Forums

on decimate filters

Started by SunLei January 1, 2007
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