ALMA Back End and Correlator

The ALMA Back End system links signals generated by Front End units installed in each antenna with the central Correlator installed in the Array Operations Site (AOS) Technical Building. Signal processing and data transfer is schematically shown in the figure on the right. Analog data, produced by the Front End electronics, is processed and digitized before entering into the data encoder, followed by the optical transmitter units and multiplexers. All these elements are installed in the receiver cabins of each antenna. Optical signals are then transmitted by fibres to the AOS Technical Building. The total distance is, in one antenna configuration, about 15 kilometres. At the Technical Building the incoming optical signals are de-multiplexed and de-formatted before entering the Correlator.

The European deliverables in the ALMA Back End project are various components, which are produced by several European institutes, closely working with ESO and NRAO. These deliverables are:

  • the digitizer chips production and assembly,
  • the digitizer clock and assembly,
  • the optical data transmission system design,
  • the fiber patch-panel,
  • the optical multiplexers (MUX) and de-multiplexers (De-MUX),
  • the photonic local oscillator photomixers.

The clock rate of the digitizer is 4 GHz, allowing an input bandwidth of 2 GHz. The digitization is 3 levels to preserve the signal to noise ratio. Comparable digitizer chips have been announced, but are not yet commercially available. During the prototype and development phase the initial layout was optimized in order to reduce the number of parts and the assembly costs. The final digitizers show an improved performance and a higher reliability. This work was carried out in close collaboration between ESO and the University of Bordeaux.

The ALMA Correlator

The ALMA Correlator, to be installed in the AOS Technical Building, is the last component in the receiving end of the data transmission. It is a very large data processing system, composed of four quadrants, each of which can process data coming from up to 16 different antennas. The complete correlator will have 2912 printed circuit boards, 5200 interface cables and more than 20 million solder points. The first quadrant was completed at NRAO in the third quarter of 2006. Work on the second quadrant is progressing on schedule.

Integral parts of the Correlator are Tunable Filter Bank (TFB) cards. The layout is such that four TFB cards are needed for the data coming from a single antenna. The TFB cards have been developed and optimized by the University of Bordeaux over the last few years.