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