The SIS junctions, the electromagnets used to suppress quantum-generated mixer noise, and the Low Noise Amplifiers (LNA's) all require noise-free, accurate electrical biasing for reliable operation. On single-pixel receivers, providing the appropriate biasing voltages and currents is handled adequately by individual boxes with manual potentiometers and LCD panels on the front. With the advent of array receivers, however, the sheer number of boxes required makes this kind of control impractical. Thusly, we have consolidated the three primary bias control functions into several Euro-style cards that feature digital potentiometers and analog multiplexers controlled by a computer. The computer-controlled IC's communicate via an I2C bus, whose signals are generated at the parallel (printer) port of the computer. Our design, outlined in Figure 15, yields the following benefits: Consolidation: Mixer, magnet, and LNA bias control cards easily fit into one rack-mounted case, as shown in Figure 16. The top rack contains the actual bias cards; the bottom rack contains 8 Acopian DC power supplies needed for the cards' operation. In Figure 17, an SIS control card is shown. This single card biases all four SIS mixers. Proven design: The SIS, magnet, and LNA circuit designs are solidly based upon the successful designs already in use at the South Pole (AST/RO), Meauna Kea (CSO) and Mt. Graham (HHT). Flexible computer control: Since the I2C bus is implemented on the standard PC parallel port, any PC can send commands to the bias cards. Monitoring the biasing signals requires only a general-purpose data acquisition card. Expandability: Since multiple I2C buses can coexist on a single parallel port interface, this design can support up to 16-channels with no modifications. We are already constructing a 7-channel, 345 GHz array receiver (called Desert Star) for the HHT, using 2 I2C buses. Both bias control systems are seen together in Figure 18: Pole Star on top, Desert Star on the bottom.

Figure 15: Pole Star array electronics outline Click on thumbnail for full-size image! Figure 16: The Pole Star bias electronics box Click on thumbnail for full-size image! Figure 17: 4-channel SIS control card Click on thumbnail for full-size image! Figure 18: Pole Star and Desert Star Bias boxes Click on thumbnail for full-size image!