out of the system and

then replaced. While the coupler was out of the setup, the supports were moved. The

coupler was then connected back into the system with normal care taken to ensure proper

alignment and the supports repositioned. Readings were taken before and after the coupler was removed.

For both the coaxial standard and the waveguide standard this operation was performed

for six connections. In the coaxial system the repeatability was 0.09 percent (2a) and in

waveguide it was 0.15 percent (2a).

i 4.10 Other Factors Affecting System Stability

The long-term stability of the system is affected by various factors even when the

physical arrangement of the component parts is left undisturbed. The more prominent factors

have been identified and steps taken to reduce their effect. For instance, temperature has an effect on stability and therefore, room temperature is maintained at 23.5°C ± 0.5°C.

Also, an effect attributable to line voltage fluctuations has been noted. The magnitude of

the effect has been reduced by operating certain critical pieces of equipment, such as generators, the rf bridge, and the intercomparison oscilloscope, from a regulated ac power

line maintained at 115 V ± 0.2 V.

23
5. SUMMARY OF ERRORS FOR EXISTING RANGES OF OPERATION

5.1 Coaxial System

The systematic error for the coaxial system in the frequency range of .95 GHz to 2.35 GHz and 4 to 4.4 GHz is as follows:

1. CW power measurement 1.20%

2. Mismatch error .40%

3. Coupling factor of directional couplers A and B 1.00 (.50 ea)%

4. Insertion loss of monitor coupler .10%

Total systematic error 2.70%

The random error associated with the coaxial system is essentially the same for all

frequency ranges covered. The component parts and their magnitudes are:

1. Precision of CW pulse intercomparison 0.16%

2. Repeatability from setup to setup 0.09%

The total random error is best estimated by the RMS sum of these values and is equal

to 0.18 percent.

The estimated maximum uncertainty associated with the coaxial system is the sum of the

systematic and random errors and equals 2.88 percent.

5.2 Waveguide System

The systematic error in the WR-90 waveguide system is as follows:

1. CW power measurement 1.10%

2. Mismatch error 1.06%

3. Coupling factor of directional couplers A and B 1.39% - 3.73%

4. Insertion loss of monitor coupler . 10%

Total systematic error 3.65% - 5.99%

The random errors associated with the waveguide system are caused by the same factors

as in the coaxial system. Their magnitudes are:

1. CW-pulse intercomparison precision 0.16%

2. Repeatability from setup to setup 0.15%

The total random error for the waveguide system is the RMS sum of these values, and is equal

to 0.22 percent.

The maximum error of the waveguide system is the sum of the systematic and random errors and is within the range of 3.87 to 6.21 percent, the exact value being dependent on the

directional couplers being used.

24
6 . REFERENCES

[1] Hudson, P. A., Ecklund, W. L. , and Ondrejka, A. R. , Measurement of rf peak-pulse power

by a sampling-comparison method, IRE Trans, on Instrumentation, Vol. 1-11, Nos . 3 and 4, December 1962.

[2] Peak-pulse power calibrations initiated, NBS Technical News Bulletin, pp. 212-213,

December 1965.

[3] Bowman, D. R. , The notch wattmeter, Proc. Nat'l Electronics Conference, Vol. II,

pp. 361-371, October 1946.

[4] Hudson, P. A., A high directivity, broadband coaxial coupler, IEEE Trans, on Microwave Theory and Techniques, Vol. MTT-14, No. 6, pp. 293-294, June 1966.

[5] Engen, G. F., An improved method for microwave power calibration with application to the evaluation of connectors, Jour, of Research of the National Bureau of Standards-C

Engineering and Instrumentation, Vol. 75C, No. 2, pp. 88-93, April-June 1971.

[6] Bramall, K. E. , Accurate microwave high power measurements using a cascaded coupler

method, Jour, of Research of the National Bureau of Standards-C Engineering and

Instrumentation, Vol. 75C, Nos. 3 and 4, July-December 1971.

[7] Engen, G. F. , Amplitude stabilization of microwave signal source, IRE Trans, on Microwave Theory and Techniques, Vol. MTT-6, pp. 202-206, April 1958.

25
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC A'l'ION OK Kl NBS TN-682

J ()RT NO. 2. Gov't Accession

No.

4. TITLE AND SUBTITLE

Implementation of the Notch Technique as an RF Peak Pulse

Power Standard

3. Rci ipicnt's Accession N<

5. Publication Date July 1976

6. Performing Organization ( ode 276.02

7. AUTHOR(S)

Philip A. Simpson and Paul A. Hudson

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit Nc 2762171

11. Contract 'Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOUS

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

bibliography or literature survey, mention it here.)

The theory and operation of a standard for measuring rf peak pulse power is

described. The standard is based on the "notch" principle. It is constructed in coax for frequencies up to 4.4 GHz, and in WR-90 waveguide (8.2-12.4 GHz). The basic

range is 1 to 10 mW but is extendable to cover 10 uW to 10 kW using directional

couplers. Risetime of the system is 14 nanoseconds. A comprehensive error analysis

is given. The uncertainty in coax is 3 percent and in waveguide is 4 to 6 percent

depending upon peak power level.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Notch wattmeter; pulse-CW equalization; pulse modulated carrier systems;

rf peak pulse power.

18. AVAILABILITY \^Z Unlimited

_ For Official Distribution. Do Not Release to NTIS

fcX 1 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13 .46 :6o2

J Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

30

22. Price

$0.80

USCOMM-DC 29042-P7

fr U. S. GOVERNMENT PRINTING OFFICE: 1976-677-347/1281 REGION NO.
NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry- It is published in two sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a

year. Annual subscription: Domestic, $17.00; Foreign,

$21.25.

• Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical

design and programming of computers and computer

systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign, $11.25. DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews such

issues as energy research, fire protection, building tech- nology, metric conversion, pollution abatement, health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NOHPERIODICALS Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies.

Special Publications—Include proceedings of confer- ences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists,

engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396). NOTE: At present the principal publication outlet for

these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements available from ACS, 1155 Sixteenth

St. N. W., Wash. D. C. 20056.

Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations.

The purpose of the standards is to establish nationally

recognized requirements for products, and to provide

all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations). Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological

marketplace.

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service (Springfield, Va. 22161) in paper

copy or microfiche form.

Order NBS publications (except NBSIR's and Biblio- graphic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington,

D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service A literature survey issued biweekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature

survey issued quarterly. Annual subscription : $20.00.

Send subscription orders and remittances for the preceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02)

Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE ANO FEES PAID US. DEPARTMENT OF COMMERCE

COM-215

Fourth Class Mail

o^t-o*

& /Si. %

'^6-l9lfe

YEARS

1901-1976 BS