Re: [Discuss-gnuradio] hi all, can I use one RX receive a wideband signal and then seperate it to many narrowband signals

[Tom Rondeau]

On Thu, Dec 23, 2010 at 3:53 AM, James Jordan
<address@hidden> 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 <address@hidden> wrote:
>>
>> On Thu, Dec 23, 2010 at 1:19 AM, James Jordan
>> <address@hidden> 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 <address@hidden>
>> > 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
>> >>
>> >>
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