6.4 The correlator on Plateau de Bure

As an example of a cross-correlator used in mm-wave interferometry, I briefly introduce the correlator system on Plateau-de-Bure. Only a spectroscopic correlator is in use. Continuum bands are synthesized by channel averages covering the desired bandwidths. Aspects concerning concrete observing projects are addressed in Chapter 8 by R.Neri.

6.4.1 The third-generation correlator

The third-generation correlator for the Plateau de Bure interferometer will allow for more flexibility, due to the following improvements:

global bandwidth: 2.56GHz (vs. 0.96GHz in the second generation system),
flexibility: 8 units with channel spacings in powers-of-2 sequence (vs. 6 units, channel spacing in powers-of-4 sequence)
global digital performance: 9.8 Teramultiplications per second (vs. 1.3TMs $^{-1}$ ).

These improvements are made possible by using new, more integrated technology at both analog and digital signal processing steps.

The cross-correlator comprises eight independent units. Each 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

MHz IF band ^6.4, in steps of 0.625MHz (by using a third frequency conversion). There are seven combinations of bandwidth and channel spacing. The channel spacing follows a power-of-two sequence. Three out of the seven modes are synthesized by the adjacent upper and lower sidebands of an image rejection mixer. 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 highest possible spectral resolution (channel spacing 0.039 MHz) 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 6.5. 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.

For further technical specifications see the Correlator Web page.

**Table 6.5:** The Complex Cross Correlator on Plateau de Bure
Bandwidth	Sub-band	Clock	Time	Number	Complex	Channel	Spectral
	of IRM $^{(1)}$	Rate	Multiplex	of Lags	Channels	Spacing	Resolution MHz
MHz		MHz	Factor			[MHz]	(3)	(4)
$2\times 160$ MHz	LSB + USB	80	4	$2\times 128$	$2 \times 64$	2.500	3.018	3.975
$1\times 160$ MHz	LSB or USB	80	4	$1\times 256$	$1 \times 128$	1.250	1.509	1.988
$2\times 80$ MHz	LSB + USB	80	2	$2\times 256$	$2 \times 128$	0.625	0.754	0.994
$1\times 80$ MHz	LSB or USB	80	2	$1\times 512$	$1 \times 256$	0.312	0.377	0.497
$2\times 40$ MHz	LSB + USB	80	1	$2\times 512$	$2 \times 256$	0.156	0.189	0.248
$1\times 40$ MHz	LSB or USB	80	1	$1\times 1024$	$1 \times 512$	0.078	0.094	0.124
$1\times 20$ MHz	LSB or USB	40	1	$1\times 1024$	$1 \times 512$	0.039	0.047	0.062