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| From: | Richard Bell |
| Subject: | Re: [Discuss-gnuradio] isolate channels from wideband |
| Date: | Tue, 21 Jul 2015 12:06:58 -0700 |
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
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, <address@hidden> 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,
https://gnuradio.org/doc/doxygen/classgr_1_1filter_1_1pfb__channelizer__ccf.htmlOn 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]
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