Spectral units of the narrow-band correlator

The narrow-band correlator has 8 independent units, which can be placed anywhere in the 100-1100MHz band (1GHz bandwidth), by steps of 0.25MHz. 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 DSB modes (320MHz/128, 160MHz/256, 80MHz/512 - 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 (160MHz/128, 80MHz/256, 40MHz/512, 20MHz/512) 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 on Plateau de Bure, 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 Mode LSB or USB for the same bandwidth but the central channels suffer from the Gibbs effect. Note, that in this context, ``LSB'', ``USB'', ``DSB'' has 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 1100MHz. The correspondence to the sky frequency depends on the parts of the 3.6GHz 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.

The spectral configuration of the narrow-band correlator is selected with the OBS program by the two commands

 OBS\NARROW Qi Qj /RECEIVER Band_Number
 OBS\SPECTRAL Unit Bandpass IF3_Frequency /NARROW Narrow_input -
                                          [/CONTINUUM Band_C IF3_C] -
                                          [/BAND Mode] -
                                          /RECEIVER Band_Number

where

$\circ$

Qi Qj are the two 1GHz wide ``quarters'' to be analyzed by the correlator (see Sect.  for more details).

$\circ$

Band_Number is the receiver band to which the unit should be connected ($1 =$3mm, $2 =$2mm, $3 =$1mm).

$\circ$

Unit is the correlator unit number (1 to 8).

$\circ$

Bandpass indicates the contiguous bandpass to be analyzed (in MHz: 20, 40, 80, 160 or 320).

$\circ$

IF3_Frequency is the center frequency of the bandpass analyzed by the correlator unit, in the third IF (100-1100MHz).

$\circ$

Narrow_input is the IF output (1 or 2).

$\circ$

Band_C is the bandpass to be used to produce the temporal data (in MHz: contiguous section of Bandpass).

$\circ$

IF3_C is the center frequency to be used for Band_C (same units as for IF3_Frequency).

$\circ$

Mode is used to indicate the sideband code: USB, LSB, DSB (see Table  for more details).

The /CONTINUUM option can be used to specify a pseudo-continuum channel to be generated from a subset of the bandpass analyzed by the correlator unit. This subset is defined by Band_C, the pseudo-continuum channel width in MHz, and IF3_C, the center frequency of the pseudo-continuum channel. If not specified, a pseudo-continuum channel containing 87.5 % of the sampled bandwidth is produced.

Here is an example of commands showing how to configure the 8 units for a (presumably) typical 3mm project.

NARROW Q1 Q3 
!     Selects the 4.2 GHz - 5.2 GHz and 6 GHz - 7 GHz windows of the 4-8 GHz 
!     IF1 band. Polarization is HH. 
!
SPECTRAL 1 160 1020 /CONTINUUM 100 1000 /NARROW 1 /RECEIVER 1
!     Correlator unit #1 covers the [940,1100] IF3 band on the first IF input 
!      (4.2 GHz - 5.2 GHz) with pseudo-continuum of [950,1050]
!
SPECTRAL 2 20 210 /NARROW 1 /RECEIVER 1
!     Correlator unit #2 samples a relatively narrow line centered at 210 MHz 
!      in the IF3 band of the first correlator input
!
SPECTRAL 3 20 770 /NARROW 1 /RECEIVER 1
!     Correlator unit #3 samples another one at 770 MHz
!
SPECTRAL 4 160 350 /CONTINUUM 50 395 /NARROW 1 /RECEIVER 1
!     Correlator unit #4 covers the [270,430] IF3 band on the first correlator 
!     input with pseudo-continuum of [370,420]
!
SPECTRAL 5 160 475 /NARROW 1 /BAND DSB /RECEIVER 1 
!     Correlator unit #5 covers the [395,555] IF3 band on the first correlator 
!     input with default pseudo-continuum [405,545]
!     Provides twice as many channels as the previous setting, but the
!     central two channels are affected by the Gibbs phenomenon
!
SPECTRAL 6 40 750 /NARROW 2 /RECEIVER 1
!     Correlator unit #6 covers the [730,770] IF3 band on the second IF input 
!     (6 GHz - 7 GHz)
!
SPECTRAL 7 320 400 /NARROW 2 /RECEIVER 1
!     Correlator unit #7 covers the [240,560] IF3 band on the second correlator
!     input 
!
SPECTRAL 8 320 925 /NARROW 2 /RECEIVER 1
!     Correlator unit #8 covers the [765,1085] IF3 band on the second correlator 
!     input

Figure  displays this example of frequency coverage.

Given the flexibility of the correlator, no general rules exist to configure it.

Figure: Example of spectral coverage by the narrow-band correlator, as produced by the ASTRO LINE command. Possible contamination by instrumental interferences are indicated