Quadrature Downconverter Gain Control

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