6.10 Base Band Converter

The base band converters (BBCs) mix the IF signals to base band and provide the final analog filtering. Each of 8 BBCs generates a final LO signal at any multiple of 10 kHz between 500 and 1000 MHz. Each BBC can select its input from any of the four IFs. The BBCs provide both upper and lower sidebands as separate outputs, allowing for a total of 16 baseband channels. (A ``channel'' means one sideband from one BBC.) Allowed bandwidths per BBC are 0.0625, 0.125, 0.25, 0.5, 1, 2, 4, 8, and 16 MHz. Thus the 16 possible baseband channels can cover an aggregate bandwidth up to 256 MHz. The BBC signals are adjusted in amplitude. With automatic leveling turned on, the power in the signals sent to the samplers is kept nearly constant, which is important for the 2-bit (4-level) sampling mode (Section 6.11). The BBCs contain synchronous detectors that measure both total power and switched power in each sideband for system temperature determination. This subsection will become obsolete when the new digital backends are installed on the VLBA later in 2010. See the beginning of Section 6 for further information.