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Re: [Discuss-gnuradio] DDC and Polyphase Channelizer

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] DDC and Polyphase Channelizer
Date: Tue, 4 Jan 2011 10:44:50 -0500

On Mon, Jan 3, 2011 at 11:25 PM, Jimmy Richardson <address@hidden> wrote:
>> Indeed. Very strange. My guess is that there is a misconception
>> somewhere in the code about sample rate. I can't quite see it in my
>> head, but I'm guessing that the channel spacing isn't exactly the
>> channel spacing you think it is.
> You mean the oversample rate O = N/i is wrong? As far as I can see, that's
> the only number that could go wrong, since pfb_channelizer_ccf only takes 3
> parameters, # of channels, taps and oversample rate, and it doesn't look
> like the other two can be wrong.
> I checked the calculation of O again, but couldn't find the problem. In the
> Matlab code, harris uses loops "for nn=1:28:5600-28" to do the processing,
> so it does seems a 28-to-1 downsampling in 40 channels.

No, not the oversample rate, just the sample rate. I'm thinking that
there is some misunderstanding somewhere about the actual sample rate
and the therefore the channel bandwidths such that the channels you
are pulling out are not covering the same frequencies that you think
they are.

On the other hand, it could be a miscalculation in the
pfb_channelizer, although all of my tests with it came out fine.

>> Have you looked at the examples in gnuradio-examples/python/pfb?
>> Specifically, channelize.py, chirp_channelize.py, and synth_to_chan.py.
> I checked the first two samples when I wrote the channelizer, although it
> seems they didn't use the oversample rate parameter. I couldn't find the
> last one (synth_to_chan.py) though.
>> Tom

The synth_to_chan.py is not in the tarball release. It was introduced
a little while ago when I added the synthesis filterbank. You'd have
to get it from the git repo.


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