NUMERICAL METHODOLOGY FOR THE EMI RISK ASSESSMENT OF VEHICULAR ANTENNAS

Transcription

1 NUMERICAL METHODOLOGY FOR THE EMI RISK ASSESSMENT OF VEHICULAR ANTENNAS Alberto Buttiglieri EMEA Product Development Electrical Electronics Unit Audio & Telematics Darmstadt, Germany

2 Content Automotive Antenna Design EMI Risks New Design Approach Basic Principles Experiment Design Case Study Results Conclusions 2

3 Automotive Antenna Design Antenna design in automotive requires a system engineering approach to satisfy the requirements balancing that in-vehicle integration implies. Different requirements lead to different technical solutions. Examples: Roof mount antennas Hidden antennas 3

4 Automotive Antenna Design FCA has used EM Simulation and CST MWS since 2007 as a development tool for antenna designs as well as the have most of our OEM antenna suppliers. When we are not willing to accept visible antennas on the roof, we are required to hide antennas somewhere else. In this case EM simulation is a fundamental tool to support the integration of hidden antennas with the hosting body parts. 4

5 EMI Risks Electromagnetic Interference (EMI) can be described as any unwanted radio frequency signal which interferes with the desired signal. These issues are affecting the whole consumer band in our daily life. The frequencies we use plus the interferences we create The increasing complexity of the Electrical/Electronics (EE) system on modern vehicles leads to a higher EMI risk, both in terms of vehicle validation and perceived product quality. 5

6 EMI Risks EMI is the most serious problem normally encountered in the vehicle validation phase for antenna reception. Design for EMC techniques applied to ECUs and harnesses design are not sufficient to guarantee antenna reception robustness against EMI. The radio antenna system and receiver have been designed for reception of very weak signals and unfortunately the receiver cannot discriminate between these weak signals and random noise if both are on the same frequency. A typical example is the audio noise induced in FM radio by other electronic equipment emitting spurious radiation on the FM band. 6

7 New Design Approach 3D EM simulation can help during the process of antenna placement in order to perform an EMI what-if risk assessment based on the available antenna options and interfering devices location. FM_Ant EMI_1 EMI_n EMI_2 The methodology presented aims to provide a good numerical estimate of the complete antenna capability to receive the wanted signal as well as decouple the unwanted noise, in order to meet the required design goals and take all the suitable countermeasures that mitigate EMI risks. 7

8 Basic Principles The EMI simulation methodology is based on the antenna reciprocity principle: The EM properties of an antenna are the same both in transmission and reception operating modes. A consequence of antenna reciprocity is the symmetry of the scattering matrix. The S-parameters that express the power coupling in a system with two or more radiators can be measured or computed by means of a single multi-port step exciting directly the victim antenna FM_Ant EMI_1 EMI_n EMI_2 8

9 Basic Principles In a complex system like a car the sources of interference can be modelled according to their coupling mechanism as: Electric-field interferes Magnetic-field interferers It is not easy to classify real equipment in E or H field interferers categories without testing/simulation, therefore both cases must be considered in order to determine worst-cases. A simplified approach consists of E-field (small monopole) and H-field (small loop) probes of compact shape compared to the FM wavelength. E-field interferer H-field interferer 9

10 Experiment Design Two S-matrices are determined for each victim antenna: E-coupling S-matrix and H-coupling S- matrix E-coupling S-matrix example Coupling parameters S FMm_EMIn and S EMIn _ FMm are equivalent, according to the Reciprocity theorem, and can be determined by directly evaluating the power delivered to a 50 Ω load connected to interfering antennas when FM antennas (victim antennas) are fed by a nominal power. 10

11 Experiment Design The proposed flowchart provides all the required data for EMI risk matrix construction by means of just three runs of the numerical solver. Run #1 Run #2 Run #3 11

12 Case Study A case study was conducted on a Fiat 500C convertible car where two in-glass antennas, Windscreen antenna vs Side window one, were compared in terms of susceptibility to EMI in FM band. Side window antenna Windscreen antenna 12

13 Case Study For the specific Fiat 500C case these ports were considered: m=2: number of FM configurations (Windscreen and side window antennas) n=7: number of E-field and H-field interference radiators Two interferers were considered to finalize the analysis and select the optimal FM antenna location: the Portable Navigation Device (PND) and the Engine Control Module (ECM). 13

14 Results For each victim antenna a list of EMI coupling factors, at the frequency of interest (f=100 MHz), is obtained versus all considered E and H interferers. Estimated advantage for side window ant. EMI coupling worst case has been proven to be of H-field type. 14

15 Results The improvement obtainable passing from windscreen to side window antenna position is consistent with the values measured in EMC chamber. Simulation Measurement H-field [db] [db] PND 8,9 11,0 ECM 4,9 7,5 EMI level difference: Windscreen vs Side window The complexity of the model and of the environment necessarily affects the simulation accuracy. In spite of that, the dataset comparison has shown a good correlation between simulation results and experimental data. 15

16 Conclusions CST MICROWAVE STUDIO has proven to be a robust instrument to study a resonant structure in FM band and above. The procedure was presented with reference to FM antennas and it can be applied to other frequency ranges/operating bands with minimal changes. This methodology can be therefore considered as a valuable tool for performing an EMI risk assessment based on the available antenna options and interfering devices location in a vehicle environment. 16

17 and thank you for your attention