The Interim Correlator

For spectral-line observing, the original post-upgrade ``interim'' correlation spectrometer provides four independent sub-correlators, each having eight chips with 1024 lags per chip. Each sub-correlator can be set up with its own independent bandwidth and configuration. The table below specifies the available configurations. The maximum bandwidth per sub-correlator is 50 MHz, with 8 other alternative bandwidths being available in decreasing octave steps. Popular configurations combine chips in groups of four to provide 9-level sampling (96% efficiency), interleaved operation (50-MHz bandwidth) or auto + cross correlation (i.e. all four Stokes parameters).

Configuration Maximum bandwidth per subcorrelator board (MHz) Polarizations per board Boards used Lags per board & channel separation (km/s at 1420 MHz, max. bandwidth)
9-level 25 1 4 2048 (2.6)
9-level 25 2 4 1024 (5.2)
3-level 25 2 4 2048 (2.6)
3-level interleaved 50 1 4 2048 (5.2)
3-level interleaved 50 2 4 2048 (5.2)
3-level Stokes 25 Full Stokes 4 2048 (2.6)

Notes:

  1. The spectrometer has its availabe bandwidth defined by the following filters:
    1. Analog filter - 50 MHz
    2. Digital filters - 25, 12.5, 6.25, 3.125, 1.563, 0.781, 0.391, 0.195 Mhz
  2. Double Nyquist sampling can be used with all configurations except interleved, but decreases the maximum bandwidth by a factor of two.
  3. 3-level, double Nyquist, 12.5-MHz bandwidth and below will give 4 sub-bands with better resolution than the corresponding 9-level configuration.
  4. 9-level operation achieves 96% of the signal-to-noise of analog correlation, whereas 3-level achieves 81%
  5. Different correlator boards can operate with different configurations to each other.
  6. The fastest dump rate for spectral-line usage is about 10 Hz.
  7. The number of lags used can be reduced by a factor of 2n down to 16 lags. The size of the output file will be proportional to the number of lags.