on decimate filters

Thank you.

"Ray Andraka" <ray@andraka.com> ??????:1Linh.251$rv1.204@newsfe21.lga...
>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.
>
>
> 

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.

Sun Lei.
"Tim Wescott" <tim@seemywebsite.com> 
??????:bKSdnfbF6IURqADYnZ2dnUVZ_vCknZ2d@web-ster.com...
> SunLei wrote:
> (top posting fixed)
>>
>> "Tim Wescott" <tim@seemywebsite.com> 
>> ??????:3pGdnfLyvYFcTgbYnZ2dnUVZ_sqdnZ2d@web-ster.com...
>>
>>>SunLei wrote:
>>>
>>>(top posting fixed)
>>>
>>>
>>>>"Tim Wescott" <tim@seemywebsite.com> 
>>>>??????:Qr-dnfY8TZaFIwTYnZ2dnUVZ_v-tnZ2d@web-ster.com...
>>>>
>>>>
>>>>>SunLei wrote:
>>>>>
>>>>>
>>>>>>hi,
>>>>>>
>>>>>>   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 
> a picture, and include a link.  There are web sites out there that let you 
> post a picture and give you a link, then force us to look at ads when we 
> look at it -- I have no idea how to use those, since I have my own site.
>
> -- 
>
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com
>
> 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