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Summary
Electromagnetic interference (EMI) has been one of the major concerns during the integration of the last generation of the Silicon Strip Tracker. Grounding and shielding problems and electromagnetic compatibility (EMC) issues arisen during the integration of the tracker sub-system have required time and important series of tests and studies to solve them. Efforts to find both the root cause and the solution of these problems have allowed learning important lessons that have to be included in the design of the integrated circuits, hybrids and system integration of the up-dated silicon tracker systems.
In general, most of the EMC problems associated with the CMS Tracker sub-systems were associated with interference generated by the power supplies and auxiliary equipment and coupled to the silicon detector through the distribution boards within the tracker area. Interference and noise currents flowing through both the power and auxiliary equipment cables penetrates into the tracker detector and propagates in that area following the distribution cables and boards. Within the detector area, these interference currents couple noise to the sensitive spots of the FEE, reducing its signal-to-noise ratio. This coupling mechanism is mainly via conduction and near field.
A new power distribution scheme based on DC-DC switching power converters is under study to overcome the problem of ultra-low voltage required by the new electronics and the conductor mass that is possible to allocate in the central part of the detector. It defines a noticeable and important noise source very close to the detector’s FEE. Although a big effort is put to reduce the primary noise emission in the distributed DC-DC switching converters based powering scheme proposed for the CMS tracker upgrade, the emitted noise levels achieved have to be directly compatible with the noise immunity levels of the new silicon detectors and associated FEE. This immunity is basically defined by the intrinsic FEE topology, the silicon strip detector-FEE connection and the integration of the unit at system level. The experience gained with the previous tracker FEE has to be seriously considered to improve future designs. For that purpose, the evaluation of the electromagnetic immunity of previous generation of silicon tracker detector- FEEs is critical to optimize and improve the performance of the future generation of detectors.
This paper presents a validated model of the noise sensitivity of the Silicon Strip Detector-FEE of the CMS tracker to quantify the impact of the noise coupling mechanisms in the immunity against electromagnetic interference. Furthermore, based on that model, it defines both strategies to improve the EMI immunity and limitations to be applied in future designs. The coupling phenomena of electromagnetic interferences with the Silicon Strip Detector-FEE has been both evaluated via simulation using finite element models and measured on prototypes. Those measurements are based on several immunity tests conducted in final prototypes of the Silicon Tracker End-Caps (TEC) and Tracker Outer Barrel (TOB) systems, to estimate the sensitivity of each system to conducted noise.
The understanding of the coupling mechanism of electromagnetic interferences to the Silicon Strip Detector-FEE and the model of the noise sensitivity have been used to identify critical elements and inappropriate layouts in the prototypes that were responsible for the degradation of the signal-to-noise of the FEE during the commissioning. Today this understanding is critical to define design recommendations and specify the electronics and system topology to increase the FEE robustness to EMI in anticipation of the challenging power distribution schemes proposed for the SLHC tracker upgrades.
