>A few points:
> 1) A CIC filter does not have pass-band ripple. It has a sin(x)/x droop,
> but no ripple.
> 2) The usual set-up is to follow the CIC with a decimating FIR filter or
> FIR filter bank to narrow the bandwidth to a small fraction (less than
> 1/4) of the CIC main lobe width and to flatten the passband response. The
> advantage of using the CIC is the CIC's response referred to the output
> sample rate is nearly independent of the decimation ratio. That allows
> you to use a fixed FIR filter or Filter bank (fixed meaning you do not
> have to adjust the coefficients when you change the decimation ratio)
> after the CIC. This scheme works fine even with small (e.g. decimate by 1
> or 2) decimation ratios in the CIC since the pass band shaping is handled
> by the FIR filter and the CIC is simply doing the pre-decimating and
> pre-filtering to get rid of images.
> IIR filters are often not appropriate in a DDC because it is difficult to
> make them stable, linear phase AND work over a large range of decimation
> ratios all at the same time. For digital comm, non-linear phase alone can
> be a deal-killer.
Reply by SunLei●January 5, 20072007-01-05
Thanks to God,
There's always a surprise to me, just like the linked software.
I will consult your suggestions and check my simulations, and report the
result to you later.
"Tim Wescott" <firstname.lastname@example.org>
> SunLei wrote:
> (top posting fixed)
>> "Tim Wescott" <email@example.com>
>>>(top posting fixed)
>>>>"Tim Wescott" <firstname.lastname@example.org>
>>>>>> On the FPGA implementation of decimation filters, which type of
>>>>>> decimate filters do you prefer?
> -- snip --
>>>Could you learn how to design stable IIR filters, and either cascade them
>>>with FIR filters or use them as-is?
>>>Could you use some CIC stages, then cascade that with a FIR filter to
>>>correct the passband ripple, either before or after sampling?
>> Yes, I will try on your suggestions later.
>> Maybe there are some misunderstanding between us, mainly because my
>> un-excactly word expressions. I don't know if there is a method to upload
>> figures, if so, I can depict the question a little more clear.
> | |
> | Try |
> --------->| this |---------->
> | one. |
> | |
> (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
> It's still ASCII text, but it will do for simple diagrams.
> If you have a web site you can also post your diagram to it in .pdf or as
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> look at it -- I have no idea how to use those, since I have my own site.
> Tim Wescott
> Wescott Design Services
> Posting from Google? See http://cfaj.freeshell.org/google/
> "Applied Control Theory for Embedded Systems" came out in April.
> See details at http://www.wescottdesign.com/actfes/actfes.html