A new bolometer readout system has been developed and tested. It consists of an analog low signal proximity circuit and an amplifier, a ``modulator'' generating two bias signals, a digital command logic, and a digital to analog converter transmitting raw data to the control computer. Original features at all stages of this system give it unique performances. 1) The periodic nature of the bolometer bias translates the measurement frequency outside of the low frequency noises of electrical origin. In consequence, measurements of radiations varying at very low frequencies (down to 0.1 Hz or less) are possible without degradation of the sensitivity. 2) Two balanced bias signals are put at the two ends of the bolometer-load bridge, which allow to adjust around zero the mean voltage of the middle point. In this way, the dynamics needed for the amplification chain is considerably reduced compared to a system which will amplify the full voltage of the bolometer (Lange et al. 1996). We are able to measure the total power on a single pixel even if the background photon flux from the telescope is important (e.g. COBRAS/SAMBA). 3) The bias current in the thermistor is a square wave, keeping the electrical power deposited in the bolometer constant, and allowing to choose the bias frequency independently of the bolometer thermal characteristics. 4) The value of the bias signals is computer controlled, which gives the complete knowledge of the resistance of the bolometer thermistor. The measurement of the power background on the bolometer is then insured. 5) The load impedance in the measurement bridge is implemented with a capacitor. This choice eliminates the additional Johnson noise produced by the traditional resistive load. It also allows to locate the load at intermediate temperatures, such as 4 K or 100 K, which limits the number of wires per bolometer to 2, instead of 3 with a conventional circuit. It reduces in the same proportion the heat load by the wiring conduction on the most critical cryogenic stage. 6) All the bias parameters are computer controlled, allowing to face unpredictable situations, and insuring an optimal use of bolometers all along the life of a space project.
This system has been experimentally tested and used on the Diabolo ground-based astronomical experiment. The results of these tests allowed to validate the electronic system with the Diabolo bolometers, i.e. to demonstrate the very good stability of the bias and the low noise of the system.
The electronic architecture for the bolometer bias and readout on the COBRAS/SAMBA HFI, described in
Sect. 6 (click here), uses the specificity of the electronic system developed on the ground-based Diabolo instrument and modified for space borne application, i.e. the digital modulator with capacitance load and the digital lock-in fully controlled by a space qualified computer based on the Thomson Inmos T805 transputer. This system has also been experimentally tested on the Diabolo experiment, and demonstrated its ability to work under operational conditions. The performances of this new readout system open new modes of measurements to instruments using bolometers, such as the slow scanning of the sky with independent pixels, as featured in COBRAS/SAMBA.
Acknowledgements
We would like to thank the referee, H. Murakami, for his fruitful comments. The support by CNES of S. Gaertner and M. Piat is gratefully acknowledged. The Diabolo experiment has been funded by INSU and the technical research associated with its development has been supported by CNES and by the different laboratories of the authors. We thank the group of Spectrométrie Thermique of IAS for the study and realisation of the bolometers used in this study, under the scientific responsibility of N. Coron. Finally, S. Gaertner gratefully aknowledges support from the TTI department of Alcatel Espace, and more precisely P. Oudard and C. Lesthievent.