+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Full Member level 4
    Points: 1,808, Level: 9

    Join Date
    Jul 2014
    Posts
    219
    Helped
    14 / 14
    Points
    1,808
    Level
    9

    doubt with cic filter

    hi,

    i just designed a cic filter to down sample input signals from 65 mhz to 10 khz. which will give a decimation factor of 6500.

    i decided to implement this in cascaded manner :: that is 3 cic filter cascaded together which will have R:20,25 and 13 ?

    where the max working register length is 38 bit width, if i selected 5 stages

    now question is how can i select the NUMBER OF STAGES in the cic filter..?????

    i read few articles and i didnt find any criteria for deciding N.. most of the papers they simply decided the N as 4,5 10 etc
    and please excuse me if this is a stupid question :(

    can anybody help me on this?

    thanks and regards

    •   AltAdvertisment

        
       

  2. #2
    Super Moderator
    Points: 249,188, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    43,363
    Helped
    13184 / 13184
    Points
    249,188
    Level
    100

    Re: doubt with cic filter

    i read few articles and i didn't find any criteria for deciding N
    Sounds like you missed a basic introduction to CIC decimators. The number of stages N is the filter order, you can visualize it's effect by looking at the CIC response either in time or frequency domain. In frequency domain, the magnitude characteristic is approximately (sin(x)/x)^N with the first zero at the output sampling frequency, the exact expression is

    Click image for larger version. 

Name:	cic-response.jpg 
Views:	0 
Size:	9.5 KB 
ID:	135521

    The filter order is usually commanded by the required suppression of out-of-band signals that are folded into the base band. N=2..4 is reasonable for many applications.

    The smooth (sin(x)/x)^N characteristic of CIC decimation filters is often unwanted in signal processing applications, e.g. for oversampling ADC. Therefore a FIR decimator with steep frequency characteristic or a CIC with compensating filter is often preferred for the last decimation stage.


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  3. #3
    Full Member level 4
    Points: 1,808, Level: 9

    Join Date
    Jul 2014
    Posts
    219
    Helped
    14 / 14
    Points
    1,808
    Level
    9

    Re: doubt with cic filter

    thanks for the replay FvM,

    I am using a fir filter in the final stage as compensation filter.
    i am doing it iin verilog. so everything i am able to calculate except "N".
    my decimation rate is 5600=65Mhz/10KHZ :: and internal width of register is Bin+N*log(RM),, where M=1;

    but i got a confusion about how i will decide value of N.

    regards



    •   AltAdvertisment

        
       

  4. #4
    Super Moderator
    Points: 249,188, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    43,363
    Helped
    13184 / 13184
    Points
    249,188
    Level
    100

    Re: doubt with cic filter

    As said, it depends on the required of band suppression by the filter. Need to know the input signal spectrum, characteristic of analog anti-aliasing filters in front of the ADC and acceptable aliased signals.


    1 members found this post helpful.

--[[ ]]--