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A real-time spatio-temporal (ST) 3D FIR Frustum filter is proposed for improving the SNR of broadband focal plane array (FPA) signals. The 3D stop band of the proposed filter attenuates non-reflected off-dish signals, such as radio frequency interference (RFI), and also attenuates noise due to local thermal sources (e.g., ground) as well as receiver noise. The frustum-shaped 3D pass band transmits, with low distortion, desired broadband reflected celestial signals, as received by the focal plane array (FPA) of receiving elements. It is shown that, for pulsar-timing and pulsar-profile studies, the pre-processing of FPA signals using the proposed 3D ST Frustum filter-based beamforming method, prior to coherent de-dispersion, yields lower distortion than the 3D conjugate field matching (CFM) method (when appropriately extended from 2D to 3D) and also lower distortion than the conventional 2D spatial-only beamforming method.