The quadrature downconverter box houses:
The downconverter takes in 14 IF signals from the ALFA receiver (100 to 400 MHz) and converts them to 14 quadrature baseband signal pairs (-40 to +40 MHz). The downconverter outputs drive the spectrometer boards, where the signals are digitized and processed.
Note that the ALFA receiver has 14 IF signals (seven beams, two polarization per beam), so the downconverter box has two spare channels.
NOTE: THE LO SHOULD BE SET TO TWO TIMES THE DESIRED LO FREQUENCY since the downconverter chips divide the input frequency by two. A 16-way splitter is used to drive the 16 LO inputs. The input power to the LO splitter should be +3 dBm, so that each of the downconverter boards receives -10 dBm.
Each of the 16 downconverter boards has an independent gain control. These gains are adjusted by a 16-channel digital to analog converter board (16 8-bit DAC's). The 16-channel DAC board is sent serial data using two RS232 control lines (RTS and DTR) from the RS232 port of the CompactPCI CPU board.
16 10-pin ribbon cables are used to distribute 5V power and the 0V to 1.25V gain control voltage to each of the 16 downconverter boards. The pinout of these ribbon cables is symmetric, so if the IDC connectors are flipped around, the power and gain control voltage will still be correct.