SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Limit of FPU RF Shield Blade supports SPIRE Optical Bench RF Filter Unit 70 37 way MDM plugs RF Filter Unit 37 way MDM sockets Spectrometer outer cover Figure 4-14 - Schematic representation of the Filter Box mounting technique showing effective limit of the FPU RF shield. The filter units are attached to the SPIRE optical bench by a blade support structure as shown in Figure 4-14. As the spectrometer cover is placed over the SPIRE optical bench, the outer cover meets with the filter units. The filter boxes are then screwed tightly to the outer cover with a 30 mm bolt spacing and become a part of the RF tight FPU enclosure. The effective outer extent of the FPU RF tight enclosure is shown on Figure 4-14 by the green dashed line. The required mechanical rigidity of the filter boxes come from the bolts attaching them to the outer cover. The first natural frequency of the filter boxes is above the required 200 Hz. 4.3 JFET units The functionality of the JFET in the read out electronics of the BDAs is adequately described in §4.4.4 below. The JFET boxes are described here as a sub-system in themselves. Electrically isolated Model U401 Silicon JFETS are used to read out the SPIRE detectors, mounted in groups of 24 on silicon nitride membranes. The contract traces and JFET source resistors are lithographed on the membranes. The JFET modules for the photometer and spectrometer are mounted in separate enclosures (illustrated in Figure 4-15) on either side of the FPU. There are two sets of JFET boxes, one set drives the signals from the photometer detectors and the other set drive the spectrometer detectors. To minimise the signal loss or phase distortion due to harness capacitance, the length of the harnesses between the JFETS and the bolometers needs to be minimised. Hence, the two groups of |JFETs are located on the Herschel optical bench as close to the FPU structure as is practical. Thermal Design: To attain the required noise performance, the individual JFET devices need to operate at around 110K. This poses several challenges to the thermal design of the modules: (i) the surrounding structure is at the same temperature as the Herschel optical bench. The thermal load from the JFETs to the JFET structure has to be minimised so as to minimise the power used to heat the JFETs. The nominal limit for all the JFETs in a single operating mode is 33 mW.; (ii) the thermal load from the JFETs to the structure needs also to be limited to ensure that the heat load to the FPU through the detector harnesses is minimised. High thermal impedance is therefore required between the JFETs and the rest of the structure. This requirement is achieved through the mounting of the devices on thin silicon nitride membranes and low cross section lithographed wires to the individual JFET terminals. The silicon nitride membrane also meets the functional requirement of providing a high resonant frequency.
Draft SPIRE Design Description Document As described above in §4.2, the length of harnesses from the SVM to the FPU provide a potential source of EMI within the FPU. Filters identical to the ones described above in §4.2 are inserted on all the inputs to the JFET boxes. Included in the structure of the JFET box is the provision of filter units identical to the ones used inside the FPU to filter the AC bias signals going to the detectors. All the harnesses passing from the JFET units to the FPU are shielded. This effectively enlarges the RF shielding provided by the FPU enclosure to include these shields. This identified as the yellow coloured section on Figure 3-27. The structure is rigidly attached to the Herschel optical bench by four bolts. It is fabricated from aluminium in order not to generate thermal stresses at cryogenic temperatures. The first mode of vibration is above 100 Hz. The structure also has a thermal strap connecting it to Level-2 of the cryostat. The structure is electrically isolated from the Herschel optical bench by Vespel washers (TBC). Figure 4-15 - SPIRE JFET module design showing the JFET modules and the filter modules. 71
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