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The RSP boards at the station level can operate in several bit modes (16,8,5,4), where a mode corresponds to the number of bits in each of the real and imaginary components of the complex number obtained by the first stage polyphase filter bank. Modes with a lower number of bits allow the transfer of more subbands to the correlator [16b->16subs, 8b->32subs, 5b->48subs, 4b->64subs]. These subbands can be any random subset of the 511 subbands spread over the 100MHz sampled bandwidth of LOFAR. This document is about the mapping of subbands as seen at the hardware, to those seen in the output of the correlator. This is important because the calibration module needs to know the frequency of a subband for calibration.
Obtained via running the command 'rspctl --sdo' on individual stations. A sample output in 16b mode is:
RCU[94].subbands=1 x 36
[ 590 592 594 596 598 600 602
604 606 608 610 612 614 616
618 620 622 624 626 628 630
632 634 636 638 640 642 644
646 648 650 652 654 656 658
660 ]
RCU[95].subbands=1 x 36
[ 591 593 595 597 599 601 603
605 607 609 611 613 615 617
619 621 623 625 627 629 631
633 635 637 639 641 643 645
647 649 651 653 655 657 659
661 ]
The RCU number corresponds to twice the number of antennas, one for each polarization. The subband numbers are contiguous between 0-1024 due to having the same running counter across both polarizations. Here, we see the AARTFAAC sdo setting 36 subbands from 295 (=590/2) to 330 (=660/2). Out of these, only 16 make it to the correlator.
The subbands in 8b sdo mode are:
RCU[94].subbands=2 x 36
[ 590 592 594 596 598 600 602
604 606 608 610 612 614 616
618 620 622 624 626 628 630
632 634 636 638 640 642 644
646 648 650 652 654 656 658
660
662 664 666 668 670 672 674
676 678 680 682 684 686 688
690 692 694 696 698 700 702
704 706 708 710 712 714 716
718 720 722 724 726 728 730
732 ]
RCU[95].subbands=2 x 36
[ 591 593 595 597 599 601 603
605 607 609 611 613 615 617
619 621 623 625 627 629 631
633 635 637 639 641 643 645
647 649 651 653 655 657 659
661
663 665 667 669 671 673 675
677 679 681 683 685 687 689
691 693 695 697 699 701 703
705 707 709 711 713 715 717
719 721 723 725 727 729 731
733 ]
These are the 72 subbands, of which only 32 make it to the correlator. Which of the subbands reach the destination are explained next.
The AARTFAAC correlator is split over two identical machines (agc001 and agc002), each correlating half of the total subbands from 12 stations. The inputs to each correlator are udp streams containing the relevant subbands from each station. The outputs are tcp streams of correlated visibilities per subband.
We name the subbands in the order they appear at the RSP board output, in groups of 7. So, in 16-bit mode:
RCU[94].subbands=1 x 36
[ sb00, sb01, sb02 ... sb06,
sb07, sb08, sb09 ... sb13,
sb14, sb15, sb16 ... sb20,
sb21, sb22, sb23 ... sb27,
sb28, sb29, sb30 ... sb34,
sb35]
In 8b mode, the subbands continue after the above:
[ sb36, sb37, sb38 ... sb42,
sb43, sb44, sb45 ... sb49,
sb50, sb51, sb52 ... sb56,
sb57, sb58, sb59 ... sb63,
sb64, sb65, sb66 ... sb70,
sb71]
Here, the list of subbands corresponds to the output order as generated by the correlator. The subband numbers are as seen in the RSP template output.
agc002 = [sb00, sb01, sb02, sb03, sb04, sb05, sb06, sb07]
agc001 = [sb18, sb19, sb20, sb21, sb22, sb23, sb24, sb25]
Here, the list of subbands corresponds to the output order as generated by the correlator.
agc002 = [sb00, sb36, sb01, sb37, sb02, sb38, sb03, sb39, sb04, sb40, sb05, sb41, sb06, sb42, sb07, sb43]
agc001 = [sb18, sb54, sb19, sb55, sb20, sb56, sb21, sb57, sb22, sb58, sb23, sb59, sb24, sb60, sb25, sb61]
Currently (05Sep16), only half the subbands are available in 8-b mode. The remaining half are corrupted at the RSP board level, and carry only the values -1, 0 or 1. This was confirmed by examining the contents of the Uniboard backnode FPGA input buffers after dumping them to disk.
Note that the LOFAR processing chain has a similar (but not the same) logic block in the RSP board to support the LOFAR 8-b mode. This block runs fine. The AARTFAAC block is different because it also supports 5-b and 4-b mode of operation. The cause of the corruption is currently being investigated. Assuming the same subband layout as detailed above, the subbands currently being corrupted are:
[ sb36, sb37, sb38, sb39, sb40, sb41, sb42,
sb43
sb54, sb55, sb56, sb57, sb58, sb59, sb60,
sb61]
Of these, sb36-sb43 go to agc002, and sb54-sb61 go to agc001.