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Digital Filters

Digital Filters. Mike Davis. Requirements. Avoid non-linearity up to and through the analog to digital (A/D) converter Use enough bits to adequately represent the signal plus noise plus interference (6 dB/bit) Use enough bits in the weighting factors not to lose dynamic range

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Digital Filters

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  1. Digital Filters Mike Davis

  2. Requirements • Avoid non-linearity up to and through the analog to digital (A/D) converter • Use enough bits to adequately represent the signal plus noise plus interference (6 dB/bit) • Use enough bits in the weighting factors not to lose dynamic range • Count only effective A/D bits (typically 6.5 to 7 for a good 8-bit A/D converter)

  3. Filter Design • Finite Impulse Response (FIR) filters provide a tapped delay line, with complex weights at each tap • The impulse response corresponds to the tap weights. In principle, any impulse response can be created with an adequate number of taps

  4. Practical Filters • Half-band filters • Commercially available • Can be cascaded for multiple octave bandwidth selection • Used in Arecibo correlator very successfully • Polyphase Filter Banks • Arrangement to get reproducibly identical filter shapes in EACH frequency bin

  5. F-X Correlator Design for the Allen Telescope Array • Uses Polyphase Filter Bank for each of 350 antennas, with Field Programmable Gate Arrays • Outputs are cross-correlated for each pair of antennas, for each frequency • Isolation of interference by steep-sided frequency bin shape • Stay tuned – Matt Dexter is designing details of the PFB at Berkeley

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