Spectral Resolution

The IRAM interferometer is equipped with 8 independent, nearly identical, correlator units. Each unit can be operated in seven modes, as shown in Table 6. 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.

Each unit can be placed anywhere in the baseband (100-680 MHz), by steps of 0.250 MHz. All connections are remotely controlled by the OBS program. Because of channel shape, the effective resolution is slightly broader than the channel spacing (by about 1.3).

The spectral correlator configuration is selected with the OBS program, by command

 OBS\SPECTRAL Unit Bandpass IF2_Frequency [/CONTINUUM Band_C IF2_C] -
                                          [/BAND Mode] -
                                          [/RECEIVER Receiver_Number]

where

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sep -1mm

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

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

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IF2_Frequency is the center frequency of the bandpass analyzed by the correlator unit, in the second IF (100-680 MHz).

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

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IF2_C is the center frequency to be used for Band_C (same units as for IF2_Frequency).

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Mode is used to indicate the sideband code for the fourth frequency conversion: USB, LSB, DSB (see Table 6 for more details).

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Receiver_Number is the receiver to which the unit should be connected (1 =3mm, 2 =1mm)

Table 6.

		SSB1 Mode		DSB2 Mode
Conf	Bandwidth3	Channels	Spacing	Channels	Spacing
	(MHz)	(MHz)		(MHz)	(MHz)
1	320	-	-	$2 \times 64$	2.5
2	160	$1 \times 128$	1.25	$2 \times 128$	0.625
3	80	$1 \times 256$	0.312	$2 \times 256$	0.156
4	40	$1 \times 512$	0.078	-	-
5	20	$1 \times 512$	0.039	-	-

¹ Default Mode is LSB. There is no practical difference between an LSB or a USB setting. ² Mode is DSB. This mode provides twice as many channels than Mode LSB or USB for the same bandwidth but the central channels suffer from Gibbs effect. ³ 5% of the passband are left out at the low-frequency and at the high frequency ends of each spectral correlator subband.

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 IF2_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 project.

!     Covers the [ 60,220] IF band with pseudo-continuum of [110,210]
SPECTRAL 1 160 140 /CONTINUUM 100 160 /RECEIVER 1
!     Covers the [200,360] IF band with pseudo-continuum of [210,330]
SPECTRAL 2 160 280 /CONTINUUM 120 270 /RECEIVER 1
!     Covers the [340,500] IF band with default pseudo-continuum [350,490]
!     Provides twice as many channels as the previous setting, but the
!     central two channels are affected by the Gibbs phenomenon. 
SPECTRAL 3 160 420 /RECEIVER 1 /BAND DSB
!     Covers the [480,640] IF band with pseudo-continuum of [490,590]
SPECTRAL 4 160 560 /CONTINUUM 100 540 /RECEIVER 1
!     Now samples a relatively narrow line centered at 340 MHz in the IF
SPECTRAL 5 20 340  /RECEIVER 1
!     And another one at 420
SPECTRAL 6 20 420  /RECEIVER 1
!     Covers the [100,420] IF band 
SPECTRAL 7 320 260 /RECEIVER 2
!     Covers the [360,680] IF band 
SPECTRAL 8 320 520 /RECEIVER 2

This configuration covers more than the full bandwidth of the receivers in broad-band mode, with some overlap at the band edges of the units. Note how the pseudo-continuum mode is selected to avoid the (presumably) strong line in the [330,350] MHz band.

Given the flexibility of the correlator, no general rules exist to configure it. Please refer to the documents ``Frequency Setup'' and ``OBS Users Guide''. for more details. Figure 5 gives another example of frequency coverage.