On Mon, 17 Aug 2009 22:32:57 -0500 Vladimir Vassilevsky <nospam@nowhere.com> wrote:> > > Rob Gaddi wrote: > > I'm primarily an analog and logic designer, so please bear with my > > lack of DSP chops. > > > > I've got an application where I'm looking to feed a DAC at a > > constant 128 MSPS from a data stream that will be a power of 2 > > fraction of that (64 MSPS down to 250 kSPS). I'd like to be able > > to get pretty close to the Nyquist limit (say 0.4 or so) of > > whatever the original data rate is. It seems so far like my two > > primary choices for interpolating the data up are: a) a CIC filter > > plus an FIR precompensator to clean up the sinc response of the > > CIC, followed by a barrel shifter to get rid of the bit growth or > > b) a single (or possibly a cascade of?) FIR interpolation filters. > > So the max. upsampling ratio is 512; you will probably need 4 or 5 > stages of interpolation for that. It could be more optimal to design > the separate filter arrangements for each mode rather then use the > same halfband filter for everything. > > > The implementation platform will be a Spartan-6 FPGA, which means my > > primary DSP tool is a dedicated multiply+add block, which can of > > course be configured as a multiply+accumulate. Ultimately, the DAC > > feeds a fixed 40 MHz LC filter; by the time I get everything out the > > door I'd like to be ~60dB down for all my aliases. > > > > Anyone have any recommendations on which of these I'd be better off > > with? > > That entirely depends on your application requirements (passband > flatness, stopband rejection) and the hardware capabilities. You may > need to try the different variants. > > > Or better alternatives that I haven't considered? > > The IIR filters can be used as well. > > > Or pointers > > to good lit on the subject? The multirate section of O&S is both > > short and impenetrable, Googling "FIR halfband" turns up lots of > > IEEE chaff and no wheat, and "multirate digital signal" on Amazon > > gives me dozens of books and no insight as to which are any good. > > I would recommend the classic book: "Multirate Signal Processing" by > Rabiner. > > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultant > http://www.abvolt.comThanks, everyone, for the help. Vladmir, you were right about Crochiere & Rabiner. The book was a pain to hunt down, but I just put away 100+ pages of it last night and it's really very well done. -- Rob Gaddi, Highland Technology Email address is currently out of order