Saturday, July 27, 2019

infinite impulse response - How to design very narrow, sharp lowpass filters - Only DC needed?


I asked exact the same here one year ago: One year ago question I asked this question again here, because after one year, I have learned more about DSP and am thinking ways to improve it.


Filter description




  • Filter input data rate is 64kHz




  • Filter output data rate is 0.5Hz - 2Hz





  • The filter is preferably to have bandwidth of 1mHz with very sharp transition area, since only DC is needed and the noises at 0-5Hz is large.




The answer I got


The averaging data is the best DC I can get. I bought this answer. However, it now becomes plausible to me, since a filter always have a bandwidth and frequency responses. Averaging is essentially a FIR with no superior characteristics.


Questions




  1. Shall the filter be designed in this way: 9 stages of halfband filters(64kHz -> 125Hz). Followed by three stages of cascaded integrator–comb (CIC) filters with R=5. Then the output will be fed into a 1st order IIR filter?





  2. Maybe 16 stages of halfband filters? (64kHz -> 0.9765625Hz) And then be fed into an IIR filter.




  3. How shall the filter be designed?






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