reverberation chamber system validation and verification

Reverberation Chamber System Validation and Verification

John Ladbury, Principal InvestigatorGalen Koepke, Project Leader (Retired)

Perry Wilson, Group Leader: RF Fields Group

2018-03-22

2

Outline

• About NIST

• Ideal Reverberation Chamber Characteristics

• Validation and Verification Plan

• Loaded Chamber Uniformity

• Fields and Field Probes

• Empty Chamber Evaluation

• Signal Quality Evaluation

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About NIST (Organization)

NIST: National Institute of Standards and TechnologyDedicated to measurement science, rigorous traceability, development and use of standards

CTL: Communications Technology LabAdvances the measurement science underlying wireless technologies

RF Technology DivisionDevelops theory, metrology and standards for the technologies upon which the future of wireless communications depends.

RF Fields Group: Measures, characterizes, and calibrates antennas and radiated fields

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RF Fields Group

Principal traceability path for rf field probe calibration in the US

Over 40 years of experience in reverberation chamber research

Helped establish international standards for calibrating and performing measurements in reverberation chambers

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Ideal Reverberation Chamber Characteristics

• Antenna placement unimportant.

• Probe placement unimportant.

• Test artifact/animal placement unimportant.

• Orientation is unimportant

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Validation and verification plan

• Uniformity (temperature in phantoms, probe field, antenna power)

• Note: cannot directly measure SAR

• Field probe/chamber calibration

• Signal quality

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Validations Performed:

• December, 2007 after initial installation

• April, 2012 after pre-chronic and thermal pilot studies

• May, 2015 well after all tests complete, and system had been off for around 1 year

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Loaded Chamber Uniformity

Replace 1 cage with an antenna at random orientation.Measure average received power during continuous paddle rotation.Repeat at 20 cage positions.Standard deviation 1.3 dBRange 2.5 dB

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RF Field Probes and calibration

Calibration Procedure for each axis1. Generate known signal

GSM, CDMA, …

2. Measure forward and reflected power.3. Radiate signal using known antenna.4. Compute field based on radiated power.5. Measure probe response6. Repeat over multiple levels to deal with

nonlinear response.

Standard uncertainty: 0.4 dB or 10 % in power

IT’IS calibration is very similar.

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Types of fields

Electric and Magnetic, also know as E-fields and H-fields. (proportional in RC)

Cartesian components: aligned with x-axis ( 𝐸𝐸𝑥𝑥 ), y-axis (𝐸𝐸𝑦𝑦), and/or z-axis (𝐸𝐸𝑧𝑧)Depends on orientation

Total Electric Field = 𝐸𝐸𝑇𝑇= 𝐸𝐸𝑥𝑥 2 + 𝐸𝐸𝑦𝑦2 + 𝐸𝐸𝑧𝑧 2 (independent of orientation)

SAR is proportional to 𝐸𝐸𝑇𝑇2 = 𝐸𝐸𝑥𝑥 2 + 𝐸𝐸𝑦𝑦2 + 𝐸𝐸𝑧𝑧 2

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Empty Chamber evaluation

Measure received power on NIST antenna and Spectrum Analyzer, field on NIST E probe, IT’IS E probe, and IT’IS H probe at 100 discrete paddle positions.

Convert all measurements to squared total E field using known equations.

Compare averages in each chamber with a target field

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Single-axis field comparison: 2007

10

100

1000

1 2 3 5 6 7 8 9 10 11 12 14 16 17 18 19 20 21

Chamber

Estim

ated

Tot

al F

ield

(V/m

)

IT'IS ExIT'IS EyIT'IS EzIT'IS HxIT'IS HyIT'IS HzNIST ExNIST EyNIST EzNIST AntennaGoal

CDMAMice

GSMMaleRats

CDMAMaleRats

Note:Field levels change over time due to animal growth, although SAR remains constant.

No entries for sham chambers.

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Single-axis deviation from goal: 2007

-5

-4

-3

-2

-1

0

1

2

3

4

5

1 2 3 5 6 7 8 9 10 11 12 14 16 17 18 19 20 21

Chamber

Dev

iatio

n fr

om G

oal (

dB)

IT'IS Ex IT'IS Ey IT'IS Ez

IT'IS Hx IT'IS Hy IT'IS Hz

NIST Ex NIST Ey NIST Ez

NIST Antenna Goal

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Total field comparison: 2007

10

100

1000

1 2 3 5 6 7 8 9 10 11 12 14 16 17 18 19 20 21

Chamber

Mea

sure

d To

tal F

ield

(V/m

)

IT'IS EtIT'IS HtNIST EtGoal

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Total field deviation from goal: 2007

-5

-4

-3

-2

-1

0

1

2

3

4

5

1 2 3 5 6 7 8 9 10 11 12 14 16 17 18 19 20 21

Chamber

Dev

iatio

n fr

om G

oal (

dB)

IT'IS Et IT'IS Ht NIST Et

Goal Uncertainty

GSM Male Rats GSM Female Rats

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CDF’s of single-axis field measurement: 2007

-15 -10 -5 0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cum

ulat

ive

Prob

abili

ty

Normalized Total Field (dB V/m)

Chamber 17, H-Field Probe

Chamber 16, H-Field Probe

Chamber 5, H-Field Probe

Chamber 6, H-Field Probe

All GSM Rat chambers

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A closer look at Chamber 6: High Male Rat GSM 2007

-30 -25 -20 -15 -10 -5 0 5 10 15 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Cum

ulat

ive P

roba

bilit

y

Normalized field (dB V/m)

Clipping

IT'IS H-Field Probe Axes

IT'IS E-Field Probe Axes

NIST E-Field Probe Axes

NIST Antenna

_______

_______

_______

_______

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Single-axis deviation from goal: 2012

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Total field deviation from goal: 2012

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CDF’s of single-axis field measurement: 2012

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A closer look at Chamber 6: High Male Rat GSM 2012

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Single-axis deviation from goal: 2015

Zeroing units in chambers 3 and 20 had failed. Data review showed no problems in chamber 20 during tests, but chamber 3 had issues for last 6 months of test.

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Total field deviation from goal: 2015

No issues seen in measurements of total electric field.

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CDF’s of single-axis field measurement: 2015

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Signal distortion

All transmitted signals have some distortionRegulations focused on preventing interference with users of adjacent spectrum.Limited guidance in main signal band.

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High-power GSM emissions, Chamber 6 (male rat)

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High-power GSM emissions, Chamber 11 (mouse)

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High-Power CDMA Spectra Chambers 11 (mouse) and 7 (m rat)

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High-power GSM in time domain

0 2 4 6 8 10-100

-80

-60

-40

-20

0

20

Time (ms)

Mea

sure

d O

utpu

t Pow

er (d

Bm

)

Chamber 6, GSM, High Power

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Overall Summary

Loaded chamber uniformity: standard deviation of 1.3 dB.

Some unusual probe behavior observed in high-power chambers for H field probes, improved for thermal pilot and prechronic, fixed for chronic tests.

After chronic tests, two zeroing units had failed. One had been working during chronic tests, the other failed approximately 6 months before end of chronic tests. Negligible impact on estimation of the total E field.

Signal quality within standard parameters for communications standards.