ASPECTS OF MODELLING A GTEM CELL USING CST

Transkript

1 ASPECTS OF MODELLING A GTEM CELL USING CST Holger Thye, Gunnar Armbrecht and Michael Koch 3rd European CST User Group Meeting Tegernsee, September 2007

2 Content 1. Introduction and motivation 2. Modelling of a GTEM cell with CST 3. Simulation and measurement results 4. Conclusion

3 Content 1. Introduction and motivation 2. Modelling of GTEM cells with CST 3. Simulation and measurement results 4. Conclusion

4 Motivation of the investigation - frequency domain GTEM = (Gigahertz transversal ElectroMagnetic) Instrument to measure emission or to test the susceptibility of electronic elements U Peak Signal: discrete frequency e.g. f = 900 MHz Testing volume: well-known electromagnetic field resonant effects at discrete frequencies t / s y Side View of a GTEM Cell testing volume enclosure (outer conductor) end of the septum septum (inner conductor) matching resistance EM- Absorber DUT feeding port h EUT 0.5 L DUT 0.5 L DUT z L 0.6 L

5 Motivation of the investigation - time domain UWB= Ultra Wideband (high frequency range) Most important factor: pulse shape remains unchanged in the testing volume U Peak Signal: UWB-pulse (100 MHz-11 GHz) E Peak? Testing volume: Pulse distortion? Ringing effects? t / s t / s y Side View of a GTEM Cell testing volume enclosure (outer conductor) end of the septum septum (inner conductor) DUT h matching resistance EM- Absorber feeding port h EUT 0.5 L DUT 0.5 L DUT z L 0.6 L

6 Object of Investigation: GTEM 3750 Location: Bern, Switzerland Very large geometries (Length: 15,51 m, Height: 5,47 m) Height of the septum in the testing volume: 3,22 m Cell width in the testing volume: 6,44 m Outer view Inner view

7 Content 1. Introduction and motivation 2. Modelling of GTEM cells with CST 3. Simulation and measurement results 4. Conclusion

8 Important features Modelling aspects concentrate on Mesh density / upper frequency limit Feeding section Septum Termination section devided in a) 50 Ω resistor network b) absorber region

9 Mesh density / upper frequency limit GTEM cell very large For CST-simulation: Min. mesh density with max. mesh edge length λ/10 λ/10 determines upper frequency limit: 1,3 GHz Number of hexahedral mesh cells 57 Mio. (extended CST-licence necessary)

10 Modelling of the septum / feeding section Modelling of septum with finite thickness (1mm), no pure surface Rotation of the cell necessary to prevent creation of critical cells Septum lies on z-axis Rotation of the portplane Creation of port extensions (3 mesh cells in a row) Few falsification unavoidable Rotation of field probes (results!)

11 Modelling of the feeding section Use of CST-port as excitation Size of port very small in relation to the cell (67 x 103 mm) waveguide port E-field distribution Usage of two input signals

12 Modelling of the feeding section/ excitation pulses Gaussian Pulse (GP) Defined CST-pulse (t r 380 ps, t fwhm 613 ps) Double Exponential Pulse (DEP) Measured pulse (t r 400 ps, t fwhm 1.7 ns) CST Import: User Defined Excitation Signal (Visual Basic Interface) normalized amplitude 1,0 0,8 0,6 0,4 0,2 0,0-0,2 t r t fwhm time [ns] Double exponential pulse Analytical double exponential pulse Gaussian pulse t r t fwhm time [ns] pulse generator attenuators oscilloscope DEP can be used for simulation? Spectrum analysis Power density 50 Ω] Double exponential pulse Gaussian pulse upper frequency limit 1,3 frequency [GHz]

13 Modelling of the 50 Ω- termination Aim: 50 Ω impedance at TEM - propagation Definition of the CST volume resistance via S-parameter optimisation Consideration of the lower frequencies (only TEM-propagation): Lowest S-parameter refers to best 50 Ω - adaptation Graphs with different volume resistor values

14 Modelling of the absorber Problem: Local dependency can t be directly modelled Solution: Successive linear modelling of the complex permittivity with 10 planar layers parallel to the back wall Usage of the Debye - equation [1]: Linearisation of the parameters with layer n [1] John PAUL: Modeling of General Electromagnetic Material Properties in TLM, 1998 ( ) 1 ) ( 10 1 ), ( ' ' + = ω ω r r n n ) ( 10 ), ( ' ' ' ' ω ω r r n n = 10 planar layers = = ² ² 1 ) ( ² ² 1 ) ( 0 '' ' e e s e e s r r r eff j j τ ω ωτ ω σ τ ω ω

15 Content 1. Introduction and motivation 2. Modelling of GTEM cells with CST 3. Simulation and measurement results 4. Conclusion

16 Simulation results in the testing volume Use of the Gaussian pulse Examination of H x (because of the highest field amplitude) Results show effect of each CST-model 6 4 Empty cell 50 Ω termination without absorber 50 Ω termination with absorber H x [ma/m] time [ns]

17 Simulation results: Field Energy Δt=50 ns: No energy loss in empty cell 10 0 Δt=100 ns: Energy in empty cell decreases rapidly, TEM propagation No change in cell with 50 Ω termination 50 Ω resistor network causes E-modes port f C E-Mode ~field energy [db] ca. 50 ns Empty cell 50 Ω termination without absorber 50 Ω termination with absorber time [ns] ca. 100 ns ca.150 ns

18 Measurement setup Usage of active field sensors Measurement of all electric and magnetic field components pulse generator UWB pulse generator U peak = 0,5-2 kv t r = 400 ps t fwhm = 1.7 ns field sensor Optical waveguide oscilloscope Optical receiver 3 parallel ports Oscilloscope Sampling rate 20 GHz/s optical receiver Active field sensor E and H-field optical 3 waveguide components

19 Comparison of measurement and simulation results Exporting the results for a measurement point in the testing volume Pulses are normalized and time-shifted perspective view point 1 Primary pulse shape quite good correlation normalized amplitude Simulation amplitude of the -0,2 reflection is higher and earlier due to absorber -0,4 modelling time [ns] 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Simulated H x at point 1 Measured H x at point 1

20 Content 1. Introduction and motivation 2. Modelling of GTEM cells with CST 3. Simulation and measurement results 4. Conclusion

21 Conclusion It is possible to create simulation model of a large GTEM cell with CST MWS with the following distinctions: The limited number of hexahedral mesh cells cause an upper frequency limit of the used excitation signals. The model must be rotated, so that the septum lies on a axis to avoid critical cells. Therefore port extension are necessary. The definition of the volume resistance can be made via S-parameter optimisation. The absorber region can be modeled with planar absorber layers with a rising complex permittivity. First measurements are comparable with the simulation results. CST-model is well suited for further investigations of transient field propagation in GTEM cells with spectral range up to 1.3 GHz.