Signal Chain

Each horn feeds a cryogenically cooled 4-stage InP HEMT amplifier operating between $26-36~{\rm GHz}$. The amplifiers were constructed at the University of Chicago, after a design developed at NRAO (Pospiezalski 1993, Pospiezalski et al. 1994). Receiver temperatures range between $18-25~{\rm K}$ at the center of the band, including the warm lens, feed horn and throat, and cold isolator and polarizer. Including CMBR and atmosphere, we achieve typical system temperatures of 25 K, for an rms sensitivity of approximately $0.8~{\rm mK~s^{1/2}}$ in a 1 GHz band on a single baseline.

The RF signal from each antenna is mixed down to 2-12 GHz IF using a local oscillator tuned to 38 GHz. The IF signal is then split into 10 1-GHz bands, each of which is further mixed down to 1-2 GHz. The 13 signals at each frequency are fed to one of 10 identical analog correlators, where the 78 complex multiplications are formed, digitized and integrated for 0.84 s in a digital accumulator. A phase switch is applied to each LO in a Walsh sequence on a $25.6~{\rm\mu s}$ clock interval and is demodulated by the accumulators, to remove any offsets or slowly varying pickup. A second level of Walsh switching is performed in software with a switching period equal to the readout interval. The multiplier gains and quadrature errors are periodically calibrated by injection of a correlated broadband noise source at the input to each receiver.

Each analog correlator is integrated onto a single full-depth VME card, and the entire 10-GHz correlator fits into a crate approximately 75 cm on a side. Filtering and downconversion of the IF signal is accomplished in a similar crate, and both rotate with the antennas on the underside of the telescope faceplate. The short fixed distance to the downconverter and correlator provides tremendous phase stability, with observed instrumental phase drifts of less than $10\deg$ over a period of weeks.

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