5 Readout electronics

The special development made for the readout electronics is described by Gaertner et al. (1997) in detail. Here we give the basic characteristics of the electronics that were specially devised for this instrument. The bolometer is biased with a square AC modulation at typically 61 Hz. The current is injected through a capacitance (in place of the classical load resistance) and an opposition voltage is applied to ensure a near-equilibrium of the bridge (Fig. 8).

Hence a small AC modulated out-of-equilibrium signal can be analysed, which is less than 10^-3 of the input voltage. The major advantages of this system are

• a constant power dissipation in the bolometer, which keeps its dynamical impedance constant (the square-wave signal does not perturb the thermal behaviour of the bolometer because it works at a constant input power);
• no additional Johnson noise due to the load (which is capacitive rather than resistive);
• a reduced low-frequency noise from the electronics, due to the modulation with a square function at frequencies above a few tens of Hertz.

A cold FET amplifier (JFET NJ132 at 100 K) is used to have an amplifier noise smaller than the bolometer noise. Shielded wire is used all the way in order to avoid electronic interferences and the cable is soldered throughout to avoid microphonics.

Version 1 of this electronics uses an analog lock-in amplifier with a slow feedback on the bias of the bolometer to force the signal to be zero (with time constant of few seconds). In this way, we measure

• at intermediate frequency (1-10 Hz), the voltage variation of the bolometer at constant current;
• at low frequency (DC below 1 Hz), the absolute power received by the bolometer. As the impedance of the bolometer is fixed by the bridge balance, the bolometer works at constant temperature and the bias power gives us directly the DC radiation input power.