Hi guys! I need to filter some samples that come in at 10MHz down to 100Hz, 1KHz and 10KHz. I thought using a CIC filter with a FIR afterwards to cut out aliased spectral components should be a good idea to do this. After all, everything should be implemented into a FPGA. But after some simulation in Scilab, this seems like a complex job. My -3dB frequency is around 3000Hz if I'm not doing sth. wrong in my Scilab program. Decimation Rate is 32, Waiting Cycles is also 32 while the number of stages is 3. Should I try some more calculations and implement it the way I planned or is there a better solution to my needs I don't see right now?

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Edited by User

Hi, CIC filter is a good way. I would try to decimate with ~3 concatenated CIC Filter, each with a FIR CIC-compensation filter. If you have access to Matlab, it is a quite easy task, you can even save the filter as vhdl.

this is the most crappy idea thinkable to stack several CICs. This all can be done witin one step if done correctly. Also stacking FIRs is not efficient and can be done in one step with appropriate numbers of TAPs.

So was my attempt with decimation down to 1KHz and a FIR to 100Hz a good approach? I have a Scilab-Program to determine the attenuation and want -3dB frequency to be at 1000Hz, but it crashes everytime I get below 5KHz ^^ Has anyone a better solution? Maybe for Octave?

1 | clear all; |

2 | z=poly(0,'z'); |

3 | R=256; // CIC divider Ratio |

4 | M=3; // CIC stages |

5 | N=1;// CIC delay stages |

6 | Scale=0; |

7 | num=(z**(R*N)-1)**M;// Based of 1-z**(-R*D) |

8 | den=(z**(R*N)-z**((R*N)-1))**M;// Based of 1-z**-1 |

9 | Pd=syslin('d',num,den); |

10 | |

11 | //Frequency Response |

12 | fsample=10e6/2**Scale;// Input sample frequency |

13 | fmin = 0.00001;// min frequency |

14 | fmax = 1; //max. Frequency (normalized), Nyquist frequency at 0.5 |

15 | //Pd=Pd**M; |

16 | Pdnom= (Pd+0.001)/(R*N)**M;//normalized transfer function |

17 | //scf(2); bode(Pdnom,fmin,fmax); |

18 | |

19 | [frq1,rep]=repfreq(Pdnom,fmin,fmax,0.0001);//Frequency response in 0.001 steps |

20 | |

21 | OutdB=10*log(abs(rep));//Amplitude in dB |

22 | frq_plot=frq1.*fsample; |

23 | scf(3); //clf; |

24 | plot("nl",frq_plot, OutdB,'black'); |

25 | xlabel('Frequency in Hz'); |

26 | ylabel('Magnitude in dB'); |

27 | xgrid(2); |

28 | title('Frequency Response of the first and the second CIC decimation Filter ') |

Tried a bit Octave, found some small programs to deal with. By building a CIC with 2048 decimation, 3 stages and 1 delay time I get a graph that tells me my -3dB frequency is at 1272Hz. Afterwards I designed a FIR with hamming window/1272Hz sampling rate and get around 104Hz @ -3dB and very steep attenuation. Does this look like a good design choice, do the values even make sense or did I screw up somewhere? ^^ Yeah, I'm fairly new to filters indeed...

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Edited by User

where is the problem using a cic with more then ratio 2048 ? What is the target for a) 10MHz filtering, ... b) ... c) 100 kHz? Which shall be the edge freuency?

If you decimate by a high ratio the number of bits needed in the CIC grow, especiially with higher order CIC stages. Therefore it might be better to cascade CIC stages.

Martin O. wrote: > If you decimate by a high ratio the number of bits needed in the CIC > grow, > especiially with higher order CIC stages. Therefore it might be better > to cascade CIC stages. you are not telling us, that some tiny little bits will increas an FPGA design much more, than tripeling it by using it three times? An FPGA decimation with CIC is notinh else than a counter and a sampler for the counter and some adds and sub. More bits will increase the counter size.

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