Figure 4: Synoptic of the digital modulator
In the new system, all parameters of the bias waves, including the
compensation of spikes and non-linearities, are controlled by a computer.
The ``digital modulator'' (see Fig. 4 (click here)) producing the bias waves
includes four high quality 12 bits Digital to Analog Converters (DACs),
used to control the following parameters:
- DAC1: the amplitude of the bias voltage,
- DAC2: the intensity of the bias current,
- DAC3: the slope of the bias voltage in each half period (this compensate for
the triangular wave non linearities),
- DAC4: the amplitude of a spike signal added to the bias current at the
beginning of each half period (compensation of transients).
A point of prominent importance in this system is to use a reference voltage
(
) with high stability and low noise, in order to feed
the four DACs. The DC output level of each DAC crosses an inverting gate
driven by the main clock, in order to obtain a square signal of very stable
amplitude. The square bias voltage is divided by a resistance bridge
(divided by 100) before being applied on the bolometer. Because a
capacitance is used to drive the bolometer current, the analogic voltage
signal is a triangular wave obtained after integration of the square wave.
An anti-saturation system is added to the integrator to stabilize the
average DC output level. This triangular voltage is applied to the load
capacitor to generate the square bias current.
DAC3 and DAC4 allow to introduce some curvature and spike to the bias
voltages in order to get a bolometer signal as flat as possible. DAC3 controls
the addition of a fraction of the triangle signal to the square signal which
will balance non linearities. DAC4 controls the addition of a fraction of the
square current to the triangle signal. This is derived in anti-spikes current by
the 
capacitance. Consequently, the bridge is well-balanced when:
where 
and 
are the capacitance and the resistor of
the integrator, 
and 
are the voltage and current
levels on the bolometer. If the voltage and current levels are the same,
and for 
, 
and 
, the value of the load capacitance must be about 10 pF.
Figure 5: Synoptic of the electronic system of the Diabolo experiment
(December 1995)
This system has been included in the electronics of the Diabolo experiment
during an observation campaign on the IRAM 30 meter radio telescope at
Pico Veleta (Spain) in December 1995. The Diabolo electronic system
(see Fig. 5 (click here)) is made of:
a Box for Each Bolometer Output (BEBO), composed of:
- a ``digital modulator'' with a capacitive load,
- four DACs (12 bits),
- a pre-amplifier (
),
- a variable gain amplifier (from 
to 
),
- a 12 bits Analog to Digital Converter (ADC),
- a digital command logic,
- several opto-couplers isolating the analog part and the digital one,
an interface, composed of:
- a multiplexer receiving 12 bolometer signals and driving the transmission
receiver,
- a controller verifying the different signals and building the telemetry
flow,
a computer based on a Reduced Instruction Set Computer (RISC)
processor, i.e. the Inmos Thomson T805, achieving the digital lock-in of the
12 bolometer signals, and a computer performing a quick-look of the signals and
allowing interactive or automatic control of the bolometers bias parameters
(DACs 1 through 4).