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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] question about bandwith B210 - USB3 |
Date: | Fri, 1 Jul 2016 12:54:45 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 |
Hi Przemek, On 01.07.2016 11:30, Przemek
Lewandowski wrote:
These are controllers known to work relatively well. I think that the problem really is in the CPU load your Flow graph has, probably Yes, but your limit checking is done in a very inefficient manner – for example, the band pass filter (which really seems mis-configured in sampling rate, considering the "16k" in the file name, also, not sure you would actually want to use a band pass here) could very efficiently do the job of the 90-fold interpolation, and thus you could completely safe the effort that the rational resampler poses (which should contribute *very* drastically to the overall CPU load). Also, when constructing a signal out of N evenly spaced subcarriers, it's recommendable to use the Polyphase synthesizer[2], which is *way* more efficient than having N interpolators and a summation. So, the problem really is that you're checking /some/ limits, but it's not inherently clear they will somehow be related to the thing you need to solve later on. This is complex baseband, so 1*f_max is sufficient (2*f_max is for the real-valued case, only). So, for the USRP side, 18 MS/s means 18 MHz of bandwidth. yes, but you seem to treat your 16kHz signal as if it was sampled at 200 kHz Yeah, as said, the parametrization of the band pass doesn't seem right. Also, SSB in baseband, by definition, doesn't have a spectrum you can represent using a real-valued signal, so that's wrong, too. Remember: real-valued signals must always be symmetric in spectrum, and every real filter can only preserve that symmetry. I happen to have just held a short lecture on that topic at a amateur radio conference[1], if the theory sounds new to you. Also: the Qt GUI graphical sinks are really your friend! Replace your USRP sink by a Qt GUI frequency sink. Set its center frequency to what you would have tuned the USRP to, and make sure it's . That's what your spectrum on the air would look like. You'll notice that it would be symmetric around the center frequency, still, because you've kept the real nature of your original signal all the way through. Maybe an example of how one can have multiple channels in a wider bandwidth quite easily with the Polyphase Synthesizer helps you[2]. you could try to replace the USRP sink by a "probe rate" block, connected to a "message debug" sink's "print" input. That way, you could see if the rate the flowgraph can run at is sufficiently high. Best regards, Marcus [1] http://marcus.hostalia.de/sdra16.pdf . Youtube Videos are promised to appear at some point in time. [2] https://gist.github.com/d98a47c29098f68628d3b32959b02432 Double-click in the Waterfall plot to change the frequency of the channel-selecting xlating FIR filter.
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