Bolometers are used in a very wide range of frequencies, from X-rays to millimetre waves. This results from the principles of thermal detection: the temperature changes of a radiation absorber are measured by a thermometer. The efficiency of this process depends on that of the radiation absorption. The theory of bolometers has been developed and refined during the last half-century (Jones 1953; Low 1961; Mather 1982, 1984). The most currently accepted assumptions are that the temperature changes are measured using a thermistor attached to the radiation absorber and electrically connected to a load resistor (Fig. 1 (click here), left).
Figure 1: Schemes showing the principles of a conventional DC bias (left),
and of a balanced periodic square wave bias with a capacitive load (right)
This scheme has been used successfully in a large number of applications,
especially in instruments dedicated to astronomical measurements in the far
infrared and the submillimetre wavelength ranges. Nevertheless, low frequency
noises in electronic components used to amplify the signal, generally gathered
under the denomination of 1/f noise, make it impossible to use this principle
for the measurement of DC optical signals. Sky chopping, which consists in
alternately orientating the beam towards two neighbour directions, and measuring
the signal difference with a lock-in amplifier, shifts the measuring frequency
around that of the sky chopping, out of the 1/f noise frequency range. This has
also the advantage of removing foreground emissions common to both positions of
the beam, such as those of the atmosphere and the telescope. The disadvantages
are that the measurement is now only a differential one (measuring the
brightness difference between two fields of view), and that the implementation
of a beam switching is sometimes a major technical difficulty, needing for
example additional mirrors, and a poorer optical efficiency. Other electrical
schemes have been studied to measure the impedance of the thermistor with an
AC bridge circuit, which shifts the measuring domain outside the 1/f noise
domain (Rieke et al. 1989; Devlin et al.
1993). In these systems, the bolometer to be measured is mounted
in a bridge with another bolometer. In order to perform a measurement of the
total power, the comparison bolometer has to be blind. Both bolometers need
to have similar impedances. This can be achieved if the background photon
flux on the bolometer to be measured is negligible. This was the case for
the IRTS instrument (Murakami et al. 1996), the
cryogenically cooled Japanese infrared telescope on which the system of
Delvin et al. (1993) has been implemented. However
in the case of the COBRAS/SAMBA project (Bersanelli et al. 1996), an important thermal background photon flux from the
telescope (
) reaches the bolometer. The blind and open
bolometer will have very different impedances which makes impossible the
balance of the bridge. To overcome this problem one usually includes in the
bridge two bolometers of the focal plane corresponding to two different sky
positions (Wilbanks et al. 1990). Such a
measurement is differential and not appropriate to a survey mission like
COBRAS/SAMBA which has to measure all the angular frequencies of the sky
emission. Another solution would be to artificially heat the blind
bolometer (Lange, private communication). The problem then is to provide a
very stable heating device.
The readout system (see Fig. 1 (click here), right) presented in this paper allows total power single pixel measurements at the focus of a warm telescope, accross a wide electrical frequency band. It is new in several respects: 1) The dynamics needed is kept reasonable thanks to a zero measurement method obtained with balanced bias voltages. 2) The load impedance is a capacitor (instead of a resistor), and then free of Johnson noise. 3) The whole system is digitally controlled, and the bias parameters can be tuned by a computer. This system has been developed by steps and implemented on the Diabolo experiment (Benoıt et al. 1996 and Désert et al. 1996) with bolometers (Coron 1976) cooled at 0.1 K. This instrument has been successfully used for high sensitivity ground-based astronomical observations in the millimetre domain. In the next section of this paper (Sect. 2 (click here)), the principle of the balanced bias with square waves is detailed. In Sect. 3 (click here), the advantages of a capacitive load are shown, and a feedback system used to control the bias level is discussed. Section 4 (click here) is dedicated to the description of a computer control of this bias system, while Sect. 5 (click here) analyses and discusses its performances. The architecture of a space qualifiable version, suitable for the High Frequency Instrument of COBRAS/SAMBA is described in Sect. 6 (click here). In the conclusion (Sect. 7 (click here)), the advantages of this new readout system are discussed.