Proposed TDR Method for Site Validation Above 1 GHz

ACIL CAS MeetingAugust 15, 2011Long Beach, CA

byGreg Kiemel, Director of Engineering

gkiemel@nwemc.comNorthwest EMC, Inc.

www.nwemc.com

Overview

Current Site Validation RequirementsProposed Alternate MethodDescription of Study that was performedData ComparisonConclusion

Site Validation Above 1 GHz

Amendment A1:2007 to EN 55022:2006 will be mandatory starting Oct 1, 2011. Requires testing above 1 GHz.

Australia and New Zealand will also require testing above 1 GHz starting October 1, 2011

VCCI required radiated emissions testing above 1 GHz starting April 1, 2010.

BSMI required radiated emissions testing above 1 GHz starting October 1, 2010.

In all cases, the sVSWR method of CISPR 16-1-4 is required.

Site Validation Above 1 GHz

FCC specifies ANSI C63.4 (2003) or (2009) as acceptable test methods. The 2003 version has no site validation requirements above 1GHz.

The updated 2009 version has two options: absorber on the floor that meets certain performance requirements, or compliance with CISPR 16-1-4.

In both cases, compliance with NSA requirements below 1 GHz is required. See FCC KDB 704992: http://apps.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=44117&switch=P

sVSWR Method

The SVSWR is the ratio of maximum received signal to minimum received signal, caused by interference between direct (intended) and reflected signals, or

TDR – Proposed ANSI Method

The TDR method is being developed as part of a working group within ANSI ASC C63® using time domain gating to evaluate only the test

environment This method requires the use of a vector network

analyzer and bore-sighted horn antennas.

ANSI Proposed TDR Method

Once in the time domain the direct path is removed numerically by a process referred to as “gating”

The time gate allows us to evaluate only the reflected signals from the test site.

ANSI Proposed TDR Method

Normalized Bore-site Trace in the Frequency Domain

ANSI Proposed TDR Method

Transform of Bore Sight Without Gating in the Time Domain

ANSI Proposed TDR Method

Transform of Bore Sight With Gating in the Time Domain

ANSI Proposed TDR Method

Max Hold trace in the Frequency Domain. With Gating this shows only the magnitude of the reflections vs. Frequency

SVSWR and TDR Method

How is TDR and SVSWR the same? Both measure phase change due to reflections

SVSWR through changing locations of the antenna TDR through direct phase measurement (VNA)

Both measurements are relative (no antenna calibrations needed)

Both methods assume a fully anechoic site

Overview of the StudyTDR vs. VSWR

Multiple FCC-listed test sites were evaluated: 10m chamber 5m chamber 3m chamber 10m Open Area Test Site (OATS) – Vinyl Cover 10m (OATS) – Wood Geodesic Dome

VSWR and TDR data taken from 1 – 6 GHz at all sites and additionally, 6 – 18 GHz at the Geodesic Dome.

Data Analysis

VSWR and TDR data were converted to “Site Error” for comparison:

CISPR spec of 6 dB VSWR is equivalent to a site error of 2.23 dB, calculated as follows: Site error = SQRT(10^(VSWR/20)^2+1)

TDR spec of –10.66 dB is equivalent to a site error of 2.23 dB, calculated as follows: Site error = 20*LOG(10^(TDR data/20)+1)

Courtesy of Zhong Chen, one site was compared by converting TDR data to VSWR.

Source: M Windler "Site Qualifications above 1 GHz," Compliance Engineering, March 2007 (www.ce-mag.com).

OATS with Geodesic DomeAcme Testing Co.

OATS with Geodesic DomeAcme Testing Co.

Comparison by converting TDR data to VSWR

Zhong Chen of ETS reviewed the TDR and VSWR data and proposed the following: “The TDR_data is in essence the reflection

coefficient (gamma), or Vref/Vdirect. The reflection coefficient is related to the VSWR by:

VSWR=(1+|gamma|)/(1-|gamma|),or |gamma|=(VSWR-1)/(VSWR+1).

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

VSW

R d

B

Frequency (MHz)

sVSWR and TDR Comparison

Worst Case (Hor)

WorstCase H TDR

Horizontal (1 – 6 GHz)

Horizontal (6 – 18 GHz)

0

1

2

3

4

5

6

6000 8000 10000 12000 14000 16000 18000

VSW

R

Frequency (MHz)

TDR_1.5_Horiz F-H-H1

sVSWR and TDR Comparison

Vertical (1 – 6 GHz)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

VSW

R d

B

Frequency (MHz)

sVSWR and TDR Comparison

Worst Case (Ver)

WorstCase V TDR

Vertical ( 6 – 18 GHz)

0

1

2

3

4

5

6

6000 8000 10000 12000 14000 16000 18000

VSW

R

Frequency (MHz)

TDR_1.5Vert F-V-H1

sVSWR and TDR Comparison

Troubleshooting

TDR Method is an excellent tool for troubleshooting site issues The first step is to orient the transmit

horn to the azimuth that produces the highest displayed peak. Record the trace for both frequency and

time domains

Worst-Case Azimuth, Failing Data

Frequency Domain

2.54 GHz, -8.5 dB

Time Domain

21.5 ns = 6.45 m

-19.7 dB

Troubleshooting in the Time Domain

The distance displayed on the screen is the round-trip distance that the pulse traveled from the transmit antenna to the reflecting object and back. In this case, the reflecting object was 3.2 meters

from where the transmit antenna was pointing. (Propagation is 1ns/ft)

A large flat piece of metal (e.g. 2 x 3 ft) can be used to confirm the reflection location. Simply hold the metal in the suspect location and confirm the peak of interest changes in amplitude.

Wall(s) of Absorber can be Placed at the Suspect Location(s)

Worst-Case Azimuth, Passing Data with Additional Absorber in Place

Frequency Domain

1.92 GHz, -15.2 dB

Time Domain

11.3 ns = 3.39 m

-31.0 dB

Conclusions

Both methods correlate in determining compliance.Absorber type, coverage area, and chamber

volume are factors in meeting site validation requirements. sVSWR method is more labor intensive and

utilizes more of the existing lab equipment. TDR method is an excellent tool in identifying

the source of non-compliance and is much faster.