FX Correlator

Figure 9.3: A spectral correlator using filter bank and complex multipliers.

Figure 9.4: Block diagram of an FX correlator.

The band limited signal can be decomposed into spectral components using a filter bank. The spectral visibility is then obtained by separately cross correlating each filter output using a complex correlator (see Fig. 9.3). The digital implementation of this method is called an FX correlator (F for Fourier Transform and X for multiplication or correlation). The GMRT correlator is an FX correlator. A schematic of an FX correlator is shown in Fig. 9.4. The analog voltages $V_1(t)$ and $V_2(t)$ are digitized first using ADCs. The geometric delay in steps of the sampling intervals (integral delay) are then compensated for. The integral delay compensated samples are multiplied by the output of NCO for fringe stopping. The samples from each antenna are then passed through an FFT block to realize a filter bank. Phase gradients are applied after taking the Fourier Transform for fractional delay compensation. The spectral visibility is then measured by multiplying the spectral components of one antenna with the corresponding spectral components of other antennas. These are then integrated for some time to get an estimate of the cross correlation. Since the Fourier transform is taken before multiplication it is called an FX correlator. For continuum observations with an FX correlator the visibility measured from all spectral components can be averaged after bandpass calibration.