DASI: A Degree Angular Scale Interferometer for Imaging
Anisotropy in the Cosmic Microwave Background
N. W. Halverson, J. E. Carlstrom, M. Dragovan,
W. L. Holzapfel, and J. Kovac
5. Correlators
The DASI correlator system is essentially identical to that designed
at Caltech for the CBI project. The 2-12 GHz IF signals from each of
the 13 receiver local oscilators are mixed down into 10 1 GHz wide
bands. Ten identical correlators then compute the complex correlation
of all baselines in each of the 1-2 GHz bands. The bandwidth of 1 GHz
is a compromise between spectral resolution, necessary for
characterization and removal of foreground sources, and cost of the
correlator. In each 1 GHz band, an array of Gilbert cell multipliers
produces the complex correlations for each of the 78 baselines. The
output of the multipliers are integrated for 25.6 ms and then
digitized. The phases of the 13 receiver local oscillators are
modulated with shifts of 180° causing the sign of the
correlation to reverse between integration periods. The digitized
signals are added into an accumulator with a sign corresponding to the
phase state of the LOs for each baseline. After a sequence of 32
integration periods, the accumulated signals have no DC offset and are
insensitive to low frequency noise. The results of the accumulator
can be be read out on time scales from .82 ms to .84 s.
A prototype unit has been measured to have an efficiency of
> 95%. The quadrature channels have been measured to have phase errors
of
less than 3 degrees. Each receiver local oscillator has a 90°
phase switch which allows the characterization of these small
quadrature errors. Each of the 10 1 GHz correlator modules is built
onto a single 9U × 340 mm VME card. The compact size of the
correlator allows it to be rigidly attached to the back of the
aperture rotation axis of the telescope. Thus, instabilities
associated with the flexing of cables carrying RF signals are
eliminated. The response of the correlator is calibrated by injecting
a stable wide-band correlated signal into the input of each of the
receivers. This allows the instrument to be accurately calibrated in
the time between absolute calibrations with planets.
Figure 6:
Cut away of the DASI telescope. A temperature controlled cabin within
the telescope structure is accessed via a ladder from a room
below.
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jc@oddjob.uchicago.edu
Last modified Apr 23, 1998