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, <[email protected]> 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 > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > -- GDB has a 'break' feature; why doesn't it have 'fix' too? _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
