SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
4.2 RF Filters
The bolometer detectors in the spectrometer and the photometer are sensitive to all frequencies of
electromagnetic radiation. Although the BDA feedhorns act as effective high pass filters, it is essential that
emissions from within the FPU unit be minimised to eliminate spurious sources of signal. The harnesses that
pass from the WE on the SVM to the FPU can act as receiving antennas. The voltages received by the
harnesses outside the cryostat could then possibly retransmit EM radiation within the FPU and corrupt the
signals from the BDA.
To eliminate this possible source of noise on the detector signal, low pass RF filters protect all harnesses
passing into the FPU. Three basic types of harnesses pass into the FPU; (i) bolometer signal wires, (ii)
bolometer bias wires, and (iii) FPU sub-system drive and instrumentation wires (eg. Coil drive current wires,
thermistor readout signals etc.). These three types of harnesses can be identified on the SPIRE system
diagram, Figure 3-1. The bolometer signals are filtered within the JFET boxes and will described separately
below in §4.3. The bolometer bias harnesses and the subsystem harnesses are conditioned by sets of filter
boxes.
There are six filter boxes for the subsystems are located on the spectrometer side of the FPU (see Figure
3-5). They have two 37 way MDM socket connectors on the WE side of the box connected to two 37 way
MDM plug connectors on the subsystems side of the box. Each pin on WE side of the box is connected to an
equivalent pin on the detector side via a muRata VFM41R01C222N1C 3-terminal varistor-capacitor EMI
filter. There are therefore 444 individual filters allocated to the subsystems. The filter requirement is that it
rejects all frequencies from 500 MHz to 10 GHz at –60 dB. The filter operates by conducting high frequency
noise signals to ground via the third terminal of the filter. The bode diagram showing the insertion losses for
the filter at room temperature and with a 50Ω source impedance is shown in Figure 4-13.
Figure 4-13 – Insertion losses for the muRata EMI filters.
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