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Re: [Discuss-gnuradio] Considerations when running a flowgraph manually

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Considerations when running a flowgraph manually from a loop
Date: Tue, 20 Jan 2015 10:06:02 -0500

On Fri, Jan 9, 2015 at 1:41 PM, Anderson, Douglas J. <address@hidden> wrote:
Hi all,

I've written a small program to capture and plot fft data from the USRP. Because it can also sweep and I have a limited knowledge (for now!) of writing GNURadio blocks, I decided to use a separate main loop to run the flowgraph and then handle the data. The flowgraph uses the head block to exit automatically when it has captured the necessary amount of samples. This setup really simplified handling retuning and plotting the data for me.

When sweeping, I expect there to be some junk samples acquired in the buffer while the LO retunes and I can handle that with the skip block, but when I set my program to only tune the USRP once and then plot the FFT constantly (basically emulating uhd_fft), I'm still getting junk data. I've double checked that the LO remains locked after the initial tune, so I think this extra junk data is a result of running the flowgraph, letting it exit, processing the data, and then running it again. uhd_fft, which is implemented as a block, does not have the same issues.

I've included 3 screenshots. 1 is uhd_fft running on my USRP N210, the other two are data from from my program plotted in matplotlib. In 1, you can see noise plotted as you'd expect, except that the LO is MUCH more visible than in uhd_fft. I confirmed that uhd_fft is not offsetting the LO so I'm not sure if this is another side effect of how I'm handing the flowgraph. Another screenshot shows the junk data that occasionally pops in and ruins my day.

My basic flowgraph setup is:

        self.connect(self.usrp, self.skip)  # USRP attached to skip block (modified to be resetable like head)
        self.connect(self.skip, s2v, self.head)  # skip -> stream_to_vector -> head
        self.connect(self.head, fft)   # head -> fft_vcc
        self.connect(fft, c2mag_sq)
        self.connect(c2mag_sq, stats, W2dBm, self.final_vsink)

In the last line, stats is just a block that can handle averaging, W2dBm translates to dBm, and then all samples are dumped into a float vector sink

My program was set to skip 0 samples for these tests.

The main loop in pseudocode:

while True:
    data = "">     process and plot data

* Is there anything tricky I should know about running a flowgraph from a separate loop like I'm doing?
* What could cause the LO to be so much more pronounced in my plots as compared to uhd_fft? For the screenshots I went to great lengths to ensure I was using the same tuning and plotting parameters. Also, although I can control the "junk" data by skipping samples with the "skip" block, it does not reduce the LO throughput.
* Any leads on what could be causing the junk data even when the USRP is not being retuned?

I'm at wits end, thanks in advance for any hints whatsoever.


In your stats block, is there any history? Are you keeping samples in there to do the averaging, and can you reset that as well? What it looks like in your last image is a sharp discontinuity between samples, so basically multiplying by a square wave in the time domain. If you are looking at two sets of data that are not continuous because of the restart of the flowgraph, that could show up here.


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