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 [1]] > > 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 > [1] > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio [2] Links: ------ [1] https://gnuradio.org/doc/doxygen/classgr_1_1filter_1_1pfb__channelizer__ccf.html [2] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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