1 1

United States Department of Commerce

I 1 I Technology Administration

I i National Institute of Standards and Technology

NISTIR 3991

A CERTIFICATION PLAN FOR A PLANAR NEARFIELD RANGE USED FOR HIGH-PERFORMANCE

PHASED-ARRAY TESTING

Michael H. Francis

Andrew G. Repjar

Douglas P. Kremer

NISTIR 3991

A CERTIFICATION PLAN FOR A PLANAR NEARFIELD RANGE USED FOR HIGH-PERFORMANCE

PHASED-ARRAY TESTING

Michael H. Francis

Andrew G. Repjar

Douglas P. Kremer

Electromagnetic Fields Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

July 1992

U.S. DEPARTMENT OF COMMERCE, Barbara Hackman Franklin, Secretary

TECHNOLOGY ADMINISTRATION, Robert M. White, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director
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CONTENTS

Page

1. INTRODUCTION 1

2. POUCY ISSUES 2

2.1. Purpose of Certification Process 2

2.2. Certification Period 2

23. Certification Costs 2

3. AUT MEASUREMENT REQUIREMENTS 2

4. TESTS BY RESIDENT PNF RANGE PERSONNEL 4

4.1 Alignment Tests 5

4.1.1 Scanner Alignment 6

4.1.2 Scan Plane Mirror Alignment 7

4.13 AUT Alignment 8

4.1.4 Probe Alignment 8

4.2 Rf Crosstalk 9

43 Rf Path Variation 10

4.4 Position Errors 10

43 Rf System 11

43.1 System Dynamic Range 11

43.2 Receiver Dynamic Range 12

433 System Linearity 12

43.4 Receiver Linearity 12

4.6 Noise Level 13

4.7 Leakage 13

4.8 Probe-AUT Multiple Refiections 14

4.9 Room Multipath 14

4.10 Truncation 14

4.11 Aliasing 15

4.12 System Drift 15

4.13 Insertion Loss Amplitude and Phase Measurement 15

5. TESTS BY NIST PERSONNEL AT THE PNF FACILITY 16

6. SOFTWARE COMPARISON TESTS 16

6.1 Test Data 16

6J2 NIST Processing 16

63 Range Data Processing 16

6.4 Comparison of Results 16
7. MEASUREMENT COMPARISONS 17

7.1 Transfer Test Antenna 17

IJ, NIST Measurements 17

73 Range Measurements 17

7.4 Comparison 17

8. SUMMARY 17

9. ACKNOWLEDGEMENTS 17

)

10. REFERENCES 18

IV
A Certification Plan for a Planar Near-Field Range

Used for High-Performance Phased-Array Testing

Michael H. Francis, Andrew G. Repjar, and Douglas P. Kremer

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

The National Institute of Standards and Technology (NIST) has written a

certification plan to ensure that a proposed planar near-field range is capable of

measuring high-performance phased arrays. Generally for a complete plan, one must

evaluate many aspects including scanner alignment, near-field probe alignment,

alignment of the antenna under test, rf crosstalk, probe position errors, rf path

variations, the receiver’s dynamic range and linearity, leakage, probe-antenna multiple

reflections, truncation effects, aliasing, system drift, room multipath, insertion loss

measurements, noise, and software verification. In this report, we discuss the

important aspects of a certification plan specifically written for the measurement of

hi^-performance phased-array antennas. Further, we show how the requirements

of each aspect depend on the measurement accuracies needed to verify the

performance of the array under test.

Key words: antennas; antenna measurements; phased array; planar near field.

1. INTRODUCTION

Recently, NIST prepared a certification plan for a proposed planar near-field (PNF) range

to be used in high-performance phased-array testing. This certification plan was drawn up based

upon information supplied to NIST on the planned application. This information included required

measurement accuracies for gain, main- and cross-polarization pattern, RMS sidelobe level, and

beam steering. Each of these accuracy requirements aftects various aspects of the range. For

example, the sidelobe level to which one must make accurate measurements will determine how well

the PNF scanner rails must be aligned and how well the probe must be positioned. The same

sidelobe specification will also determine what levels of leakage and room scattering can be accepted.

In this report, we discuss those factors one must consider in demonstrating that a range is

capable of making PNF measurements to the desired accuracy. The proposed PNF range to be

tested (hereinafter called the Range) has the design shown in figure 1. The antenna under test

(AUT) and the probe both can move in X and Z. This feature allows for easier determination of

the effects of probe-AUT multiple reflections and room scattering on the AUT parameters.

In section 2 we discuss some policy considerations, and in section 3 we indicate the phased

array measurement requirements that drive the specifications of the PNF Range. In sections 4

through 7, we discuss the tests required for certifying the PNF Range. Tests are divided into four

1
categories: (1) tests performed by resident PNF Range personnel, (2) tests performed by or under

the direct supervision of NIST personnel, (3) software comparison tests, and (4) measurement

comparison tests. Section 8 contains a summary.

2. POLICY ISSUES

} 2.1. Purpose of Certification Process

NIST intends that the certification process, when successfully completed, will verify (1) that

the planar near-field (PNF) technology has been successfully transferred, and (2) that the PNF

Range and its personnel are capable of measuring antenna parameters of high- performance phased

arrays of a specified size to the required accuracy over a specified frequency range.

One of the major goals of NIST is technology transfer. We hope therefore that any problems

that become apparent during the certification process can be resolved so that the certification

process can be successfully completed. Toward this end NIST will provide advice and consultation

to a customer, whenever possible, to help resolve such problems. NIST certification does not

guarantee that accurate measurements will be performed, and only indicates that accurate

measurements are possible.

2.2. Certification Period

Planar near-field measurements are complex and involve detailed theory and complex

measurement techniques. Accurate PNF measurements require both a good PNF range and

qualified personnel to perform the measurements and process the data. For these reasons, the PNF

range and its personnel are certified as a unit.

The average electronics engineer in industry changes jobs about every 5 to 7 years. Much

of the equipment used on the PNF range requires a major repair or replacement in roughly the same

time frame. Thus, it seems appropriate to make the certification valid for 7 years.

2J. Certification Costs

Since NIST is required by law to recover the cost of its services, the cost will necessarily vary

from range to range and will also depend on the PNF Range’s required measurement accuracies.

3. AUT MEASUREMENT REQUIREMENTS

The PNF Range is required to measure the gain of a 4.5 m diameter phased array to an

accuracy of 0.2 dB. The Range must also have the ability to measure -55 dB sidelobes to an

accuracy of 5 dB and beam steering angles up to 60° to an accuracy of 20". This phased array will

operate at frequencies up to 4 GHz.

To ensure that the above measured parameters meet the required accuracies, we need to

evaluate the individual error sources and place limits on them so that their combinations meet the

2
requirements. Preliminary error budgets for gain, sidelobe level, and beam steering are shown in

tables 1 through 3. These error budgets come primarily as a result of NIST error analyses. These

budgeted errors correspond to worst-case errors (3a values for the random errors). Note that we

have used a root sum square to combine the errors in each budget. In combining errors in this way,

we have assumed that the errors are independent and therefore are not likely to all add in the same

direction. From experience this is usually a valid assumption. However in a rigorous sense, the only

way to guarantee that the total error is below a required value is by ensuring the arithmetic sum of

the individual errors is less than the required total error.

The error budgets found in tables 1 through 3 will drive the specifications for the PNF

measurement system either through the error equations of Newell [1] and Yaghjian [2] or through

rules that are derived from NISTs exp>ertise. TTiese are discussed in the next section.

Table 1. Error budget for AUT gain measurement.

Error Source

Probe relative pattern

Probe polarization ratio

Probe gain measurement

Probe alignment

Normalization constant

Impedance mismatch factor

AUT alignment

Data point spacing (aliasing)

Measurement area truncation

Probe position errors Probe-AUT multiple reflections

Receiver nonlinearity

Receiver dynamic range

Flexing cables

Temperature effects

Room scattering

Leakage and crosstalk

Random errors Root Sum Square

Error (dB^

0.18

0.00

0.00

0.11

0.02

0.05

0.04

0.02

0.00

0.05

0.02

0.10

0.01

0.00

0.01

0.01

0.00

0.00

0.05

3
Table 2. Error budget for -55 dB sidelobe measurement.

Error Source Error

Probe relative pattern 0.5

Probe polarization ratio 0.5

Probe gain measurement 0.0

Probe alignment 0.5

Normalization constant 0.0

Impedance mismatch factor 0.0

AUT alignment 0.5

Data point spacing (aliasing) 0.8

Measurement area truncation 0.5

Probe position errors 1.8

Probe-AUT multiple reflections 2.4

Receiver nonlinearity 0.2

Receiver dynamic range 0.5

Flexing cables 1.5

Temperature effects 1.5

Room scattering 1.4

Leakage and crosstalk 0.8

Random errors 0.8

Root Sum Square 4.4

Table 3. Beam steering measurement error budget.

Error Source Error

Probe alignment 5

AUT alignment 5

Scanner alignment 5

Other^ 10

Root sum square 14

4. TESTS BY RESIDENT PNF RANGE PERSONNEL

There are several major types of tests that resident Range personnel must perform to

characterize the errors associated with measurements on the range. They include tests to determine

alignment accuracy, errors caused by the instrumentation, and errors due to rf energy traveling

undesired paths. In a rigorous sense, most of these tests should be performed for each unique

probe-AUT combination. Because of the nature of these tests, they will demonstrate not only if the

PNF range is capable of making measurements to the desired accuracies but also if resident Range

personnel are able to determine the errors associated with a particular measurement.

' Currently, there are no equations or rules of thumb that predict how most error sources affect

the measurement of the beam-steering angle. However, errors in the measurement of the beamsteering angle have been observed for probe-AUT multiple reflections, truncation, and aliasing.

4
4.1 Alignment Tests

For this particular AUT, the sidelobe measurement accuracy, the beam steering angle, and

beam steering accuracy are the important conditions that drive the mechanical requirements of the

scanner. Note that both translational and angular requirements apply to the alignment of the rails,

rotators, scan plane mirror, the AUT, and the probe. Evaluation of these conditions leads to the

scanner alignment requirements in table 4. All alignment tests shall be performed twice. The

minimum time interval between performances of the same test shall be one week. This is necessary

to ensure the stability and repeatability of the measurement system.

Specifically for the case of a steered beam, X-, Y-position errors cause errors in the sidelobe

direction % given by eq (58), [1]

where A(k) and 5^(k) are the Fourier transforms of the position errors AP(P) and 5Z(P). g(K) is

the ratio (D '(^)/D^(k)) with Kq being the direction of the main beam, and is the beam-steering

angle.

The largest errors occur for periodic errors in SZ and Ap, see [1]. For example, using eq (2),

assuming a wavelength of 7.5 cm, and a probe alignment error of 0.5 dB (table 2), then a Z-position

error with a period in X of will cause errors in the directions corresponding to = ±2'jrf£^ with

magnitudes (in decibels) of (8.77t/2^)5Z^ cos 0,, g(K^ = ±2w/€J. In all other directions the error

will be zero. The pattern errors caused by periodic position errors lead to the AX, AY, and AZ

alignment requirements of table 4.

Angular alignment errors lead directly to errors in determining the beam-steering angle.

Thus a 5" error in the scanner alignment will cause a 5" error in the location of the beam. This leads

to the angular requirements of table 4.

(1)

and Z-position errors cause errors in the sidelobe given by eq (61), [1]

(2)

5
4.1.1 Scanner Alignment

To meet the requirements of table 4, one may align the PNF range to satisfy the

requirements discussed above or one may use software compensation to correct for any residual

inadequacy (provided that it is much less than a half wavelength). If personnel cannot make

alignments to the required accuracy, software compensation still requires the ability to measure

alignment errors with sufficient precision. If the position errors are measured with sufficient

accuracy, one can make corrections using the k-correction technique [3], or the technique of Muth

and Lewis [4]. The accuracies given below can be met using one of these software compensation

methods. It will be difficult to meet the mechanical requirements without software compensation,

though it is possible.

Table 4. Scanner alignment requirements.

Translation

AXvs.Xl,X2,Y 0.01mm

AY vs. X1,X2,Y 0.01 mm

AZ vs. X1,X2,Y 0.01 mm

Angle

Z1 perpendicular to X1,X2 T

22 perpendicular to X1,X2 2"

Y perpendicular to X1,X2 2"

XI parallel to X2 2"

A file which is compatible with the computer system and which contains a digital map of all

the alignment errors shall be saved for future use and for comparison with NIST alignment test

results.

The alignment of the scanner shall be determined using the following procedure. First, the

straightness accuracy of the front XI rail (the rail closest to the AUT, see fig. 1) shall be measured

using a laser measurement system with straightness interferometer optics and a separate laser

position measurement system. The straightness interferometer optics, part of which is on the probe

transport mechanism, should be located close to the XI axis when the mechanism is at the bottom

Y position. The laser straightness measurement system must be set up parallel to the average of the

XI axis and is used along with the laser position measurement system to measure the AZl versus

XI along this rail at intervals of 2 cm (about A/4 at 4.0 GHz) or less in XI. This quantity we

designate as AZh- Second, the laser straightness measurement system, the laser position

measurement system, and an optical square which has an accuracy of 1" or better should be used at

the mid-range, Xq, of the XI rail to measure AZl along the Y rails at intervals of 2 cm or less. This

quantity we designate as AZy. Third, the laser position measurement system and an electronic level

are used to measure the rotation about the front XI rail (i.e., the forward-backward tilt in the Y-Zl

plane with a tilt away from the AUT being positive) at intervals of 2 cm or less in XI. We

designate this angle by e^iLT. Fourth, the laser position measurement system and the electronic level

shall be used to measure the rotation about the Z1 axis at intervals of 2 cm or less in XI. This

angle is €roll is measured in the XI-Y plane. As viewed from the AUT, €roll is positive when

measured counterclockwise from Y toward XI. Note that in steps 3 and 4, the tilt and roll are

6
measured with the level located on the probe transport mechanism and with the mechanism in the

bottom Y position. Fifth, using this information we can determine AZ, AY, and AX as a functions

of XI and Y. These quantities are given by the following equations.

A similar procedure is employed to determine the alignment for the X2 rails.

The parallelism of the XI and X2 axes is measured using the laser straightness measurement

system, the laser position measurement system, and two optical squares. The laser straightness is

set up parallel to the XI axis. One of the optical squares shall be used to turn the beam 90® toward

the rails. The second optical square shall be used to turn the beam another 90® so that it is antiparallel with the XI rails at one of the X2 rails. The straightness shall be measured at intervals of

2 cm or less in X2. A least squares line shall be fit to this data as a function of X2. The angular

departure of X2 from a line parallel to XI is given by the arctan of the slope of this least squares

fit line.

The orthogonality of the XI, Y, and Z1 axes is measured using a procedure similar to that

in the above paragraph but using only one optical square. The orthogonality of X2 and Z2 are

likewise determined.

4.1.2 Scan Plane Mirror Alignment

A precision, optically flat mirror, capable of being adjusted in azimuth and elevation shall

be installed on the seismic isolation slab provided in the Range facility. The face of the mirror shall

be parallel to the face of the scan plane within ±2”. This mirror shall look along the -Z axis. It

shall be mounted 2 m (±10 cm) above the slab. The mirror face shall be accessible to a transit

mounted behind and to one end of the XI rails. The scan plane mirror shall be used to transfer the

scan plane alignment to the probe and the AUT. The normal to the mirror shall be aligned parallel

to the normal of the scan plane. The normal to this mirror shall be transferred to the probe which

in turn shall be used to align the AUT.

(3)

AY(X1,Y) - COS(e„LT(Xl)-eTO,^)

- COS(euQjj_(Xl)-EuQjj^(XQ))],

AXCX1.Y) = Y[SIN(eKou,(Xl)-eRoiL(Xo))]- ®

7
The alignment of the mirror shall be set using the following collimation techniques. First,

the azimuth alignment shall be done by aligning a transit with collimation optics along the X axis.

Then an optical square shall be used to turn the collimation of the transit 90° to the face of the

mirror. Last, the azimuth angle of the mirror shall be set parallel to the collimation. Second, the

elevation alignment of the mirror shall be accomplished using the transit's level. This shall be done

by setting the transit in front of the mirror, leveling the transit, and setting the mirror’s elevation

angle to that of the scan plane.

4.13 AUT Alignment

The AUT mount shall be capable of adjusting the antenna to within 0.05 mm in translations

and T in rotations. Adjustment capabilities on the AUT in azimuth, elevation, and along the phi

axis of the AUT (see fig. 1), shall be provided.

The AUT shall be aligned parallel to the scan plane mechanically by use of a depth gauge

mounted in the probe aperture. A fixture will be made to adapt the depth gauge for the probe. A

minimum of three alignment points will be defined on the AUT aperture face. The depth gauge will

be capable of making linear displacement measurements to an accuracy of ± 0.01 mm or better.

The phi axis of the AUT will be defined with an electronic level. After aligning the AUT, the PNF

range personnel shall record the residual alignment errors for Z (depth gauge reading at the

reference points), azimuth, elevation, and phi (electronic level reading) and provide them to NIST.

4.1.4 Probe Alignment

The flex (angular deviations) of the probe over its X, Y, and Z translation limits shall be less

than 2" at the probe face. Angular gimbles will be provided to allow adjustment in elevation and

azimuth (see fig. 1) of the probe in two separate and independent planes. The probe shall be

rotatable in phi from 0 to 90° (to provide for both vertical and horizontal polarization

measurements) within an accuracy of 0.05°. The probe face shall be perpendicular to the waveguide

axis to an accuracy of ±03°. The probe face shall be aligned to the scanner to an accuracy of ±2".

The probe alignment shall be accomplished as follows:

* A collimation fixture shall be built that has three mirrors mounted on it in different

orientations (fig. 2). This fixture shall have an adjustable mirror mounted on the face that

has been aligned to the plug insert which defines the inside walls of the probe. It shall have

an additional two mirrors mounted approximately 90° from each other on its sides. These

two mirrors shall have been characterized so that the angle between them is known.

* The angular flex of the probe as it moves in X and Y shall be determined by using a

collimation transit mounted to the side (along the X axis) of the probe. The collimation

fixture shall be mounted with one mirror in the downward direction. The transit shall be

collimated on the X mirror of the collimation fixture directly, and through an optical square

for the Y mirror. The angular deviations as the probe moves in X and then in Y shall be

characterized through the transit. The maximum magnitudes of these angular deviations

shall be recorded.

8
* The linear translation of the probe as it moves along the Z1 axis shall be characterized by

aligning a transit along that axis, by using the transit*s crosshairs and the crosshairs of the

face mirror on the collimation fixture. Measurements of the deviation in X and Y shall be

taken every 2 cm along the entire probe travel.

* Measurements of the phi rotation error shall be accomplished with the probe at a desired

Z1 position. The transit shall be collimated and fixed on the crosshairs of the face mirror

of the collimation fixture. Another transit shall be collimated to the side mirror (along the

X axis) of this fixture. The probe shall be rotated 90® in phi so that the other side mirror

is rotated in view of the X transit. Collimation of this mirror shall be compared to the

original collimation to determine the phi rotation error. The collimation on the face mirror

shall be compared to its collimation in the initial orientation to determine the azimuth and

elevation errors. Deviations in the crosshairs on the face mirror represent the X and Y

translation errors.

* The probe face to waveguide error shall be measured by aligning a transit along the inside

edge of the rectangular waveguide portion of the probe. Aprecision flat/parallel mirror shall

be mounted on the face of the probe. Deviations in azimuth and elevation of this mirror

represent this error.

* The probe’s azimuth and elevation shall be compared to the azimuth and elevation of the

scan plane mirror. A transit shall be collimated to the scan plane mirror. The probe, with

the collimation fixture inserted in its aperture shall be moved in front of this transit.

Differences in azimuth and elevation of the face mirror represent this error.

4.2 Rf Crosstalk

The rf isolation shall meet the requirements of table 5.

Mixer LO-to-rf port-to-port isolation, coaxial switch port-to-port isolation, the isolation of

all isolators, and coupler directivity shall be measured using a network analyzer which has a

calibration traceable to NIST.

Receiver reference channel to measurement channel isolation shall be measured using the

following procedure. First a signal source shall be used to put a -25 dBm signal into the reference

channel. Next all measurement channels shall be terminated with matched loads at the rf input

ports. Then the reference channel shall be set to 0 dB. The attenuation in each measurement

channel shall be set to the same value as that of the reference channel. An amplitude measurement

shall be made at the rf output port of each measurement channel using the maximum available

averaging. The resulting amplitudes shall be less than -90 dB.

The isolation between receiver measurement channels shall be measured using a procedure

similar to that described above. A -25 dBm signal shall be fed into each measurement channel in

turn and the amplitude out of each of the other measurement channels recorded.

9
Table 5. Isolation requirements.

Isolation requirement

(dB)

Isolation measurement

accuracy requirement

(dB)

Mixer LO-to-rf port-to- ^ 25 within ±2

port isolation

Receiver referencechannel to

measurement-channel

isolation

> 90 within ±5

Isolation between two

receiver measurement

channels

> 90 within ±5

Coupler directivity > 30 within ±3

Isolation of isolators > 30 within ±3

Coaxial-switch port-toport isolation

> 80 within ±5

4.3 Rf Path Variation

The transmission line shall provide phase and amplitude stability of the rf signal during

motion of the probe. The stability performance shall be less than ±0.02 dB and ±0.2° over the

entire scan plane or sufficiently repeatable to allow for correction.

The rf path variation shall be determined using the following procedure illustrated in figure

3. A 20 dB coupler and tuner assembly shall be inserted at a point between the signal source and

the probe where there is no cable flexing. A load shall be inserted in place of the probe and the

tuner adjusted for a null. Then a short shall be inserted for the load. The signal reflected by the

short shall be fed into the receiver. The short shall be positioned at the center of the scan area and

the amplitude and phase set to zero. A full two dimensional scan of the entire available scan area

shall be performed with AX = AY = 2.0 cm and the amplitude and phase recorded. The recorded

phase shall be divided by 2 and saved in a file along with the amplitude. This file now contains a

map of the phase and amplitude variation over the entire scan plane.

4.4 Position Errors

The probe servo positioning system for XI and Y shall be capable of placing the probe to

within 0.01 cm or less of its commanded position. The AUT servo positioning system for X2 and

Z2 shall be capable of placing the AUT to within 0.01 cm or less of the commanded position.

10
The positioning accuracy shall be determined by the following procedure.

a. For the XI position accuracy, the probe shall be placed 10 cm above its bottom limit. Then

the probe shall be commanded to go to XI = 2.00 cm, 4.00 cm, and so forth until XI =

900.00 cm is reached. This procedure shall be repeated at the central Y position and at 10

cm below the top limit. The actual XI position shall be recorded using the laser

measurement system. The difference between the commanded position and the actual

position shall be 0.01 cm or less.

b. For the Y axis the probe shall be commanded to go to 10 cm from the right limit (as viewed

facing the scanner). Then the probe shall be commanded to go to Y = 2.00 cm, 4.00 cm,

and so forth until Y = 900.00 cm is reached. This procedure shall be repeated at the central

XI position and at 10 cm from the left limit. The actual position shall be recorded using the

laser measurement system. The difference between the commanded and actual position shall

be 0.01 cm or less.

c. For the X2 axis the AUT shall be placed at 10 cm from its forward position. Then the AUT

shall be commanded to go to X2 = 2.00 cm, 4.00 cm, and so forth until X2 = 900.00 cm is

reached. This procedure shall be repeated at the central Z2 position and 10 cm from the

back limit. The actual position shall be recorded using the laser measurement system. The

difference between the commanded position and the actual position shall be 0.01 cm or less.

d. For the Z2 axis the AUT shall be commanded to go to 10 cm from the right limit. Then the

AUT shall be commanded to go to Z2 = 2.00 cm, 4.00 cm, and so forth until Z2 = 100 cm

is reached. This procedure shall be repeated at the central X2 position and 10 cm from the

left limit. The actual position shall be recorded using the laser measurement system. The

difference between the commanded and actual position shall be 0.01 cm or less.

4^ Rf System

The dynamic range of the system must be adequate to measure the low sidelobes and the

cross polarization. The receiver d^amic range should be an order of magnitude better than the

system dynamic range to ensure that the system meets the requirements. The system dynamic range

is primarily limited by system noise.

4.5.1 System Dynamic Range

The system dynamic range shall be at least 75 dB relative to the near-field peak of the main

component and shall be determined using a receiver averaging time of no more than 10 ms. The

probe shall be placed at the main component near-field peak. The receiver shall be set to 0.0 dB.

Then to determine the noise floor, at least 90 dB of attenuation shall be inserted in the

measurement channel (at any convenient point along the rf line) using a rotary vane attenuator

(RVA) or by inserting a fixed attenuator until the signal disappears in the noise. The receiver

reading shall be recorded and the difference between this receiver reading and 0.0 dB is the system

dynamic range.

11
4^^ Receiver Dynamic Range

The receiver dynamic range shall be at least 85 dB and shall be determined using a receiver

averaging time of no more than 10 ms. The receiver dynamic range shall be determined by setting

the input signal to the mixer at its maximum input level (as specified by the manufacturer, typically

-20 to -30 dBm). The receiver reading shall be set to 0.0 dB. Then to determine the noise floor,

at least 100 dB of attenuation shall be added to the rf line until the signal disappears in the noise.

The receiver reading shall be recorded and the difference between this reading and 0.0 dB is the

receiver dynamic range.

433 System Linearity

We can determine the requirements for system and receiver linearity by using eqs (64) and

(67) of [1]. The system linearity shall be measured over a 75 dB dynamic range. The residual

amplitude error shall be less than 0.001 dB/dB below the maximum signal level of this dynamic range

and the residual phase error less than 0.17dB.

The system’s amplitude linearity shall be determined with the probe at the near-field peak

of the main component using the RVA calibrated by NIST. The RVA shall be inserted in the

system between the signal source and the transmit antenna. The RVA shall initially be set to 0.00

dB and the receiver to 0.00 dB. The RVA shall be set to 2 dB, 4 dB, and so forth in 2 dB

increments to 20 dB. Then the RVA shall be set to 25 dB, 30 dB, and so forth in 5 dB increments

to 75 dB. At each RVA setting, the receiver reading (amplitude and phase) shall be recorded. This

procedure shall be repeated 10 times and the average of the 10 measurements determined. The

averages shall be compared to the calibrated values of the RVA to determine the system linearity

correction.

43.4 Receiver Linearity

The receiver linearity shall be measured over an 85 dB dynamic range. The residual

amplitude error shall be less than 0.001 dB/dB below the maximum signal level of this dynamic range

and the residual phase error less than 0.17dB.

The receiver’s amplitude and phase linearity shall be determined using an RVA calibrated

by NIST for both amplitude and phase versus attenuation. The RVA shall be inserted in the system

just prior to the rf input of the receiver. The RVA shall initially be set to 0.00 dB and the receiver

to 0.00 dB. The RVA shall then be set to 2 dB, 4 dB, and so forth in 2 dB increments to 20 dB.

Then the RVA shall be set to 25 dB, 30 dB, and so forth in 5 dB increments to 85 dB. At each

RVA setting, the receiver reading (amplitude and phase) shall be recorded. This procedure shall

be repeated 10 times and the average of the 10 measurements determined. These averages shall

then be compared to the RVA calibration values to determine the receiver linearity correction.

12
4.6 Noise Level

The average noise level of the far field shall be less than -70 dB relative to the far-field peak

of the AUT main component and shall be measured as follows. A full two=dimensional near-field

scan of the AUT main component shall be made at 4 GHz using AX = AY = 2 cm and the far-field

determined. The average noise level shall be computed using the far-field amplitude at the points

where

k

(6)

using the following equation.

Average noise level

[amplitude(k^, ky)]^

(7)

where N is the number of data points in the average.

4.7 Leakage

The maximum leakage for the entire scan plane shall be 75 dB or more below the maximum

measured near-field signal of the AUT main component.

Rfleakage shall be measured at 4 GHz using the following procedure. First, a complete scan

shall be performed to determine the maximum near-field signal. Second, a complete scan shall be

performed with the probe radiating normally, and with a matched load on the AUT-to-receiver

transmission line where it normally connects to the AUT output ports. Third, a complete scan shall

be performed with the AUT receiving normally, and a matched load on the probe-to-rf source

transmission line where it normally connects to the input port(s) of the probe. No level of the

second and third scans shall exceed -75 dB relative to the peak of the first scan.

13
4^ Probe-AUT Multiple Reflections

The effects of the probe-AUT multiple reflections on the far field shall be no greater than

the levels shown in table 6.

Table 6. Multiple reflection level.

Far-field pattern level

relative to the main beam

m

0

-55

Maximum multiple reflection

error level relative to the main beam

m

'

-39

-65

The effects of these multiple reflections shall be determined by using four full two

dimensional planar near-field scans taken at Z©, Zq + A/8, Zq + 2A/8, and Zq + 3A/8, where Zq is

established to optimize both the effects of probe-AUT multiple reflections and truncation. Then the

far field for each scan shall be calculated and a phase adjustment applied to account for the

differences in Z position. A complex average of the four adjusted far fields shall be made, and a

complex difference between the average far field and each of the individual far fields shall be

computed. These differences shall not exceed the values specified in the above table.

4.9 Room Multipath

The multipath, the signal due to the reflection of rf energy off the walls and other structures

excluding only the probe-AUT multiple reflections, shall be no greater than -75 dB relative to the

peak of the far-field signal.

The room multipath shall be verified as follows. A centerline y-scan of the AUT shall be

measured at different x-positions relative to the scanner but the AUT and the probe shall have the

same x-position relative to each other. These scans shall be measured 2 cm apart (relative to the

scanner) at a frequency of 4 GHz and shall cover the entire X and Y scanning range. A complex

average with respect to x shall be determined for each y-position so as to obtain an average y

scan. The complex difference between this average scan and each individual scan shall be determined.

This two dimensional near-field difference (multipath) pattern shall be transformed to obtain an

error spectrum. This error spectrum shall be compared to the AUT far-field spectrum and no point

in the error spectrum shall be greater than -75 dB relative to the mainbeam peak.

4.10 Truncation

We have found from our experience that the truncation error spectrum bears some

resemblance to the true spectrum, e.g., the truncation error spectrum is higher in the mainbeam than

in the sidelobe regions. For the normal X- and Y-scan dimensions of the AUT, the truncation error

shall meet the requirements in table 7.

14
Table 7. Maximum truncation error.

Far-field pattern level

relative to the main beam

m

0

-55

Maximum truncation error level

relative to the main beam

m.

-45

-80

The truncation error shall be determined using the following procedure. First, a normal two

dimensional near-field scan shall be performed on the AUT and the far field calculated. Next, all

the amplitudes of near-field data shall be set to zero except for the outer rectangular boundary. The

far field of this resulting data is calculated to yield the truncation error spectrum.

4.11

Aliasing

The aliasing error shall not exceed -75 dB relative to the peak of the main beam for 1 0

1

< 75®. Aliasing errors shall be determined at 4 GHz by (1) making a two dimensional scan with A/8

spacing in X and Y, (2) computing the far field for this full set of data, (3) computing the far field

using every other data point in X and Y (A/4 spacing), (4) computing the far field using every fourth

data point in X and Y (A/2 spacing), (5) performing a complex subtraction between the A/4-spaced

andA/8-spaced far fields, and (6) performing a complex subtraction between the A/2-spaced andA/8-

spaced far fields. The resulting differences obtained in (5) and (6) shall be compared to the aliasing error requirements and a sampling spacing determined. It should be noted that before the

subtractions in (5) and (6), all three sets of near-field data must be zero-filled to the same array size.

4.12

System Drift

System drift, the variations in amplitude and phase read on the receiver as a function of time,

shall be less than 0.1 dB in amplitude and 10® in phase over a four-hour period, and must be

correctable to 0.01 dB and 0.1®.

The probe shall be set to the peak of the near-field amplitude. The receiver amplitude and

phase shall initially be set to 0.0 dB and 0.0®. The amplitude and phase of the receiver shall be

recorded every 2 min for the 4 h period.

4.13

Insertion Loss Amplitude and Phase Measurement

Insertion loss amplitude and phase measurement shall be repeatable to an accuracy of ± 0.05

dB in amplitude and ±2® in phase for the main component. For the cross component, the amplitude

shall be repeatable to within ±0.2 dB in amplitude and ± 10® in phase. Ten repeated insertion loss

amplitude and phase measurements shall be taken at four different times separated by a week.

15
5. TESTS BY NIST PERSONNEL AT THE PNF FACIUTY

NIST will perform the following tests using the same procedures described in section 4.

Their results will be used to verify those obtained by Range personnel.

1. Scanner Alignment (under 4.1).

2. Scan Plane Mirror Alignment (under 4.1).

3. Leakage (under 4.7).

/ 4. Room Multipath (under 4.9).

5. Truncation (under 4.10).

6. NIST will perform at least three additional tests chosen from section 4 to verify the

test results obtained by Range personnel.

6.

SOFTWARE COMPARISON TESTS

The purpose of these tests is to ensure that the Range software works correctly. Results

from data processed at the PNF range shall agree with NIST results to within the round-off error of the computers. The following apply.

6.1 Test Data

NIST will provide sample data from its PNF Data Library to Range personnel.

6.2 NIST Processing

NIST will process the sample data through its PNF software.

63 Range Data Processing

Range personnel shall process the sample data through its PNF software. All results shall

be supplied to NIST on an appropriate medium.

6.4 Comparison of Results

NIST will compare the Range results to the NIST results and will provide the results of the

software tests to Range personnel.

16
7.

MEASUREMENT COMPARISONS

7.1 Transfer Test Antenna

A transfer test antenna will be measured on both the PNF Range and the NIST PNF range.

The results from the Range shall agree with NIST results to within the combined uncertainties of

both measurements. The antenna shall be transportable and measurable at both facilities. It shall

have low sidelobes to properly test the low sidelobe measurement capability of the Range. The

customer shall be responsible for acquiring an antenna that is acceptable to NIST for these

measurements.

7.2 NIST Measurements

NIST will measure the gain, co-polarization pattern, and cross-polarization pattern of the

transfer antenna over ± 75° in azimuth and elevation. NIST will determine the error budget for its

measurements. Individual entries in the error budget will correspond to the 3cr value for random

errors and the worst-case error for systematic errors.

7J Range Measurements

Range personnel shall measure the gain, co-polarization pattern, and cross-polarization

pattern of the transfer antenna over ± 75° in azimuth and elevation. Range personnel will

determine the error budget for its measurements. Individual entries in the error budget shall

correspond to the 3a value for random errors and the worst-case error for systematic errors. Range

personnel shall provide all results to NIST.

7.4 Comparison

NIST will compare the results of the two sets of measurements. Range results shall agree

with NIST results to within the combined errors of the two measurements. NIST will provide the

results of the comparisons to the customer.

8.

SUMMARY

This certification plan is based in part on information supplied by Range personnel regarding

the types of antennas Range personnel will measure and the required measurement accuracies. Any

significant changes or additions in measurement frequency, antenna size, or measurement accuracies

will require modifications to the certification plan.

9.

ACKNOWLEDGEMENTS

The authors thank the Naval Surface Warfare Center Crane Division for their contribution

to this effort.

17
10. REFERENCES

[1] Newell, A.C., "Error analysis techniques for planar near-field measurements," IEEE Trans.

Antennas Propagat., vol. AP-36, pp. 754-768, June 1988.

[2] Yaghjian, A.D., "Upper-bound errors in far-field antenna parameters determined from planar

near-field measurements part 1: analysis," Nat. Bur. Stand. (U.S.) Tech. Note 667, October

1975.

[3] Corey, L.E. and Joy, E.B., "On computation of electromagnetic fields on planar surfaces

from fields specified on nearby surfaces," IEEE Trans. Antennas Propagat., vol. AP-29,

pp.402-404, March 1981.

[4] Muth, L.A. and Lewis, R.L., "An iterative technique to correct probe position errors in

planar near-field to far-field transformations," Nat. Inst. Stand. Technol. Tech. Note 1323,

October 1988.

18
Figure 1. Coordinate axes for the proposed PNF range.

19
Waveguide insert

Figure 2. Schematic of collimation fixture.

20
From Source

Tuner

1—

r

II IS To Receiver

Directional Coupler

(High Directivity)

]=0

Flexing Coble

=4 1

1

Notes:

Drawing shows type

’N’ connectors for

illustration only.

Coupler used as a

reflectometer.

Termination mma;*

Short\

Probe

Figure 3. Schematic for testing rf amplitude and phase path variation.

21
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BL-114A U.S. DEPARTMENT OF COMMERCE

(54« NATIONAUNSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NISTIR 3991

2. PERFORMING ORGANIZATION REPORT NUMBER

B92-0196

S. PUBUCATION DATE

July 1992

4. TITLE AND SUBTITLS

A Certification Plan for a Plan Near-Field Range Used for High-Performance Phased-Array

Testing

S. AUTHOR(S)

Michael H. Francis, Andrew G. Repjaur, and Douglas P. Kremer

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST. SEE INSTRUCTIONS)

UP. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER. COLORADO e0303-332S

7. CONTRACT/GRANT NUMBER

S. TYPE OF REPORT AND PERIOD COVERED

9. SPONSORINQ ORGANIZATION NAIIE AND COyPLETE ADDRESS (STREET. CITY. STATE. ZIP)

10.

SUPPLEMENTARY NOTES

11.

ABSTRACT (A 20O-WORD OR LESS FACTUAL SUMMARY OF MOST SIQNIRCANT INFORMATION. IF DOCUMENT INCLUDES A SIQNinCANT BIBUOQRAPHY OR

LITERATURE SURVEY. MENTION IT HERE.)

The National Institute of Standards and Technology (NIST) has written a certification

plan to ensure that a proposed planar near-field range is capable of measuring highperformance phased arrays. Generally for a complete plan, one must evaluate many

aspects including scanner alignment, near-field probe alignment, alignment of the

antenna under test, rf crosstalk, probe position errors, path variations, the

receiver's dyneunic range and linearity, leakage, probe-antenna multiple reflections,

truncation effects, aliasing, system drift, room multipath, insertion loss measurements

noise, and software verification. In this report, we discuss the important aspects of

a certification plan specifically written for the measurement of high-performance

phased-array antennas. Further, we show how the requirements of each aspect depend on

the measurement accuracies needed to verify the performance of the array under test.

12.

KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

2mtennas; antenna measurements; phased array; planar near field

13. AVAILABILITY

T

UNLiMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

ORDER FROM SUPERINTENDENT OF DOCUMENTS. UP. GOVERNMENT PRINTING OFFICE.

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECWIICAL INFORMATION SERVICE (NTIS), SPRINGWELD.VA 22161.

14. NUMBER OF PRINTED PAGES

26

IS. PRICE

A03

ELECTRONIC FORM
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