SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Figure 4-12 – Schematic block diagram of the detector This is achieved with three different cards mounted in the DCU, viz. the LIA Card, DAQ+I/F Card and the Bias Card. Each LIA Card takes 32 differential signal inputs from the detectors and preamplifies the positive and negative signals with four OP-400 operational amplifiers. They are then fed into a DG303A CMOS Analogue switch Table 4-1 – Detector driver electronics specifications. Number of channels 48 96 144 24 48 68 PLW BDA PMW BDA PSW BDA SLW BDA SSW BDA Signal AC amplitude 10 mV Dark condition Signal DC level 5 mV JFET VOsmax Common mode offset 1 V DC Noise allocation 7 nV/Hz 1/2 0.64 pA/Hz 1/2 0.05 to 25 Hz Input capacitance < 100 pF Input impedance > 1 M Base band signal bandwidth 0.03 to 5 Hz Photometer 0.03 to 25 Hz Spectrometer 5 Hz Thermometry Input noise impedance > 7kΩ Common mode rejection - 60 dB 30–300 Hz
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. 69
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