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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Last modified Apr 23, 1998