NBSIR 73 -350

ADAPTER EVALUATION BY AUTOMATED MEASUREMENTS

R. L. Jesch

G.F. Engen

M. P. Weidman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Final Report

December 1973

Prepared for:

Department of Defense

Calibration Coordination Group - 72-72A

Newark, Ohio 43055

NBSIR 73-350

ADAPTER EVALUATION BY AUTOMATED MEASUREMENTS

R. L Jesch

G. F. Engen

M. P. Weidman

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Final Report

December 1973

Prepared for:

Department of Defense

Calibration Coordination Group - 72-72A

Newark, Ohio 43055

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts. Director

ADAPTER EVALUATION BY AUTOMATED MEASUREMENTS

ABSTRACT

A problem of long standing interest in the UHF

and microwave field is that of adapter evaluation.

For many purposes, the adapter property of major

interest has been its efficiency. However, a more

complete description in terms of scattering parameters was required for the present adapter evaluation project. The approach to the problem was to

automate the manual measurement technique for

adapter efficiency. In achieving this goal, it was

found that the entire set of scattering parameters

could be obtained with little further effort. As a further by-product, an alternative procedure for

determining correction factor for automated network

analyzers was obtained.

Key Words: Adapter, power meters, test set.

Automated Network Analyzer (ANA) , reflection

coefficient, scattering parameters.
CONTENTS

Page

1. INTRODUCTION 1

2. INSTRUMENTATION 1

3. THEORETICAL BACKGROUND 3

4. ADAPTER MEASUREMENT SYSTEM - 6

5. SCATTERING PARAMETER MEASUREMENTS 8

6. CONCLUSIONS AND RECOMMENDATIONS - 8

LIST OF FIGURES

Figure 1. Test set 2 Figure 2. Test set for adapter evaluation 2 Figure 3. Test set with adapter inserted 5 Figure 4. Adapter measurement system 7
1. INTRODUCTION

This is the final report on CCG project 11-llk, This

project was the primary interest of the CCG R,F. Measurements

Working Group (D.H. Caldwell, E.A. Fitzgerald, J.H. McAdams

and R.D. Moyer) . The objective of this project was to reduce

to practice a proposed theoretical method for characterization

of adapters. The approach to this problem was to provide a complete characterization of the adapter with emphasis on

the efficiency. In particular, the ability to measure efficiency is enhanced by utilizing recently developed theories

for non-ideal short-circuit performance and exploiting recent

additions to the automated system which were made in conjunction with the development of a power calibration capability.

The theoretical approach was reduced to a computer program

for adaptation on the NBS automatic network analyzer. This

final report gives a brief description of the basic equations

used in the computer program, in addition to the instrumentation

and procedure required to make adapter measurements. A more

complete description of the mathematical theory and its reduction to measurement practice is now being prepared in a paper

by one of the authors (Engen)

.

2. INSTRUMENTATION

Existing, commercially available automated measurement

systems are generally designed around the reflectometer concept

and include a complex ratio detector as shown in figure 1.

Although this suffices, in principle, for the desired efficiency measurement, the performance of the complex ratio

detector generally falls substantially short of what is

required to achieve the stated accuracy goals.
COMPLEX

RATIO (W)

WW—X X

Figure 1. Test set.

At the National Bureau o£ Standards, the test set of

figure 1 has been replaced by that shown in figure 2. The

key feature is the addition of two power meters of high

accuracy which permit a measurement of the absolute levels

at the sidearms in addition to the existing complex ratio

detector. Historically,

(5>

X

r- (W)

X X

Output Port

Figure 2. Test set for adapter evaluation.

the test set of figure 2 was developed for power meter calibra

tion. Here, the net power at the output port is approximately

proportional to P^. The complex ratio detector provides a cor

rection factor, which is typically no more than a few percent.

Thus the error in its determination has only a small effect on

2
the total accuracy. In this application the second power

meter, P^, is not required. The test set of figure 2 thus

represents an extension of one initially developed for another

applicat ion

.

Although the power meters , P^ provide an absolute level

indication, it is only their ratio which is of interest in

adapter evaluation. In short, the power meters permit an

order of magnitude, or more, improvement in the accuracy with

which the magnitude of the complex ratio is measured. Although

the dynamic range over which this improvement is realized

is only 20 dB or so, this is more than adequate for the

immediate application.

3. THEORETICAL BACKGROUND

The well known relationship between the complex ratio, w,

he T' given by,and the reflection coefficient, r^, at the output port is

w = cr^ * 1

(1)

where a, b, c are complex constants. The relationship between

P^, P^ and w may be written,

P.

|w|2 = K (2)

^4

where K is a real constant. The evaluation of a, b, c is

generally referred to as a calibration of the system and is

basic to automated measurements. This problem will be considered in greater detail below. The parameter, K, can be

easily evaluated by comparing the values |w|^ and P^/P^ for

several values of w and averaging. Provided that is such

that P^ and P^ are within the dynamic range of power meter

operation, |w| is determined from (2), and the only role of ^

3
the complex ratio detector is to provide the argument. As |r^|

becomes small, however, also becomes small and it is

necessary to obtain both magnitude and phase from the complex

ratio detector.

The measurment of efficiency calls for observing P^, P^

,

and w in response to certain terminations, which are connected

to the output port in figure 2, and a (usually) different set

connected to the "output" port in figure 3. If one of these

terminations is an ideal moving short, the locus of w in the

complex plane is a circle, which may be characterized by its

radius, R, and the position of its center, R^. It is possible

to express the efficiency in terms of the values of R and R^

obtained with an ideal sliding short connected to the output

ports in figures 2 and 3. To a good approximation, the efficiency is merely the ratio of the two radii. Because ] R^ | is

usually small in relation to R the accuracy with which R is

measured depends primarily upon the accuracy in measuring |w|.

The dependence upon the argument of w is small, and vanishes

as R^ goes to zero. Moreover, the total expected range of

variation in the value of |w| , in response to the short motion

or adapter insertion, is perhaps no more than 10-201. On the

basis of these considerations it should be apparent that the

modified test set is ideally suited to this particular application. As was the case in power calibration, the procedure

has only a small sensitivity to errors in the complex ratio

detector.

If an ideal (lossless) sliding short were available, no

other terminations would be required. In reality, of course,

no such device exists. Moreover, if the performance of the

traditional manual method is to be equaled, some means of

accounting for the non-ideal short performance must be devised. In order to accomplish this, it is convenient also to

observe the system response to an impedance standard (typically

a quarter-wavelength short) and a low reflection sliding

termination

4
Returning to (1), a, b, c are complex constants which

characterize the measurement system o£ figure 2. Moreover,

R = be

C3)

and

R

ac

1

- (4)

In figure 3 the relationship between w and may be written

w

aTj^ + b'

(5)

where a', b', c' are a new set of constants, which in turn

determine R' , R^ as in (3) and (4)

r- (W)

i

X

X X

Adapte r

Output Port

Figure 3. Test set with adapter inserted.

An alternative way of viewing the problem is that of

using the impedance standard, non-ideal sliding short, and

sliding load to determine the calibration constants a, b, c and a', b', c'. Quite apart from the immediate application,

this procedure is of considerable general interest as an alter

native calibration technique for automated network analyzers

5
in which only one impedance standard is required. (Existing

calibration routines often require two or more standards and

include approximations which are inconsistent with the accuracy

goals in this problem.)

The model to be assumed for the non- ideal short is that

of a lossy uniform waveguide, of unknown attenuation, containing a moving piston of large but unknown reflection. The

moving load is assumed to be contained in either the same

waveguide, or at least one of the same attenuation constant,

and possesses a small but unknown reflection.

4. ADAPTER MEASUREMENT SYSTEM

The adapter measurement system utilizes an automatic network analyzer which consists of a four-arm re f lectometer , shown

in figure 4, with the incident and reflected arms terminated

with thermistor mounts. Provisions are made to allow the computer of the ANA to measure substituted dc powers in the

thermistor mounts and test to reference channel complex ratios.

NBS Type II bridges are used for measuring dc powers. With

the standard termination and sliding short connected to the

test port, ratios are measured using the thermistor mount

power readings. The ANA provides phase information for both

the high and low reflection terminations , and also the low

reflection magnitude of reflected to incident signals at the

test port.

6
ANA and Computer Interface

NBS Type

II Bridge

NBS Type

II Bridge

Incident Signal

Arm 4 Thermistor

Arm 3 ThermistorReflected

Signal

0- j

/*" ANA

Ref.

ANA and Compute r A

Test

Parameters Measured:

S . S , ir 22' Phase Shift, ri2i

Efficiency

Test

Port

Parameters of Ref If^ctomete

r

(a, b, c)

I— Adapter j

Parameters of Reflectometer plus Adapter

(a", b', c')

Figure 4. Adapter measurement system.

In order to use the adapter measurement system, sliding

low reflection terminations, sliding short circuits, and one

standard high reflection termination are required in each of

the types of transmission lines or waveguides which terminate

the adapter to be measured. In order to measure a coax-towaveguide adapter, a coaxial sliding load, sliding short and

fixed short along with a waveguide sliding load, sliding short

and fixed short are required. Terminations for most waveguide

sizes are presently available along with 7 mm and 14 mm coaxial

terminations. In order to obtain phase information from this

system, the phase of the fixed "standard" termination must be

known. In general, a nominal quarter-wavelength short or open

circuit will suffice for the "standard" (for multiple frequency

measurements, the propagation constant and depth of the quarterwavelength short must be accurately known and accounted for)

.

7
5. SCATTERING PARAMETER MEASUREMENTS

The design o£ the hardware and software for the adapter

measurement system is arranged to measure the scattering

parameters of any adapter or two-port device regardless of

input and output connections. The system also provides for

measuring S^^ only, the magnitude and phase of reflection

coefficient, for a termination.

The scattering parameters for the adaptor are obtained

from the previous equations and are related by equations (6) , (7) and (8)

S,, - (6)

a - b'c

-S22 = asLj_Jl£ (73 a - b ' c

and

'^12^21 "11^22

a - b ' c

The assumption in the computer program is made that S-^^ ~ ^21'

The software is designed to lead an operator through the

measurement procedure without his having a knowledge of the

basic theory.

6. CONCLUSIONS AND RECOMMENDATIONS

This report has described the basic theory, equations

and instrumentation leading to the first step in adapter

characterization. The principal accomplishment was the reduction of theoretical concepts into a computer program. A

test set was successfully modified and adapted to the automatic network analyzer.

This effort is by no means complete. Additional funding

is needed to perform a number of adapter measurements that

are required to examine and verify the parameters of interest.

8
Many varieties of "non-precision" connectors and adapters are

used with various types of equipment with losses in accuracy.

One cannot forever refuse to measure the performance of such

equipment just because its connectors don't mate with precision connectors . The advent of the automatic network analyzer make this problem even more acute, since one must

necessarily know the transition from one connector system to

another. The funding on the part of CCG for adapter evaluation prompted the Electromagnetics Division to initiate a formal progiam on connector and adapter evaluation. The EMD

effort was designed primarily for modelling and measuring

selected types of adapters and connectors, especially SMA

type connectors

.

9
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA ^gglR 73-350

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Adapter Evaluation by Automated Measurements

5. Publication Date

6. Perfonning Organization Code

7. AUTHOR(S)

R.L. Jesch, G.F. Engen. M.P. Weidman

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No. 2724462

11. Contract/Grant No. CCG 72-72A

12. Sponsoring Organization Name and Address

Department of Defense

Calibration Coordination Group

Newark, Ohio 43055

13. Type of Report & Period Covered ti- n t-> Fmal Report

M^y to D^Q, 1973

14. Sponsoring Agency Code

IS. 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 problem of long standing interest in the UHF and microwave field is that of

adapter evaluation. For many purposes, the adapter property of major interest

has been its efficiency. However, a more complete description in terms of

scattering parameters was required for the present adapter evaluation project.

The approach to the problem was to automate the manual measurement technique

for adapter efficiency. In achieving this goal, it was found that the entire set

of scattering parameters could be obtained with little further effort. As a further by-product, an alternative procedure for determining correction factor

for automated network analyzers was obtained.

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

Adapter; automated network analyzer (ANA); power meters; reflection

coefficient; scattering parameters; test set

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