REDUCING EMC VERIFICATION AT SATELLITE LEVEL

Patrice Pelissou (1) , Laurent Trougnou (2), Jean-Claude Pourtau (3) , Michel Terral (3)

Alain Reinex (4) , François Torres (4), André Schaffar (5), Yannick Herlem (5) (1) EADS Astrium SAS, 31 Avenue Des Cosmonautes, 31402 Toulouse (France)

E-mail: patrice.pelissou@astrium.eads.net

(2) ESTEC, Keplerlaan 1 , Postbus 299, 2200 AG Noordwijk (The Netherlands)

(3) THALES Alenia Space, 26, avenue J.F. Champollion31037 Toulouse Cedex 1 – France

(4) XLIM, 123, avenue Albert Thomas - 87060 LIMOGES CEDEX

(5) EADS Astrium Space Transportation , 66, Route de Verneuil 78133 Les Mureaux Cede Summary. For the present Telecom satellites and for the future high power satellites, the EMC tests are more and more difficult to implement at satellite level due to technical and schedule constraints. Moreover the EMC qualification test at system level is questionable. So a new EMC process has been elaborated taking all qualification efforts upstream of the tests at satellite level combining more simulations and EMC tests at unit and subsystem level. This paper presents the main results of the ESA EMC study “Reducing EMC verification for future Telecom platforms”. This study was conducted in 2005/2007 under ESA contract 18722/05/NL/AG. I. SCOPE

The EMC tests in clean room at system level for current & future high power platforms represent high AIT preparation effort (thermal, mechanical, accessibility, safety,…)and so has an impact on the satellite schedule. As the AIT schedule is more and more short, actions are taken to improve and reduce the EMC testing at satellite level. A new EMC process need to be elaborated taking all qualification efforts upstream of the tests at satellite level combining more simulations and EMC tests at unit and subsystem level. The figure here below shows a typical Telecom Control Plan from the specification to the validation tests at system level. The colored parts identify the improved tasks during this study.

II. CRITICAL REVIEW OF THE EMC SPECIFICATION

This task is based on the review of the documentation and experience of the main European telecom satellites manufacturers : Alcatel Space with Spacebus 4000 platform and EADS Astrium with Eurostar 3000 platform. The requirements are compared to the original MIL-STD-461 requirements from which they have been derived. They are also compared to the requirement of the ECSS-E-20-07 Draft 03. The comparison is synthesized in tables with in particular the recall of the requirement (limits, frequency range, test method ….), the origin of the requirement / involved physical phenomena, the E3000 and SB4000 experience (occurrence of NC, retrofit), the requirement value assessment and the possibilities of improvement. In general, the EMC requirements applicable to E3000 and SB4000 satellites units have a good level of maturity since they have been evolving fro many

EMC qualification tests (EMC tests in conducted and radiated mode)

EMC/ESD Specification

EMC Control Plan

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Review of EMC design (bonding, grounding…) Review of EMC analysis Review of EMC test plan, procedure

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Conducted mode analysis (power and signal) Radiated mode analysis Magnetic analysis ESD analysis Harness constraints analysis

EMC analysis of unit RFD/RFW and system impact assessment EMC solution implementation at unit/system level Design rules implementation checking

EMC tests at sub-system (payload) and system level in order to check the good behaviour of the S/L, correlate the simulations and check the workmanship.

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years. Most requirements coming from the original MIL-STD-461C have been adapted in their frequency range, applicable limits and levels to actual needs. III. CONDUCTED MODE MODELING

Concerning the conducted EMC analysis at system level, the ESA study outputs allow to simulate a complete EPS bus model in time domain. The different models have been elaborated The advantages of this modeling method are : - To predict ripples voltage at star point and users level in time domain - All useful informations are available (amplitude, phase, frequential content) - to simplify the EMC measurements (oscilloscope, current probe) - to use of a commercial tool (PSPICE). III.1 Unit CE modeling With the objective to predict with good accuracy the ripple level on power lines at system level due to all units contribution, the first step is to develop a unit model from measurement in time domain [1]. The advantage of this measurement is to have all useful informations (amplitude, phase, frequential content) and to simplify the EMC measurements (oscilloscope, current probe). The DC-DC converters are designed to operate on a low source impedance of the power supply. When the source impedance increases, the converter stability margin decreases. Therefore, the Voc parameter is not accessible to a direct measurement that would require very high source impedance. As a consequence, the Norton model appears more appropriate because the Icc parameter can be directly measured when the converter is supplied with very low power source impedance. Nevertheless, to determine the impedance parameter of the noise source (Z), it is necessary to perform a second CE measurement with increased power supply impedance. This impedance shall remain compatible of the DC-DC converter stability.

As only time domain measurements are available, a solution was found to use a Fourier transform of the time-domain currents [1]. But the time window of the acquisition could lead to unwanted artifacts in the spectral domain since the signals are not damped. Hopefully, the generated noise is mainly originating from the equipment’s DC/DC converter. Thus it exhibits periodic features, the periodicity depending on the power supply’s switching frequency. This is verified on measurements and the follwoing figure shows a typical time-domain result for the current. In order to validate the software results, we have compared the computed input differential impedance “Zdm” to the measurement made with a network analyser. The comparison is summarized in the following figure. The comparison shows a good correlation between measurement and simulation.

III.2 Fuse blowing modeling

The fuse model developed by EADS ASTRIUM is based on the physical behaviour of the device: when a current is applied at fuse inputs then the fuse temperature increases within the time up to the open circuit when the metal temperature reaches the gold melting temperature (around 1000°C). The fuse operating is based on two principles : electrical and thermal. The power dissipated during the circulation of a high level current in a metallic material (resistivity effect) gives a temperature elevation which creates 2 phases : - Phase 1 (before fuse blowing) : The material temperature keeps under its fusion temperature. The material resistance increases with the temperature (so with the dissipated energy RI²t) creating the fuse resistor variation : the fuse is similar to a variable resistor.

User

Battery

Solar array

PSR User

User

User

Fuse

Primary power bus

� −<×≈ inmIZV PSR argCS userusers ripplepointstar

Impedances : Zdm Modulus

- Phase 2 (fuse blowing) : the material temperature reaches the fusion temperature (about 1320°K). The fuse blows up and an electrical arc is maintained during some µs by the circuit inductances current. Once the arc switched off, the electrical contact is broken : the fuse is assimilated to an open circuit. The fuse model based on its physical behaviour gives a good correlation between the simulations and the tests. IV. RADIATED MODE MODELING

IV.1 Oversized cavity theory limits The general principle of the experimental checking is to carry out a series of measurements of coupling between two antennae placed in a cavity mock-up, in order to show that the received power in any place inside of the cavity is statistically independent of the location of the antennae inside the cavity (homogeneity), the polarization of the antennae and their orientation (isotropy), the distance between antennae and of their position inside the cavity. Each measurement corresponds to the link budget between a transmitting antenna and a receiving antenna in a given frequency band. The measurements are performed in the frequency range [250 MHz – 40 GHz] in the following mock-up. In parallel to the measurements, the mock-up model is meshed using the ASERIS-FD software in a 3D grid made up of cubic elementary cells of 4,5 mm side length, which leads to a computational volume of 100 million elementary cells being given the dimensions of the model. This allows us to carry out reliable calculations until frequencies of approximately 7 GHz taking into account the condition of meshing in �/10.

The Lehman statistical theory of the electromagnetic field in the complex cavities provides the expressions of the probability density functions of the 6 frequential components of the field, of the squared partial fields |Ex|², |Ey|² and |Ez|² and of the squared total field |E|². Taking into account that the 6 components of the electric field are ruled by the same normal law (Gaussian law), centred into 0 and of standard deviation �, Lehman deduced by mathematical ways that the squared partial fields are ruled by an exponential statistical law (law of the Chi-squared with 2 degrees of freedom) and the total field |E|² a law of the Chi-squared with 6 degrees of freedom, with � for single parameter. ���� The measurements and simulations performed in the mock-up have shown that the results fit the Lehman theory for the frequency range [1-40GHz]. . In summary : the measurement data in the frequency range [1 - 40GHz] show good statistical correlation with the theory. The electromagnetic environment in the cavity can be considered statistically uniformly homogenous (space), isotropic (direction) and randomly polarized (polarization). Comparison of theoretical and measured insertion loss factor The insertion loss factor XC of the cavity is characterized with a quite good accuracy regarding the theoretical value (see the following figure). IV.1 Trade-off of RE&RS methods As for the cavity insertion loss factor, the Shielding Effectiveness of the RF units at the operational frequencies is one of the key parameters for the payload self-compatibility analyses. The RF unit shielding effectiveness is deduced from RE & RS measurements as shown in the follwoing figure.

I fuse V fuse

V bus

Time

0s 40us 80us 120us 160us 200us1 I(R7) 2 V(R7:2,R1:1) 3 V(L8:2)

-800A

-400A

0A

400A

800A1

>>-200V

-100V

0V

100V

200V2

40V

50V

60V

70V

3

I fuse V fuse

V bus

A trade-off has been performed on 3 different RE & RS tests methods. The main results are presented here below. a) In anechoic chamber according to the MIL-STD-462 (methods RE02 and RS03): the measurements are performed at one meter distance from the 4 principal faces of the EUT. Complementary RE and RS measurements are carried out with the test antenna at 1m in the direction and with the same polarization of the maximum RF leakage. b) Using specific “sniff” RE and “spray” RS test procedure : the reduced RE “sniff” and RS “spray” tests are performed with a coaxial-to-waveguide transition (TGC) fitted with a dielectric spacer duly calibrated in order to ensure a constant distance between the probe and the equipment under test. c) Using the reverberation chamber This method consists in diving the equipment under test into an electromagnetic field, statistically homogeneous and statistically isotropic. In such a case, the coupling between the internal and external space of the EUT does not depend either on its orientation or on the polarization of the transmitting antenna. A mechanical tuning/stirring device which unceasingly modifies the inside geometry of the chamber enables to approach the conditions of

uniformity and of isotropy of the field during a period which corresponds to one of its rotation. The following figure gives the comparison of the Shielding Effectiveness values of one unit using the three methods. The measurements performed in anechoic chamber on 4 principal faces of the EUT are not satisfactory because for microwave frequencies the electromagnetic field is directed in beam, which can be compared with optical rays. Therefore the field which is transmitted through the shielding material is focussed in small rays. Large measurement errors can result when the points of largest leakage (= lowest SE) are not searched for. We observe up to 27dB error on SE measurements during our tests. The reduced RE "sniff" & RS "spray" test method practically always gives the worst-case shielding effectiveness measurement results of few dB with regard to the other methods. The reduced RE "sniff" & RS "spray" tests is simplest to implement, can be carried out apart from a shielded enclosure (directly in clean room for example) and allows to quickly measure the shielding at the operational frequency range of all flight models of the same type of equipment. The use of reverberation chamber for determining the shielding effectiveness of RF units has the advantage over other test methods in that the reverberation chamber exposes the EUT to a more realistic electromagnetic test environment met into the cavity of a satellite. Moreover the reverberation chamber test method gives the best repeatability of SE measurement results and allows to carry out easily and quickly SE measurements by automatic sweeping through a large frequency band at the contrary of the other testing methods where the measurements are carried out for discrete frequencies.

RF Unit Generator

Radiated E-field

P out P in

Radiated Isotropic Power

P isotropic = (E²/120π) (4πd²)

SE = P isotropic / Pout

Rx antenna

d

Load

V. CONCLUSIONS

The following ESA outputs have demonstrated significant advantages : - the development of fuse PSPICE models in order to determine the power bus behaviour in transient mode and thus to avoid further fuse blowing/CE/CS tests on recurrent programs; - the development of analog and digital I/F PSPICE model in order to treat waivers; - the validation of the oversized cavity model from 1GHz to 40GHz for typical Telecom spacecraft; - the advantages of using reverberating chamber for shielding effectiveness of RF unit. REFERENCES

[1] F. Torres, A. Reineix, P.Bisognin, P.Pelissou « Analyse expérimentale et modélisation théorique des perturbations conduits sur les bus d’alimentation des satellites », CEM06 St Malo