Interferometer Backends

There are 2 interferometer backends :

• A 12-channel continuum detector (2 IFs, 6 antennas). Output from these detectors is read once per second (except when the acquisition software is waiting for tracking information).
• A 2-bit, 4 level spectral correlator.

The spectral correlator is entirely configured by software It consists in 8 independent, very flexible, units. Each unit can operate in 5 modes providing different bandwidth/resolution capabilities

1.

320 MHz bandwidth, with 128 channels and channel separation 2.5 MHz.

2.

160 MHz bandwidth, with 128 channels and channel separation 1.25 MHz.

3.

80 MHz bandwidth, with 256 channels and channel separation 0.312 MHz.

4.

40 MHz bandwidth, with 512 channels and channel separation 0.078 MHz.

5.

20 MHz bandwidth, with 512 channels and channel separation 0.039 MHz.

Each unit can be connected anywhere in the second IF band (100-800 MHz) by tunable local oscillators (LO3s), in step of 0.25 MHz. The 320 MHz bandwidth is obtained by concatenating the USB and LSB of an image rejection mixer (IRM) at the 3rd LO frequency. All the others are obtained by selecting either the USB or the LSB of the IRMs. These IRMs have sideband separation of about 30dB, and at most 10 kHz is lost around the LO3 frequency. The 160 and 80 MHz bandwidths may also be obtained like the 320 MHz bandwidth in DSB mode; then the channel separation is halved at the expense of a central hole arond the LO3 frequency.

The correlator is typically run with 4 units in broadband mode, and 4 units as a ``zoom'' on interesting spectral line(s) in either receiver.