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CORRELATOR BOARDS

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).

The top two images show the possible configurations of the correlator boards -- acting as four independent boards and acting together, respectively, while the bottom panel exemplifies an observing strategy not allowed with the current system.

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 full 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 channels.

• 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 (to 2048). 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 record dump. Your options for the number of channels to use are: 4096 (only with one polarization and 3-level interleaved sampling), 2048 (available for the 3-level A and B polarization set-up or the 9-level sampling with one polarization), 1024 (available for all set-ups), 512, 256, 126, 64, 32, or 16 channels.

Table 3: Possible Correlator Configurations

Sampling	Max BW	Pol/sbc	lags/sbc	Resolution
	per sbc			at 1420MHz
9-level	25MHz	1	2048	2.6 km/s
9-level	25MHz	2	1024	5.2 km/s
3-level	25MHz	2	2048	2.6 km/s
3-level interleaved	50MHz	1	4096	2.6 km/s
3-level interleaved	50MHz	2	2048	5.2 km/s
3-level Stokes	25MHz	-	2048	2.6 km/s