Hi Tom, This is very old topic. I have read a DSP book. But I find that I still not very understand channelizer. Why in channelizer use low pass filter, in my imagine channelizer will use band pass filter to filter each channel like that: 600Mhz baseband signal have 4 channel each channel have 100Khz bandwidth. so the channel freq is: 599.8Mhz 599.9Mhz 600Mhz 600.1Mhz. So I need 4 bandpass filter to filter each channel but actually the channelizer use lowpass filter, so why?
Thanks! On Thu, Dec 23, 2010 at 10:16 PM, Tom Rondeau <trondeau1...@gmail.com>wrote: > On Thu, Dec 23, 2010 at 3:53 AM, James Jordan > <james.jordan....@gmail.com> wrote: > > Thanks Tom. > > if I really don't know how pfb_channelizer_ccf and pfb_decimator_ccf do, > but > > they seem > > use the same principle. And how to set the taps? > > > Yes, they are based on the same principle, but the decimator just > extracts the 1 channel while the channelizer produces all channels. > > To create the taps, you want to build a prototype filter that will > have the bandwidth of the channelized signals at the input sampling > rate. So if the input to the channelizer is fs and the bandwidth of > each channel is B, you can build the taps with: > taps = gr.firdes.low_pass_2(1, fs, B/2, B/10, 80) > > The "B/10" is the transition width, which you can make as tight as you > need to, this is just a random guess right now. The 80 is the stopband > attenuation. This should make a rather long filter, then each channel > uses a filter that is len(taps)/N. > > In the examples directory, you can see how this is done in > pfb/cahnnelize.py and pfb/decimate.py. > > Tom > > > > > On Thu, Dec 23, 2010 at 2:49 PM, Tom Rondeau <trondeau1...@gmail.com> > wrote: > >> > >> On Thu, Dec 23, 2010 at 1:19 AM, James Jordan > >> <james.jordan....@gmail.com> wrote: > >> > Hi Martin, > >> > pfb_channelizer_ccf will seperate all channels, But I dont need each > >> > channel. > >> > I only need the channel I am interested in. Seperating all channels > will > >> > eat > >> > a lot of CPU resource. > >> > >> > >> Not really. It's a very efficient algorithm and won't cost you that > much. > >> > >> > >> > I have check pfb_channelizer_ccf source, it finally use fftw to > process > >> > channelizer. > >> > So can I directly use fftw to do my work. > >> > >> Not quite. The PFB channelizer uses a filterbank where each filter is > >> specifically generated with a phase relation. The FFT part isn't doing > >> exactly what you expect it's doing. We'd have to go through the math, > >> though. > >> > >> If you are looking to just get a single channel out, then use the > >> pfb_decimator_ccf(N, taps, channel) to split the bandwidth into "N" > >> channels, using filter taps "taps," and you can specify which channel > >> you want to take by specifying the "channel." Here's the way to > >> translate the "channel" into the physical Nyquist zone you are looking > >> for N=7 (hopefully this format survives): > >> > >> Channel: 4 5 6 0 1 2 3 > >> Frequency: -3B | -2B | -1B | 0 | 2B | 2B | 3B > >> > >> Tom > >> > >> > >> > On Tue, Dec 21, 2010 at 6:19 PM, Martin Braun <martin.br...@kit.edu> > >> > wrote: > >> >> > >> >> On Tue, Dec 21, 2010 at 06:02:44PM +0800, James Jordan wrote: > >> >> > Hi all, I need to receive many narrowband signals, but usrp hard > ware > >> >> > only > >> >> > provide 4 RX, > >> >> > so I need to receive more than one narrowband signals per RX. Is my > >> >> > idea > >> >> > possible? > >> >> > I dont want to use more than one usrp to achieve that, anyway which > >> >> > will > >> >> > be an > >> >> > option if my first idea can't work. > >> >> > >> >> If your total bandwidth (sum of all bandwidths) does not exceed a > >> >> couple > >> >> of MHz, you can use the polyphase channelizer (pfb_channelizer_ccf). > >> >> The result will be an equally spaced set of narrowband channels. > >> >> > >> >> Happy DSP'ing, > >> >> MB > >> >> > >> >> -- > >> >> Karlsruhe Institute of Technology (KIT) > >> >> Communications Engineering Lab (CEL) > >> >> > >> >> Dipl.-Ing. Martin Braun > >> >> Research Associate > >> >> > >> >> Kaiserstraße 12 > >> >> Building 05.01 > >> >> 76131 Karlsruhe > >> >> > >> >> Phone: +49 721 608-3790 > >> >> Fax: +49 721 608-6071 > >> >> www.cel.kit.edu > >> >> > >> >> KIT -- University of the State of Baden-Württemberg and > >> >> National Laboratory of the Helmholtz Association > >> >> > >> >> > >> >> _______________________________________________ > >> >> Discuss-gnuradio mailing list > >> >> Discuss-gnuradio@gnu.org > >> >> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >> >> > >> > > >> > > >> > _______________________________________________ > >> > Discuss-gnuradio mailing list > >> > Discuss-gnuradio@gnu.org > >> > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >> > > >> > > > > > >
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