doubt with cic filter

[dipin]

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

 

[FvM]

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



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.

 

[dipin]

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

 

[FvM]

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.