The front end system (Figure 1) consists of a broadband (1-18 GHz) dual-polarization log-periodic crossed-dipole feed, and a thermally-controlled RF box (called the FEM, or Front End Module) that houses the amplifiers, calibration noise source (noise diode), attenuators, and optical transmitters needed to control the gain of the system and supply the two RF signals multiplexed onto a single optical fiber output.
The block diagram for the FEM is shown in Figure 2.
Each ROACH has a set of four registers named 'eq_x0_coeffs', 'eq_x1_coeffs', 'eq_x2_coeffs', and 'eq_x3_coeffs'. The 'x0' and 'x1' coefficients correspond to the first antenna's X and Y polarization, respectively, while the 'x2' and 'x3' coefficients correspond to the second antenna. I will refer to each of these as one of the ROACH's four IF channels, two for each antenna. The correlator can tune successively to different 500 MHz IF bands, and each is divided into 4096 subchannels.