SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
and sensitivity to microphonic disturbance. The detector noise is typically 10-20 nV/Hz -1/2 , and the noise of
the readout amplifier must be of this order or less. With current transistor technology, this requires the use of
silicon JFETs, which must operate at a temperature of around 100 K, and must also be located as close as
possible to the detectors. This is why SPIRE employs JFET amplifier modules as part of its cold FPU. The
essential features of the readout electronics are shown in Figure 3-26. The bolometer is biased (heated to its
optimum operating temperature of around 1.3To) by a sinusoidal current bias at a frequency of at least 100
Hz, applied via the 10-MΩ load resistors. The bias excitation is much faster than the thermal time constant,
so that bias itself does not produce a temperature modulation. This is preferred over DC bias as it upconverts
the signal information to the bias frequency, getting well above the 1/f noise knee of the JFET
readout amplifiers. With this arrangement, because of the inherently low 1/f noise of the bolometers, the1/f
noise knee of the system can be very low (less than 0.1 Hz).
AC bias
~ 100 Hz
300 mK
10 MΩ
Bolometer
~ 5 MΩ
10 MΩ
4.4.5 Feedhorns and bolometer cavities
74
~ 100 K
50 kΩ
50 kΩ
Figure 4-18 Bolometer bias and cold readout circuit
JFET
power
Differential
output
to warm
amplifier
The size of a composite bolometer (typically mm diameter) is small compared to the telescope diffraction
spot (in the case of SPIRE, with f/5 final optics, the spot size is 2.44λF = 12.2 mm at 500 µm) and the firstpass
absorption efficiency of the absorber is only ~ 50%. In order to couple the telescope beam onto the
detectors in the array, conical feedhorns are used, with a short section of waveguide at the end of the horn to
feed the radiation into a cavity containing the bolometer, as shown schematically in Figure 4-19. The
waveguide acts as a low-pass filter as it does not propagate radiation of free space wavelength > 3.4a where
a is the radius. For high efficiency, the absorber is located in the centre of the cylindrical cavity with λ/4
spacing between the front of the cavity and a reflecting back-short at the back. The feedhorns are packed in a
hexagonal arrangement in the focal plane to fit as many as possible into the area available.