Re: [Discuss-gnuradio] isolate channels from wideband

[Marcus Müller]

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