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Figure 25.3: Block diagram of the delay-DPC unit of the GMRT correlator

The block diagram of the delay and data preparation card (delay-DPC) is shown in Fig. 25.3. Each basic unit of the delay-DPC takes the four outputs of ADCs corresponding to (see Fig 25.1) the signals $R_U, R_L, L_U$ and $L_L$ from a given antenna. These 6 bit quantized signals are rounded off to 4 bits and then sent to a multiplexer. The multiplexer has various modes; for example any one of the four inputs of the multiplexer can be mapped to all four of its outputs ($A,B,C,D$ in Fig. 25.3). Other mappings include (a) $A = R_L$, $B = L_L$, $C = L_L$, $D = R_L$, and (b) $A = R_U$, $B = L_U$, $C = L_U$, $D = R_U$, which are used for polarization observations with the correlator. The multiplexer outputs are passed through a memory based integral delay compensation circuit (see Chapter 9). The delay compensated outputs are then fed to the FFT subsystem.

The rate at which data is written to the memory in the dly-DPC card is tunable. In particular it can be any one of $32/2^k$ MHz, where $k = 0~{\rm to}~k = 7$. This rate is chosen to be the Nyquist rate for the input signal bandwidth, i.e. for bandwidths smaller than 16 MHz, the rate is less than 32 MHz. However, the data is always read out at a constant rate of 32 MHz25.3. To maintain the data throughput, data from the memory hence has to be read out in an `overlapping' fashion. This way of reading the data provides the facility to perform `overlapping' FFTs (and hence an improvement in the signal to noise ratio) when the input bandwidth is less than 16 MHz.

The two other functions of the delay-DPC system are (a) gain measurement (b) Walsh demodulation.


... 32 MHz25.3
In the final correlator it will be at 32.25 MHz.

next up previous contents
Next: FFT Up: An overview of the Previous: ADC   Contents