SPIRE Design Description - Research Services

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.