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Spectral units of the narrow-band correlator

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.


Table: Configurations of the narrow-band correlator units
Spacing Channels Bandwidth$^1$ Mode$^2$
(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
$^1$:
Note that 5% of the passband are lost at both ends of each subband.
$^2$:
Default mode is LSB. There is no practical difference between an LSB or a USB setting. DSB Mode provides twice as many channels as each of the SSB modes for the same bandwidth but the central channels suffer from the Gibbs effect. Note, that in this context, ``LSB'', ``USB'' and ``DSB'' have nothing to do with the tuning of the receivers; it is just an unfortunate coincidence of the same terms.

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.


next up previous contents index
Next: ASTRO Up: Correlators Previous: IF processor and narrow-band   Contents   Index
Gildas manager 2015-03-19