Each of the 16 downconverter boards has an independent gain control. These gains are adjusted by an 16-channel digital to analog converter board (16 8-bit DAC's). The 16-channel DAC board is sent serial data using two control lines on the RS232 port of the CompactPCI CPU board.
The gain controls are 8 bits each, where 0 is maximum gain, 255 is minimum gain.
Gain in dB is roughly linear with control voltage (ie., gain is logarithmic with control voltage). There is about ~45 dB gain change over full scale.
See Figure 12 on the AD8348 data sheet (available from Analog Devices) for a plot of gain vs. control voltage.
DAC Number to GALFA Channel Mapping:
|DAC 0||Beam 0||POL-A||Real & Imag|
|DAC 1||Beam 0||POL-B||Real & Imag|
|DAC 2||Beam 1||POL-A||Real & Imag|
|DAC 3||Beam 1||POL-B||Real & Imag|
|DAC 12||Beam 6||POL-A||Real & Imag|
|DAC 13||Beam 6||POL-B||Real & Imag|