The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Nov 27th, 2020, 5:48pm
Pages: 1
Send Topic Print
Sigma-Delta Decimation Filter: CIC (Read 3001 times)
Christoforos
New Member
*
Offline



Posts: 2

Sigma-Delta Decimation Filter: CIC
Jun 13th, 2014, 6:30am
 
Hello everyone,

I have modelled a decimation filter after a Sigma-Delta converter in MATLAB. The passband frequency is 10 Hz and the downsampling ratio is 512. The decimation filter I am using is the Hogenauer CIC filter. The filter is stable up to and including the 3rd order. From 4th order it becomes unstable. Also, when I look at teh zplane the poles of the filter are just outside or the unit circle. Can someone help me to fix this?

Thank you
Back to top
 
View Profile   IP Logged
Burt Harris
New Member
*
Offline



Posts: 1
Woodinville, WA
Re: Sigma-Delta Decimation Filter: CIC
Reply #1 - Aug 29th, 2015, 6:04pm
 
I've been researching 4th order CIC filters as well.   I found a really good article here where Richard Lyons explains CIC filters http://www.embedded.com/design/configurable-systems/4006446/Understanding-cascad...

He says Although recursive, happily CIC filters are guaranteed stable, linear-phase shown in Figure 4b, and have finite-length impulse responses.    So, even without reviewing your model, I suggest the only problem might be that round off errors in Mathcad introduced small errors in the location of the poles and zeros.  It seems intuitively reasonable to me that they must be stable given their time-series behavior.

Lyon's book, Understanding Digital Signal Processing, 2Ed, apparently contains more detailed description.   Personally, after reading his very clear article, I'm ready got go buy a copy of his book.

Now, I'm interested in implementing a 4th order CIC filter for decimating input from a PDM digital Microphone using a low-density FPGA.    Can anyone help express such a filter in Verilog, I'm a novice with this language?

Back to top
 
 

/Burt
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2020 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.