The MAC subsystem is divided into six subracks-one in each FX rack. Each subsystem accommodates 11 MAC cards and a control card within a subrack. A total of 192 x 11 signals are required to be accepted by this subrack from externally located FFT subsystems, and distributed to the 11 cards after some re-ordering of bits to suit the MAC card pin assignments. In view of the large number of signals, wire wrapping would have been very cumbersome and also difficult to maintain. Hence, A PCB was specially fabricated instead. In view of the large power dissipation ( about 550W per subrack ) to be accommodated, and in view of the complexity of the signal routing, a 16-layer PCB was designed. Keeping in mind the relatively small number of such PCBs required in the system, a cost-effective solution was arrived at by making all the MAC backplanes identical.The distribution of MAC cards adapted for this purpose is shown in figure. In this figure, the numbers '1' through 'A' denote the 10 FFT cards. Numbers without a prime (') denote FFTs from second rack while two primes ('') denote FFTs from the third rack. The resulting arrangement ensured that no cable needed to cross a rack to connect an FFT output to a MAC card. Also, the cable lengths were kept to be minimum, but of equal length. This set of three racks processes one of the two side- bands. The other side-band is processed by an identical set of three FX racks. Both sets of the three FX racks thus realise a 30-X-30 array of multipliers shown schematically in the figure. It can be seen that second rack has the maximum number of cross correlators and also that most of its inputs are derived from the neighbouring racks. The backpanel design was thus based on the second rack, since it is the most generic one. The arrangement for first and third racks can be derived from the second. The figure shows the actual 30-by-30 array realisation according to the arrangement of the backpanels. Ideally, all results should have occupied the same triangle. The mixture is the price paid for the optimission. A few of the results will thus require a sign flip in their imaginary parts to obtain their complex conjugates so that all numbers can be thought of as originating from the same triangle of the 30-by-30 matrix.
GOBACK TO MAC SUBSYSTEM