4.4.1 The current system

The cross-correlator currently in use on Plateau de Bure, called CAMEMBERT^4.3, comprises six independent units. It consists of three parts: an IF processor (frequency setting, low-pass filter selection, oscillator phase control - i.e. the analog functions), a digital part, controlled by a master processor (i.e. delay steps, clipping correction, FFT, small delay corrections, bandpass correction), and a satellite micro reading out and further processing the correlations.

Each unit can be placed in the [100,600] MHz IF band, in steps of 0.625MHz (by using a third frequency conversion). The 40, 80 and 160MHz bandwidths are synthesized by the adjacent upper and lower sidebands of an image rejection mixer. Low-pass filters select the desired bandwidth. Note that these bandwidths show the Gibbs phenomenon right in the middle of the band (i.e. at the edges of the IRM sidebands). The central two channels are flagged by default, the observer should avoid to place the most important part of the line there. The 10 and 20MHz bandwidths do not have this restriction: for the 20MHz bandwidth, only the upper sideband of the IRM is used. The 10MHz bandwidth (only unit 5) is produced by slowing down the clock rate from 40 to 20MHz. The spectroscopic capabilities of the cross-correlator at Plateau de Bure are summarized in Table 4.5. Part of the flexibility is achieved by using the ``time-multiplexing'' technique. For example, a time-multiplexing factor four means that the data, arriving at a rate of $160\times 10^6$ samples/s, are alternately put into four shift-registers. The shift registers are read out at the clock frequency of 40MHz, thus creating four data streams taken at a rate that is lower by a factor of four (as compared to the sampling speed). Equivalently, a time-multiplex factor two means two data streams at a rate of 80MHz each.

 

Table 4.5: The Complex Cross Correlator on Plateau de Bure

Bandwidth	Sideband	Clock	Time	Number	Complex	Channel	Spectral
	of IRM(1)	Rate	Multiplex	of Lags	Channels	Spacing	Resolution $[\mbox{MHz}]$
$[\mbox{MHz}]$		$[\mbox{MHz}]$	Factor	(2)		[MHz]	(3)	(4)
160	USB + LSB	40	4	64	64	2.500	3.025	4.000
80	USB + LSB	40	4	128	128	0.625	0.756	1.000
40	USB + LSB	40	2	256	256	0.156	0.189	0.250
20	USB	40	2	512	256	0.078	0.094	0.125
10	USB	20	1	512	256	0.039	0.047	0.062

 
Notes: (1) image rejection mixer (2) with negative & positive time lags (3) box-shaped time-lag window (4) Welch time-lag window