Keywords: pdbi-freq-setup

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## Choice of Correlator Setup

The correlator offers 6 units, of up to 160 MHz width each, with a total of up to 256 channels per baseline per unit. Each unit is connected to a variable frequency local oscillator (LO3) followed by an image rejection mixer for conversion of the IF band (100-600 MHz) into baseband (0-80 MHz). The LO3 frequencies are adjustable by step of 0.625 MHz, with a small offset of 50/512 MHz added because of the general frequency scheme.

40, 80 and 160 MHz bandwidths are obtained by using the USB and LSB outputs of the LO3 image rejection mixer (separation better than 30 dB), with a filter of 20, 40 and 80 MHz respectively. Sampling is done at 160 MHz. At 160 and 80 MHz bandwith, correlator boards operate with a time multiplex factor (TMF) of 4 and 2, producing 64 and 128 channels respectively. At 40 MHz, 256 channels are obtained. The 20 MHz band is obtained by using the LSB output of the mixer, and cascading two correlator boards in pass-along to obtain also 256 channels, with a resolution of 0.078 MHz.

The frequency coverage of the correlator is thus extremely flexible, but care must be taken of the compromise between bandwidth and resolution. If more than one line has to be observed simultaneously, the observer should be careful to choice the appropriate setup, but many lines can be observed simultaneously, specially when one takes into account coincidences between the Upper and Lower sidebands (lines separated by 2.6 to 3.5 GHz). Note that in practice, 3 or 4 units are left in broad band mode to provide continuum flux measurement (and accurate amplitude and phase calibration), so that the setup currently concerns 2 or 3 units.

The `ASTROLINE` command, in the ASTRO program implements a graphic display of the frequency setup, with overlaid line frequencies. The spectral correlator bands are displayed according to the setup specified in the `/SPECTRAL` option. The syntax of this option is identical to that of command `OBSSPECTRAL` in the OBS program.

Command `ASTROLINE/SPECTRAL Unit Band LO3` can be used to overlay a spectral correlator setup on a plot produced by a previous `ASTROLINE/SPECTRAL Unit Band LO3`.

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