CIC filter design issue in MATLAB

[kataria0]

cic filter

Hello
I want to design CIC filter in MATLAB with just 3 variables:
Order, Decimation factor, & input data rate.

The problem is when I use MATLAB's "filterbuilder"and select CIC I can specify decimation factor and input data rate only. The order is determined for you automatically based on passband and attenuation specifications.
2 questions:
1. Is there a way for me to design CIC without worrying about passband and attenuation specifications but rather based on the 3 variables I mentioned in the beginning?

2. How is passband defined in MATLAB?

Thanks for reading. I hope to get a reply soon.

 

[FvM]

matlab cic

I don't use MATLAB CIC tool, but generally (e. g. in Hogenauers paper) passband/attenuation would be just a point on the filter characteristic. It's (sin(x)/x)^n, so it's easy to calculate the relation to CIC order, I think.

 

[kataria0]

cic filter matlab

Hello FvM
The problem is I don't understand their passband definition and therefore I can't figure out from the equation what roll-off to expect at so called "passband".
Their roll-off at passband is not fixed. Its weired.

 

[wqy1985]

cic filter design

try FDATOOL in matlab

 

[FvM]

cic matlab

In Hogenauers paper, passband is an arbitrary choosen frequency, usually given by the CIC filter application. Filter characteristics are calculated related to this frequency.

 

[wqy1985]

designing cic filter in matlab

the frequency response is determined by the R(decimation facor),N(the order of the CIC) and the M (defferential delay). the passband is determined by them~~~
is the passband edge is the at the -3dB ?

 

[FvM]

matlab cic filter

In ususal CIC terminology, passband isn't the -3dB bandwidth.

As part of the filter design process, R, M and N are choosen to provide acceptable
passband characteristics over the frequency range from to zero to a predetermined
cutoff frequency fc expressed relative to the low sampling rate.

The design examples in Hogenauers paper have passband attenuations at fc between 0 and about 6 dB.

 

[wqy1985]

cic filter

you are right. It is not at the -3dB. In my project, it is about -0.22dB.