The narrow-band correlator can process the signals from up to 6 antennas. It has 8 independent units which
can be placed anywhere in the 100-1100 MHz band (1 GHz bandwidth), by steps of 0.25 MHz. Each unit can be operated in seven modes, as
shown in Table . Each mode is characterized in the following by couples of total bandwidth/number of channels. In the 3
double-sideband modes (DSB, 320 MHz/128 channels, 160 MHz/256 channels, 80 MHz/512 channels - see Table
) the two central
channels may be perturbed by the Gibbs phenomenon if the observed source has a strong continuum. When using these modes, it is recommended
to avoid centering the most important part of the lines in the middle of the band of the correlator unit. In the remaining SSB modes
(160 MHz/128 channels, 80 MHz/256 channels, 40 MHz/512 channels, 20 MHz/512 channels) the two central channels are not affected by the Gibbs
phenomenon and, therefore, these modes may be preferable for some spectroscopic studies. Because of signal apodization, the effective spectral
resolution is slightly broader than the channel spacing (by about a factor 1.6 in the standard case for NOEMA, that uses a Welch time-lag
window).
In addition to the spectra produced every integration time (subscan in the OBS terminology), the correlator units output every second the visibility for a pseudo-continuum channel created by averaging data from several spectral channels.
Spacing | Channels | Bandwidth![]() |
Mode![]() |
(MHz) | (MHz) | ||
0.039 | 1 x 512 | 20 | SSB |
0.078 | 1 x 512 | 40 | SSB |
0.156 | 2 x 256 | 80 | DSB |
0.312 | 1 x 256 | 80 | SSB |
0.625 | 2 x 128 | 160 | DSB |
1.250 | 1 x 128 | 160 | SSB |
2.500 | 2 x 64 | 320 | DSB |
The 8 units can be independently connected to the first or the second correlator entry, as selected by the IF processor (see above).
Please note that the center frequency is expressed in the frequency range seen by the narrow-band correlator, i.e. 100 to 1100 MHz. The
correspondence to the sky frequency depends on the parts of the 3.6 GHz IF1 bandwidth which have been selected as correlator inputs and
on the selected receiver side band (LSB or USB): Use the ASTRO software (see Sect. ) to display the relation between
sky- and IF1 frequencies.