PdBI Correlator Status Report

The installation of the new correlator in September 2000 represents a major step in the technical development of the Plateau de Bure Interferometer. Since the last report in Newsletter 45 (August 2000), the installation and testing phase has been completed. A small review is given below.

The correlator has been flown to the Plateau de Bure on September 22nd, 2000 (Fig. 2, 3), and has been rapidly connected to the system (Fig. 4).

Its first light revealed a few minor problems:

1.

The analog IF level was higher than expected, and called for the insertion of extra coaxial attenuators. A residual slope across the IF band is present (2-3dB excess at the high end of the band). Part of the problem comes from the recent installation of some length of higher quality cables. A new coaxial cable equalization policy is under study. It will allow some stretching of the usable bandwidth of the coaxial cable signal transport system, since the optic fiber installation is postponed.

2.

The VVAs (Voltage Variable Attenuators, IC reference MA/COM AT65-0283) that define the sampling levels come from a poor batch and need a few seconds for stabilizing. The calibration sequence timing has been modified to take this effect into account, until the problem is electrically solved. As usual, this problem was not present on the prototype unit batch, so it remained undetected on the series units.

3.

A few microprocessor crashes are still present and being investigated.

  

Figure 2: Transport of the new correlator.

  

Figure 3: The crates have arrived.

  

Figure 4: The new correlator after its installation in the PdBI correlator room.

In spite of these small difficulties, a substantial improvement in bandwidth and resolution has been achieved, as can be seen from the spectrum in Fig. 5, showing amplitude and phase from 100 to 700 MHz, obtained with only 3 units (out of 8).

  

Figure 5: The new bandpass. Note that the practical use is currently limited to below 680 MHz.

The previous correlator has been flown down and is being refurbished in Grenoble to serve as an enhanced line backend for the new receiver setup on Pico Veleta (Fig. 6). More information on this project can be found at the URL http://iram.fr/TA/backend/concepts.html#3.

  

Figure 6: View of the rack to be used in the 30-m backend room.

The new backplanes have been designed and are being manufactured. The control board modifications are set and a small piggyback board needs to be designed. A coaxial distribution box prototype has been built and tested. Many coaxial devices are being ordered. The IF processor rack mechanical and power supply modifications are under design.

Marc TORRES