SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Figure 3-28 – SPICE model of the detector driving electronics used for EMC analysis. 54
3.7.3 Microphonics The impedances of the NTD bolometer elements are on the order of 5 MΩ. This represents a compromise between high responsivity (which requires high impedance) and immunity to EMI and microphonic disturbance, which require a lower impedance). The JFETs located outside the FPU convert the impedance of the detection circuit to approximately 7 kΩ. So-called microphonic effects are due to the physical motion of the detector wiring that result in either capacitative or inductive injection of voltages into the signal lines. The magnitude of microphonic induced noise increases with the magnitude of vibration of the harness and the support structure. With the low signal levels to be detected, very small vibrations of the wires cause a serious problem. Microphonic effects can be strongly suppressed by use of differential wiring. However, charge can build up due to the physical solicitation of asperities on the conductor and insulator interface. As the harness connecting the bolometer elements to the JFET units represents the most vulnerable part of the detection system, it will be strapped down to the structure to give a minimum resonant frequency of any part of the harness of 1 kHz. In practice this means running the harnesses via the cold detector box supports, onto the optical bench and along the FPU covers. The harnesses will be strapped to the structure at intervals of approximately 1 cm. This routing is not optimum for the electrical requirements as the capacitance of the harness will be near to 100 pF, meaning that the bias frequency may have to be lower than initially desired to prevent roll off. However, the need to prevent induced noise from microphonics is felt to be an overriding concern as the bias frequency can be varied sufficiently in the electronics to optimise the performance of the detection system. 3.8 System-level criticality A top-level analysis has been conducted into the effects of a failure or partial failure of one of the SPIRE subsystems (Assessment of System Level Failure Effects for SPIRE, Swinyard). In this analysis the following failures are shown to be mission critical – i.e. a failure of one of these sub-systems will cause major loss of scientific capability for the SPIRE instrument: (i) total loss of the cooler; (ii) Structural failure in the 300-mK system leading to thermal short; (iii) total loss of the photometer long wavelength array; (iv) total loss of either spectrometer array; (v) total loss of the FTS mirror mechanism. All other sub-system failures will lead to a greater or lesser degree of loss of performance and difficulty of operation, but they do not lead to a total failure of either the photometer or spectrometer scientific goals. The redundancy and reliability of these sub-systems will be addressed as a first priority. For most sub-systems cold redundancy can be provided to ensure a high probability of avoiding total failure in any part of the sub-system implementation (see section 3.8.1). However in some cases this is not possible, for instance there will not be multiple detector arrays and only a single cooler will be fitted. In the case of the detectors reliability is achieved by having many pixels arranged in blocks for the purposes of power supplies and multiplexing into ADCs etc. In the case of the cooler, and the 300-mK thermal architecture, either large safety margins will be implemented backed up by testing or “soft” failure modes will be designed to prevent dead thermal shorts in the event of structural failure. An additional method of providing operational reliability is to define backup operational modes for the subsystems and instrument. The following instrument backup operating modes are required in event of subsystem or system failure:
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