The FFT card employs a two-tier structure consisting of a FFT main board and pipeline board. The main board contains all the logic circuitry of the FFT card while the pipeline board contains only the ASICs. Both boards are 6 layer PCBs, 1.6mm thick with 35µ copper plating on all layers. The main board is 100mm by 280mm while the pipeline board is 100mm by 246mm. The layer assignments for both boards are same. Both the PCBs are designed with 8mil track width and 8mil track-to-track spacing design rule. The via size used is 16mils.
The main board which plug into the backpanel, has one 128-pin EURO ( 4 rows X 32 pins ) as the backpanel connector and one 26-pin FRC for the input from Delay Subsystem. In addition, it has three 100-pin surface mount connectors for the pipeline board. The FFT backplane is a 3.2mm, 6 layer PCB. Its construction is the same as that of the FFT Card. Like all others, two Berg pins are provided for ECL clock connections.
A total of 60 FFT cards are required for the entire system. Each background supports 10 FFT cards in addition to two FFT control cards. The inter-card spacing is 7T (= 1.4 inch ). Thus, a total of 6 sub-racks make up for the entire requirement. The 10 FFT cards in each subrack are split into two groups of five each. Each group has its own control card. These two groups are independent of each other, except for the power and ground layer of the backpanel. Each FFT subrack is housed in one of the six FX racks.
The interconnection of the FFT with the Delay subsystem is already described in the discussion on the Delay subsystem. The FFT cards are connected to the MAC subsystem by 50-core round twist-n-flat jacketed shielded cables. Two FFT card outputs are combined into one 50-pin FRC connector and is transmitted to a distribution system, as TTL signals interleaved with ground.