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The delay corrected signal from each channel is then Fourier transformed in real-time. The
functions of FFT card can be broadly classified in the following :
- Fringe stopping : As the earth rotates, each antenna experiences different radial
velocities with respect to the source. Hence, the signal at each antenna is Doppler shifted
in frequency by different amounts depending on the position of the antenna in the array.
This effect is corrected.
- Data Windowing : At the first stage of FFT pipeline fringe stopped data is multiplied
by desired window function (512 independent sample) having applications in controlling
the side-lobe level of FFT.
- FFT : A 512 point FFT is achieved (one every
) by using a pipeline of
5 ASICs that can perform Radix-2 and Radix-4 FFT butterfly after suitable twiddling
operation of 32 MHz rate. This gives a continuous stream of 256 complex spectral channels.
- FSTC : Fractional sampler time corrections (i.e. delay corrections that are
fraction of the sampling interval) are achieved by multiplying the complex spectra output of
the FFT pipeline by the relevant phase gradients. Strictly speaking this is achieved by
adding suitable phase to the final stage twiddle phase (
) just before the last
radix-2 operation.
Next: Multiplier and Accumulator
Up: Correlator
Previous: Delay-DPC
Contents
Manisha Jangam
2007-06-19