FFT CONTROL CARD
The FFT Control-card performs the following tasks:
- Supplies Twiddle Factor Values
- The ASICs need external twiddle factors to perform the DFT. These are required by all stages in the FFT pipeline except stage 0. This is so because in stage 0, the twiddle factors are incorporated in the hardware logic of the ASIC. The Stages 1.2 and 3 get their twiddle factors from the control card. In the last stage, Stage 5, the twiddle factors are combined with the FSTC and hence are supplied from the on-board RAM.
- Supplies INITs
- The FFT INITs to all stages, 0 through 5 are generated and distributed by the control card.
- Loads Fringe Values
- The NCO for each pipe-line on every FFT card is programmed with new values of initial rate and increment every STA cycle to accomplish fringe stopping.
- Loads the FSTC Values
- Similar to the fringe values. One number per FFT card, ranging from 0 to 31, pertaining to the almost of FSTC is loaded once every few FFT cycles to compensate for the change in fractional delay.
- Initialisation of the FFT Cards
- At the beginning of every observing session the control cards perform the initialisation of all the FFT cards. The following tasks constitute this initialisation:
- Loading Control Words into the ASICs
- All ASICs in the FFT cards are loaded with required control words. These control words, each 32 bit wide, define parameters such as the length of FFTs to be performed, sequence of polarisation in the FFT output, amount of scaling in the experiment fields of the numbers etc. All the ASICs within a card form a shift register 320 bits ( 10 ASICs X 32 bits each ) long, into which the FFT control card shifts the control word.
- Loading on-board RAMs
- The on-board RAMs on the FFT card contain
- The External Addresses for Stages 0 and 5 of the FT pipeline
- A user definable time domain window
- The FSTC tables
- A few NCO control signals
BLOCK DIAGRAM OF FFT CTRL CARD
SCHEMATIC
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