Localization and Identifying EMC interference Sources of a Microwave

Transmission Module

Ph. Descamps1, G. Ngamani-Njomkoue2, D. Pasquet1, C. Tolant2, D. Lesénéchal1 and P. Eudeline2

June 2012

1LaMIPS, Laboratoire commun CRISMAT, UMR 6508 CNRS, 6 bd Maréchal Juin, Caen, France.

2Thales Air Systems SA, Technical Unit Radio-Frequency (TU-RF), Technology

and Innovation (REIRI-Y), Z.I. du Mont Jarret, 76520 Ymare, France

OUTLINE

Motivation

Description of the Demonstrator

Expression of the electromagnetic fields in a cavity

3D Electromagnetic Simulations of the demonstrator

Electric field distribution propagation of the energy in the cavity

S parameters measurements

Electrical field measurements with GTEM cell

Conclusion

BACKGROUND

French national projet : AUDACE (Analyse des

caUses de DéfaillAnces des Composants des systèmes

Electroniques embarqués)

Part of AUDACE project: ”Measuring stress and their effects”

Aim of this work : Reliability of components and circuits against

electromagnetic disturbances :

Identify problems of EMC ⇒ identify sources of disturbance.

Synoptic of the radar

digital

processing

Microwave

stage (LNA ..)

Receiver

Antenna

Transmitter

Receiver

Transmitter Management

of radar

Signal

generator

duplexer

Consideration of EMC in the realization of transceiver modules.

Motivation

Measuring of the electromagnetic field radiated

Identifying critical high frequencies : parasitic frequency noise and resonance frequencies

Identifying areas of strong field and components generating high emissions.

Studying the effect of the metal shielding on the electromagnetic field.

Presence of different signals in the same

transmission module Transmission/receivers (T/R) are usually shielded by metal

cavities to be immune from external disturbances and to

avoid disturbing close circuits.

Very high radiated power emission due to high output

power of transmitters (radars) (hundred of kilowatts).

Transmitters modules are built in a confined area

containing several radiating elements (ex : inductances,

transmissions lines active circuits..)

Complete transmission module too complex to study in terms of EMC

Module simplified reproducing the main effects of EM

Description of the Demonstrator

5

Microwave Feeder coupler

inductance

Using a

circuit bulk

Patch

antenna

Circuit representation of the simplified demonstrator

A demonstrator in case of reproducing the

maximum electromagnetic effects found on

the circuit : measurement of different

signals as microwave signals, power,

digital, low frequency signals.

Layout of the PCB

The microstrip line is a 150 mm-long

Zc = 50 Ω

Coupler = 20 dB at 3 GHz

The two antennas have been calculated for a 5 GHz resonance frequency

Feed line w = 3mm, l = 7.07mm

1 square patch antenna and 1 patch antenna with notches

Length of the package: 150mm, Width : 55mm, Height : 75mm

Substrate : FR4, h = 1,6mm and εr = 4,6

The complete demonstrator has been simulated and measured as a

6-port with and without the shielding box.

Cross section

Photography of the cavity

222

r

p,n,mc

p

b

n

a

m

2

Cf

Electromagnetic fields in a cavity

The calculation of cutoff frequencies of the modes allows us to

identify each mode and we can then select the TE mode

(Transverse Electric) or TM mode (Transverse Magnetic)

using Mapping of electromagnetic fields.

3D Electromagnetic Simulations of the demonstrator

Two degenerate modes observed correspond to the modes TE11 and TM11. (confirmed by

dispersion diagram).

Cartography of the electric field

for a half-filled cavity with air.

Dimensions : 55 x 37,5 x 150

mm

Propagation of electromagnetic fields

Propagation of electromagnetic fields of the fundamental mode for F = 3GHz

E field – Mode TE 10 H field – Mode TE 10

Distribution of electromagnetic fields for F = 1GHz

E field – Mode TE10 H field – Mode TE 10

At 1 GHz, the electric field does not propagate in the guide: it is rapidly attenuated.

Over the cutoff frequency of the fundamental mode F = 3GHz, the electric field

propagates along the cavity.

Propagation of the electric field of higher modes at F=3GHz

There is a stationary phenomenon below

the cutoff frequency and propagation

above the cutoff frequency .

The higher modes are propagated in the

cavity when the frequency is higher than

cutoff frequency

Propagation of the electric field of higher

modes for F = 3GHz

10

Identification of index modes

Mode TE101 TE102 TE103 TE011 TE012 TM110 TE104 TM111 TE111

Calculated 2.90 3.38 4.05 4.12 4.47 4.84 4.84 4.94 4.94

Calculated with correction 2.85 3.32 3.98 4.05 4.39 4.76 4.76 4.86 4.86

Simulated (3D simulator) 2.85 3.32 3.97 4.12 4.46 4.74 4.83 4.88 4.93

Cutoff frequencies for the half empty box

Cross section

a = 5.5 cm ; b = 3.75 cm ; c = 15 cm

(Correction : )

Frequencies of the resonance modes in GHz (1st line calculated for the empty box, 2nd line calculated

with the correction of εrequ , 3rd line simulated with substrate and without printed metal, 4th line measured

on S41)

The resonance frequencies have been calculated, simulated and measured

22

r

n,mb

n

a

m

2

Cf

Electric field distribution propagation of the

energy in the cavity

Electric field distribution

Propagation of the energy in the cavity

(F = 3GHz)

Substrate FR4 : permittivity 4,5 ; height 1,6mm

The energy is spread in the substrate and symmetrically with respect to the axis line

Propagation of the electric field along the line

at F = 3GHz. (Vector representation)

Measurements of the coupler : S31 and S41

Coupling : C = 20 log(S31) = 20dB at 3GHz

Isolating : I = 20 log (S41) = 30dB at 3GHz

Directivity : D = 10dB

All the S-parameters between SMA ports were extracted with open and closed box (with

shielding)

Mode TE101 TE102 TE103 TE011 TE012 TM110 TE104 TM111 TE111

Calculated 2.90 3.38 4.05 4.12 4.47 4.84 4.84 4.94 4.94

Calculated with correction 2.85 3.32 3.98 4.05 4.39 4.76 4.76 4.86 4.86

Simulated (3D simulator) 2.85 3.32 3.97 4.12 4.46 4.74 4.83 4.88 4.93

Measured (S parameters) 2.85 3.32 3.85 3.95 4.33 4.67 4.72 4.8 4.83

Parasitic modes appear as perturbations for the

measured S31 and S41 between the line and the

coupled access of the coupler. Resonance

frequencies have been identified in the table

below :

Measurement of the square patch antenna : S51

Return loss S55 of the

square patch antenna

The same parasitic modes

appear with S51 as

perturbations between the

line and the square patch

antenna.

Measurement of the square patch antenna : S61

Measured resonance frequencies are very close to calculated and simulated resonance

modes of the cavity.

Resonance frequencies appear only when the cavity is closed.

Return loss S66 of the square

patch antenna

Comparison of S parameters of the circuit with

opened and closed shielded box .

All perturbations find by the resonance frequencies when the cavity is closed

are identified as parasitic modes.

16

Description of GTEM cell for electric field measurements

The GTEM cell is a frequency extended variant of the traditional TEM (Transverse Electro-Magnetic) cell. The GTEM

cell is, in principle, a tapered coaxial line (offset septum plate), from a coaxial feeding point, having an air dielectric

and a characteristic impedance of Zc = 50 Ω.

This coaxial line is terminated by a combination of discrete resistors and RF absorbers to achieve a broadband match.

The outer conductor of this “coax line” is created by the metal walls of the cell which provide screening for both

internal and external electromagnetic fields.

Typical Test Set-up for RF Emission

Specifications

Septum height: 500 mm

Dimension (LxWxH in m): 2.95 x 1.48 x 1.61

Door (LxH in m): 0.44 x 0.38

EUT max. size (LxWxH in m): 0.41 x 0.41 x 0.31

EUT size (3 dB criteria, LxWxH in m): 0.30 x 0.30 x 0.15

Max input power: 100 W

RF-input connector: N-type

Nominal impedance: 50 Ω

Frequency range: DC up to 20 GHz

Test Cells for EMC Radiated &

Immunity Testing DC to 20GHz

Volume for testing

1,0

4

m

Electric field measured in GTEM cell

Electric field component Ez is higher than the two other Electric field components

(Ex and Ey).

The closed demonstrator has been put into a GTEM cell.

The input port 1 has been fed with a 20dBm RF signal. The amplitude of the electric

field has been measured by the septum in the three XYZ directions as defined in Figure

below :

222EzEyExE

Comparison between the S41 of the coupler and

the measurement of the electric field.

The same parasitic modes appear as perturbations between the line and the

coupled access of the coupler with the measurement of the electrical field.

Comparison between the S51 and S61 between the line and the two

square patch antennas and the measurement of the electric field.

PIERS 2011, Marrakech

Measured resonance frequencies are very close to the calculated and simulated

resonance modes.

Other parasitic modes appear as perturbations between the line and the two

square patch antennas with the measurement of the electrical field.

Mode TE101 TE102 TE103 TE011 TE012 TM110 TE104 TM111 TE111

Calculated 2.90 3.38 4.05 4.12 4.47 4.84 4.84 4.94 4.94

Calculated with correction 2.85 3.32 3.98 4.05 4.39 4.76 4.76 4.86 4.86

Simulated (3D simulator) 2.85 3.32 3.97 4.12 4.46 4.74 4.83 4.88 4.93

Measured (S parameters) 2.85 3.32 3.85 3.95 4.33 4.67 4.72 4.8 4.83

Comparison between the S41 of the coupler and the

measurement of the electric field.

The same parasitic modes appear as perturbations between the line and the

coupled access of the coupler with the measurement of the electrical field.

Comparison between S parameters of the coupler, two

patch antennas and the measurement of the electric field

By measuring the electric field in the GTEM cell, it is possible to identify

parasitic frequencies (modes) sources of perturbation

Correlation between electric field orientation

(xoz plane) and S41, S51 and S61.

The orientation of the electric field in xOz plane is more significant (0° corresponds to

Oz axis) than in the other planes. Peaks appear for the identified resonance frequencies.

Parasitic Electric field : Contribution of cables

55dB

The contribution of the radiated electrical field by cables connected to

the module (empty cavity + circuit) is negligible.

Conclusion & Perspective

Electromagnetic behavior of a demonstrator including several elements in

case of reproducing the maximum electromagnetic effects found on the

circuit has been studied.

Parasitic resonances can be detected from measurements of the electric field

outside the closed box identify sources of disturbances.

As the field patterns of all the modes are known, it is thus possible to know

what the field repartition inside the box is and to know where it is adequate

not to put components that are liable to radiate.

Better understanding the interaction between the cavity and high

frequency circuits.

Appropriate probes to scan the surface of the circuits can be built to measure

electromagnetic near fields and to validate the method.

THANK FOR YOUR ATTENTION