# CIC or FIR decimation filter

1. ## CIC or FIR decimation filter

Hi,

I'm currently writing my bachelor's thesis. I read many research papers and came up with a very efficient and fast FIR filter structure. However, yesterday I received the parameters of the filter needed at the institute in order to program it on an FPGA. And to my surprise, they need a high-pass filter with a pass band from 10Hz to 1MHz! Which means most of my work so far has been for nothing, since the filter order required to design such a filter is around 50000. My idea is to filter the high frequency signal from the original signal (narrow band low-pass) and subtract the resulting filter from the original. Am I on the right track?

The narrow band LPF, must be designed using a decimation filter and in my case since fs/2 is huge (at least 1 000 000 Hz), very large decimation factors must be used. I did a bit of research, and it seems CIC-filters are recommended for such decimation. However , I'm looking for a way to use FIR filters in the design, so my whole work during the past 4 months won't go to waste (at least not completely). Any thoughts?
So to sum it up: CIC filter or FIR decimation filters to achieve decimation factors of 50-100 ?

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2. ## Re: CIC or FIR decimation filter

Recited from MATLAB Help:
Cascaded Integrator-Comb (CIC) filters are a class of linear phase FIR filters comprised of a comb part and an integrator part.

So CIC is a FIR filter too.
I think its no matter what filter to use, but the best design is to represent your overall decimation factor as a multiplication of prime numbers and build a cascade of filters with these decimation factors. For example, 100 = 2*2*5*5.

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3. ## Re: CIC or FIR decimation filter

Originally Posted by Mityan
Recited from MATLAB Help:
Cascaded Integrator-Comb (CIC) filters are a class of linear phase FIR filters comprised of a comb part and an integrator part.

So CIC is a FIR filter too.
I think its no matter what filter to use, but the best design is to represent your overall decimation factor as a multiplication of prime numbers and build a cascade of filters with these decimation factors. For example, 100 = 2*2*5*5.
Thanks for the reply. Yes I know CIC are special multiplierless FIR filters. What I meant was, I developed a new efficient structure for multipliers and would like to use it in my design. However I think I'm going to use a CIC filter and an FIR compensation filter, so I can apply it to the conventional compensation FIR.

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4. ## Re: CIC or FIR decimation filter

As you mentioned that you came up with efficient and fast FIR Filter structure, since you didn't mention more details about your FIR filter structure so can't say much about that but CIC filters are multiplier less filters but doesn't have sharp transition so CIC filters are mostly followed by compensation filters BUT if your FIR filter structure is efficient and really fast one (as you mentioned) than implementing low pass filter using that won't be a bad idea.

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