On Tue, Jul 21, 2015 at 3:00 PM, Marcus Müller <marcus.muel...@ettus.com> wrote:
> That would be extremely awesome. Especially for people with some DSP > filter experience, the Polyphase filterbanks are somewhat hard to > understand, and I think Tom's article / presentation (which I can't seem to > find right now) on the PFBs can only profit from a real-world example > accompanying their message. > > --Marcus Here's my presentation from last GRCon: http://gnuradio.squarespace.com/grcon14-presentations#tut-rondeau I can't find a link anywhere to the WSR'14 papers/presentations. I'll need to put that on my website and the gnuradio.org Academic Papers page sometime. Tom > On 21.07.2015 20:56, Chris Kuethe wrote: > >> Maybe I'll do up an illustrated example on this using NOAA weather >> radio, or the pager band >> >> On Tue, Jul 21, 2015 at 11:42 AM, <mle...@ripnet.com> wrote: >> >>> I just use the built-in firdes stuff, rather than using an external >>> designer. >>> >>> >>> >>> >>> >>> >>> >>> On 2015-07-21 14:38, Marcus Müller wrote: >>> >>> Hi Rich, hello Markus, >>> >>> On 21.07.2015 19:51, Richard Bell wrote: >>> >>> GNU Radio has channelizers built-in, but I've not used them yet, so I >>> don't >>> know how far they take you into this kind of task. >>> >>> the Polyphase channelizer is actually an implementation derived from that >>> school of thought, and it works amazingly well. >>> In fact, in preparation of a presentation at a certain ham conference, I >>> tried using it to get 20 PMR/LPD channels out of a 1MS/s signal in real >>> time, and then just shuffle them around, before feeding them back into >>> the >>> inverse synthesizer PFB. >>> >>> It's pretty easy: >>> Design a single low pass filter, as if you just wanted to filter out the >>> channel which is centered exactly at your RF center frequency, i.e. 0Hz, >>> with the full sampling rate [2], using the gr_filter_design tool. Play >>> around with the different window types[1], and bear in mind that the >>> suppression outside your desired passband needs to be high enough so that >>> the sum of the energy in all other channels don't hurt your channel too >>> much, but don't overdo it (60dB suppression should be enough). >>> Now you get a long filter. Copy and paste the filter coefficients from >>> gr_filter_design to your PFB filter taps property. >>> Set your channelizers number of channels according to your plans -- 40, >>> if >>> you want to get all the 40 25kHz channels in 2MHz. You get a block with >>> 40 >>> outputs! >>> Explaining things like channel mapping is best done by pointing you at >>> the >>> official documentation: [3] >>> >>> >>> Greetings! >>> Marcus >>> >>> [1] Hamming is not always the best choice, I'd try that, Blackman-harris, >>> and Kaiser. I personally like harris in this case -- we want to get a >>> full >>> channel, two adjacent channels are usually not occupied, and as soon as >>> we >>> pass the stopband frequency, we're basically at -100dB. >>> [2] assuming you want to use 2MS/s for your 2MHz wide band, 2MHz sampling >>> rate, and assuming 25kHz wide channels, 12.5kHz cut off frequency, 25kHz >>> start of stoppband. I get something like 440 taps. >>> [3] >>> >>> https://gnuradio.org/doc/doxygen/classgr_1_1filter_1_1pfb__channelizer__ccf.html >>> >>
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