Hi Geoff, might be a good idea to ask this on usrp-us...@lists.ettus.com, because these CICs are not part of GNU Radio :)
Anyway: There's nothing much to interpret about CIC responses; they are well-known. The length (and potentially order, as in exponent) of the CIC used depends on your specific USRP model. (and, potentially, UHD version) It's rare that one actually designs an equalizing filter for the CIC – in all USRPs, the CIC comes before the halfband filter(s), and thus, you'd typically only see "the flattest part" of the frequency response of the CIC. Designing a compensating filter is mathematically impossible, but you can alleviate certain aspects of the CIC impulse response. But this would really require you to state what the target metric is over which bandwidth. Also, it's almost certain that this filter would be computationally much worse than just oversampling at the USRP end and then decimating in software with a filter that fits what you need better. Notice that I very much believe that "what you need" is the critical aspect here: Occam's razor tells me that unless you can define why the CIC is your problem very well, then it's likely you're trying to solve something that doesn't need solving (or that equalizing parts of the CIC response isn't the answer you're looking for ;) ). So, if you asked me: Describe what your system need is, and how you came to the conclusion that the CIC is a problem! This mailing list is very prone to having interesting discussions about real-world problems where every party learns a lot. Best regards, Marcus On Wed, 2018-04-25 at 16:39 -0400, Geoffrey Mainland wrote: > I hope you will forgive a few naive signal processing questions :) > > 1) What is the frequency response of the CIC and halfband filters in the > USRP? This was addressed long ago on the mailing list > (https://lists.gnu.org/archive/html/discuss-gnuradio/2007-05/msg00191.html), > but the associated MATLAB script is no longer available now that the > nabble archive is gone. > > 2) How do I design a filter to compensate appropriately? > > I know there are other methods for avoiding CIC droop, like > oversampling, but I'd like to understand how to compensate with a filter > as well. > > Thanks! > Geoff > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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