On Fri, Oct 14, 2011 at 7:47 AM, John Ackermann N8UR
<address@hidden> wrote:
Marcus D. Leech said the following on 10/13/2011 09:18 PM:
On 10/13/2011 09:11 PM, John Ackermann N8UR wrote:
Is there a (hopefully simple) example of using the polyphase
channelizer block in grc?
I'm trying to do a simple case of dividing a 250 kHz wide stream from
USRP into a small number (initially 4) of contiguous channels. Given
my generally ignorant status, I'm stumped on setting the parameters.
[ snip ]
If it's only 250Khz wide, and you only need four, have you tried 4
discrete freq-xlating FIR filters? They're fairly easy to use, and can
produce a baseband output at a decimated rate.
I'm starting with 4, but if my harebrained scheme works, want to increase to whatever the CPU will allow.
I've attached a screenshot of my test flowgraph. It's a linear translator that takes a chunk of spectrum down to baseband, then retransmits it on a different frequency range -- similar to what the amateur satellites do.
What I'm playing with is whether I can improve the transmit efficiency by dividing the passband into multiple, independently squelched channels so that I'm not amplifying and retransmitting noise where there is no real signal present. For best results, you'd want to have many channels (and possibly some means to select adjacent channels), but to start with 4 is enough to see whether this idea makes any sense at all.
John
I haven't yet figured out how to use the polyphase channelizer because I
almost never have the N equi-spaced channels problem in
my fields of interest. So, discrete filters it is (sometimes a hierarchy
of discrete filters).
John,
You've almost got it right except for the taps. The "taps" parameter is the FIR taps of the prototype filter, which then gets partitioned among the N channels of the channelizer.
While there is no example in GRC readily available, you can look at the channelize.py example in: gnuradio-examples/python/pfb/
Specifically, line 49 is where the filter taps are created. Notice that the sample rate is the sample rate at the input to the channelizer while the bandwidth and transition width are defined for the channel bandwidths. This makes a fairly long filter that is then split up between the N channels of the PFB.
Tom