SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document 3.6.3 Temperature stability The temperature of the bolometric detectors must be highly stable to achieve the required sensitivity, and there are corresponding requirements on the thermal stability of the instrument itself. A detailed analysis of the effects of temperature drift on the sensitivity of the instrument in various operating modes has been conducted (Bock - Temperature Stability Requirements for SPIRE) and shows that the detector temperature must have a stability of 0.6-1.2 µK/Hz -1/2 in the detection band of the detectors and, further, must drift by no more than 1 mK per hour over the course of an observation. There is a very weak direct dependence on the temperature stability at the 1.7 K level and the corresponding figures for the 4-K level are, for the case of drift scanning to detect point sources, 5 mK/Hz -1/2 and 0.3 K per hour. 3.7 EMC Bolometric detectors are sensitive to any form of power input to the device. Unwanted electromagnetic radiation (from any part of the spectrum) absorbed by the detector will generate a spurious signal. Over and above this, any spurious RF power reaching the detector through conduction along the bias or signal wires will be dissipated through ohmic heating of the thermometer element, generating a spurious signal. It is critical therefore that bolometers be protected from EMI via an effective RF shield and a grounding scheme that prevents ground loops and protects the sensitive parts of the system from injected EMI. 3.7.1 Signal quality The signal, gain and noise figures expected at each portion of the detection chain are shown schematically in Figure 3-26. The basic signal level is of order of a few mV at the detector and, after current amplification, at the output of the JFETs. The voltage equivalent noise at the detectors is of order 25 nV/Hz -1/2 so all amplification and digitisation stages must contribute insignificant noise compared to this. The value of the basic detection noise also represents the sensitivity of the system to injected EMI. 3.7.2 Grounding and RF shield In order to maintain the very low noise required at the detector it is necessary to create an RF shielded environment around the detectors and to filter all wiring entering the enclosure. In the SPIRE instrument the RF shielded enclosure is created using the FPU box and the JFET boxes together with harnesses running between them – this is shown in yellow in Figure 3-27. The only significant aperture into the RF shield will be the optical entrance aperture of the instrument itself. The RF rejection performance of real enclosures is difficult to model and predict as it is very sensitive to the detailed physical implementation and the necessary electromagnetic modelling is complex and computer-intensive. Whether there is sufficient attenuation of RF frequencies by the apertures along the optical path of the instrument remains to be seen. If tests prove that that further attenuation is required, a “chicken wire” type RF filter will be fitted to the instrument entrance aperture. At this location, the optical beam is defocused and the presence of a fine wire mesh in the beam would not cause significant distortion to the final image at the detector plane. The grounding of the SPIRE electrical system is complex, even more complex than shown in Figure 3-27. The basic grounding scheme treats the two halves of the instrument, the photometer and spectrometer, as entirely separate electrical systems with their own power supplies and grounding. The analogues grounds of the two systems can be brought to a unipoint connection to the system ground through the thermal straps to the cooler – as shown in Figure 3-27. However, there is a separate ground and power supply for the digital part of the detector control electronics (not shown in Figure 3-27) which also has to be connected to system ground. There is some concern that this may lead to injection of noise into the analogue system via the system ground. Another possibility therefore is to isolate the RF shield at the cold end and connect to system ground in the warm electronics. Both these possibilities are open to us providing the mechanical structure of the FPU is electrically isolated from the cryostat. This will be implemented by electrical isolation washers 50
Draft SPIRE Design Description Document on the feet of the FPU mounts and sapphire plates on the thermal straps between SPIRE and the Herschel cryostat. In order to study the relative effects of various noise injection and signal cross talk, a SPICE equivalent circuit of the detectors and the read out and AC biasing electronics has been developed. This model can be seen in Figure 3-28. 51
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