There are four independent correlator
boards available in the correlator spectrometer. Each board can be assigned any available bandwidth, center
frequency, sampling level, polarization, and number of channels.
Note though that, for purposes of Doppler correction,
the boards can only be configured to
work as four independent boards or as one joined system. That is,
two boards cannot be overlapped and centered on one frequency while the
other two are set to another frequency (Figure 2).
Figure 2: The top two images show the possible configurations of the correlator
boards - acting as four independent boards and acting together, while the bottom
panel exemplifies an observing strategy not allowed with the current
The options are detailed below:
Center Frequency: This is the frequency on which the
individual board is centered. This can either be the rest frequency
or observed frequency of your lines. (A description is given below of the
possible velocity corrections which can be made to the entered frequencies.)
Bandwidth: The available range in bandwidths is 50.0 - 0.781 MHz.
Bandwidth is only available as a binary fraction of 50.0MHz. That is, you can choose
from 50.0, 25.0, 12.5, 6.25, 3.125, 1.562, 0.781, or 0.195 MHz.
Sampling level: Your data can be sampled with either 3-level,
3-level interleaved, or 9-level
sampling. 9-level operation achieves 98% of the signal-to-noise of analog
correlation whereas 3-level sampling achieves 81%. One advantage
of 9-level sampling is to limit the effects of RFI (usually) to just a few channels.
You cannot use 9-level sampling with a 50-MHz bandpass. If you wish
to use the 50 MHz bandpass you must use 3-level interleaved sampling.
Polarization: For each board you have the option of having
either A, B, A & B, or Stokes polarizations. If you use either
only A or only B polarization and 3-level interleaved sampling
you can have access to the entire 4096
channels on each board. If you choose to split a board into
both A & B polarizations or increase the sampling levels, you naturally
cut down on the the number of available
No. of Channels (Lags) Used: The total number of channels available on
each board is 4096. This number can only be used, though, if you
are using 3-level (interleaved) sampling. Increasing your sampling to
9-level cuts the total number of channels available in half. A complete listing
of the options is given in Table 3. Note that the smaller your
number of channels the smaller (in size) your data file will be for a given
Table 3: Possible Correlator Configurations
are: 4096 (only with one pol. & 3-level sampling), 2048 (available for
the 3-level A & B set-up or the 9-level sampling with only one polarization),
1024 (available for all set-ups), 512, 256, 128, 64, 32, or 16 channels.
The smaller the number of channels used, the smaller (in size) you data file