Measurement of PCB’s Radar Cross Section in a GTEM Cell...Int. Zurich Symp. on EMC - Munich, 25 Sept. 2007 - D. Pouhe` – p.7/19 Beneﬁts of Using the GTEM Cell The GTEM Cell can

Measurement of PCB’s Radar CrossSection in a GTEM Cell

David Pouhe

[email protected]

Technical University Berlin

Inst. for High-Frequency- and Semiconductor Technologies

Antennas & EMC Group

- Germany -

Int. Zurich Symp. on EMC - Munich, 25 Sept. 2007 - D. Pouhe – p.1/19

Outline

Motivation.


Outline

Motivation.

Necessity of Investigating the RCS of PCBs.


Outline

Motivation.

Necessity of Investigating the RCS of PCBs.Physics behind the RCS and Definition.


Outline

Motivation.

Necessity of Investigating the RCS of PCBs.Physics behind the RCS and Definition.Why RCS as Main Parameter?


Outline

Motivation.


Benefits of Using a GTEM Cell as Test Facility.


Outline

Motivation.



Modelled Geometries.


Outline

Motivation.




Measurement Procedure and Results.


Outline

Motivation.




Measurement Procedure and Results.

Conclusion.


Motivation


MotivationImportance in modern microelectronic



Complexity of the layout: Bends, Vias,Traces, etc. are sources of unintended radia-tion




Radiated emission can alter the EMC perfor-mance of PCBs.





In presence of an RLC-circuit on the PCB for example, resonancesare carried over.





In presence of an RLC-circuit on the PCB for example, resonancesare carried over.

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

x 109

−150

−100

−50

0

Frequency [Hz]

Nor

mal

ized

RC

S [d

B]

Resonances are carriedover to higher frequencies

Bare PCB

PCB with an RLC circuit


Motivation - count...

These resonances carry interference over much longer distanceswhich can not be predicted by field or galvanic coupling=⇒ Resonance behavior of radiated field with RCS being mainparameter of investigation is therefore a good figure of merit indiagnosing radiation problems on PCB




Real EMI:s from a PCB during the design phase are best predicted bymeasurements.




Real EMI:s from a PCB during the design phase are best predicted bymeasurements.

A PCB with known resonances may behave differently bysusceptibility and emission measurements.


Necessity for Investigating the RCSPhysics behind the RCS and Definition





Pint: power intercepted by a target at a distance r.Si [W/m2]: incident power density at this scatterer.

Pint = σ · Si (2)

where σ [m2] is the target’s cross section.


Necessity for Investigating the RCS - count...



Assumption: Pint is re-radiated as scattered power uniformly inall 4πr2-direction of the space.The power density, Ssc back to the source is given by

Ssc =Pint

4πr2=

σ · Si

4πr2(4)



Assumption: Pint is re-radiated as scattered power uniformly inall 4πr2-direction of the space.The power density, Ssc back to the source is given by

Ssc =Pint

4πr2=

σ · Si

4πr2(6)

Making σ subject of (4) for r large enough, yields

σ = 4πr2Ssc

Si

= 4πr2|Esc|

2

|Ei|2(7)

Ei and Esc are incident and scattered electric fieldsrespectively.



Why RCS as Main Parameter?




Frequencies are becoming inexorably high. As a consequence,PCB are becoming smaller. Contacting is therefore beset withimmense difficulties.





Unavoidable currents on the feed line are in the near field of thePCB and represent a radiation source which interacts with andcontaminates that of the PCB.






RCS measurement dispenses the need for a feed line thusrendering the attenuation of surface waves pointless.







RCS measurement dispenses the need for separate susceptibilityand immunity measurements.







RCS measurement dispenses the need for separate susceptibilityand immunity measurements.

Diagnosing radiation problems on PCBs is a cumbersome task,since they are commonly of arbitrary layout geometries.


Benefits of Using the GTEM CellThe GTEM Cell can be used simultaneously for susceptibilityandimmunity measurements.



This possibility is an extension of the features of the GTEM cell toRCS Measurements.




The accessibility and flexibility.





Low installation cost.






Minimization of drawbacks in outdoors RCS measurements.







The large frequency bandwidth of the cell.







The large frequency bandwidth of the cell.

Its highly accuracy and the good stability and reproducibility ofmeasurements. Even light and low-density targets with lowest RCSlevel can be measured.


Benefits of Using the GTEM Cell - count...No need for specific signal processing tools to achieve RCS evaluation.


Benefits of Using the GTEM Cell - count...No need for specific signal processing tools to achieve RCS evaluation.

The RCS level of the EUT is determined by evaluating the change inthe reflection coefficient,Γ, at the cell output.

Lr =6πh2

s

5λΓ (11)

where Lr = RCS-radius = Radius a sphere of surface area equal tothe RCS

σ = 4πL2

r . (12)

or analyticallyLr =

rhs

heff · ZL

Zij (13)

Zij is the impedance of the PCB as defined in [3]. hs is the height ofthe cell, heff the effective height of the PCB and λ the wave lenght.


Measurement ProcedurePerform the calibration of the VNA using the empty cell itself asload, since PCBs are generally small EUTs with relatively smallbackscattered fields, when compared to the reflection from thehybrid terminal.



This leads to a gain in dynamic range of more than 25 dB.




First connect the open and short standards and finally the cell itselfas standard load.





Once the cell and the VNA are connected, block the trolley wheelsto avoid any movement of the connecting cable. The calculatedcorrection coefficients may be assumed to be valid for themeasurement.






For PCB with a ground plane, it is judicious to subtractmeasurements of a bare metal plate having the same dimensionsas the PCB from measurements of the EUT.






For PCB with a ground plane, it is judicious to subtractmeasurements of a bare metal plate having the same dimensionsas the PCB from measurements of the EUT.

For PCB with active elements it is appropriate to carry out bothin-vivo and in-vitro measurements.


Measurement Procedure - count...

Photograph of Modelled Antenna Geometries


Measurements Procedure - count...

Measurement Setup. DUT in a GTEM cell.


Some Results

0 250 500 750 1000 1250 1500 -0,16

-0,12

-0,08

-0,04

0,00

0,04

0,08

0,12

0,16

Real part Imaginary part

Rea

l and

Imag

iner

y pa

rt

Frequency / MHz

Measured S11 at the cell’s output. The cell was loaded with the Bentwire.


Some Results

0 250 500 750 1000 1250 1500 -0,20

-0,15

-0,10

-0,05

0,00

0,05

0,10

0,15

0,20

0,25


Rea

l and

Imag

inar

y P

art

Frequency / MHz

Measured S11 at the cell’s output. Cell loaded with the Hertzian

dipole.


Some Results

0 200 400 600 800 1000 -0,04

-0,03

-0,02

-0,01

0,00

0,01

0,02

0,03

0,04


Rea

l and

imag

inar

y P

art

Frequency / MHz

Measured S11 at the cell’s output. Cell loaded with the patch

antenna.


Some Results

0 250 500 750 1000 1250 1500 -0,10

-0,08

-0,06

-0,04

-0,02

0,00

0,02

0,04

0,06

0,08

0,10


Rea

l and

imag

inar

y pa

rt

Frequency / MHz

Measured S11 at the cell’s output. Cell loaded with the rectangular

loop.


Some Results

Structure fr [MHz] R [dB] Lr [cm]

Theory Meas. Theory Meas.

Patch antenna 815.89 802.20 -21.23 -26.29 5.23

Hertz. dipole 908.51 838.5 -12.40 -12.52 15.67

Rectangular loop 964.82 956.78 -22.01 -24.27 5.53

Bent wire 697.34 691.5 -11.77 -14.57 13.01

Table 1: Calculated and measured resonance frequencies

and RCS-radius.


Conclusion

- The usefulness of investigating radiation from PCBs withRCS being the main parameter of interest has beendemonstrated.- The necessity of using the GTEM cell as test facility tocarry out these measurements has been underlined.- The proposed technique offers a quick and precise meansof investigating EMI from PCBs.- The procedure is based on the application of relationbetween the backscattering cross section and the cell’sreflection coefficient which enables the theoreticalprediction of resonances.- Comparison between experimental and calculated resultsshow good agreement, thus testifying the validity of themethod.


ReferencesReferences

[1] D. Pouhè et al., ”Assessment of resonance properties ofelectrically small PCBs via radar cross sectionmeasurements in a GTEM Cell”, Proc. 16th Int. Symp. on

EMC, pp. 357-362, Zurich, 13-16 Feb. 2005

[2] D. Pouhè, G. Mönich, ”Radar Cross SectionMeasurements in a GTEM Cell”, Proc. AMTA Europe

Symp., pp. 130-135, Munich, 1-4 May 2006

[3] D. Pouhè, ”RF Radiation properties of printed circuitsboards in a GTEM Cell”, IEEE Trans. EMC, Vol. 48, N0. 3,pp. 468-475, Aug. 2006


Measurement of PCB’s Radar Cross Section in a GTEM Cell...Int. Zurich Symp. on EMC - Munich, 25 Sept. 2007 - D. Pouhe` – p.7/19 Beneﬁts of Using the GTEM Cell The GTEM Cell can

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