I guess in my mind, once the usrp_source device and flowgraph instances were
created and connected, run() would just be pulling samples through the blocks.
I read through a presentation on the GNU Radio flowgraph yesterday on
gnuradio.squarespace.com (can't find the exact link ATM) but failed to clear up
my understanding of this issue.
Here is the code I was referring to:
from gnuradio import gr, uhd, blocks
import numpy as np
import matplotlib.pyplot as plt
class topblock(gr.top_block):
def __init__(self, freq=700e6, rate=1e6, nsamps=100):
gr.top_block.__init__(self)
self.u = uhd.usrp_source(device_addr="",
stream_args=uhd.stream_args('fc32'))
self.u.set_center_freq(freq)
self.u.set_samp_rate(rate)
self.head = blocks.head(gr.sizeof_gr_complex, nsamps)
self.vsink = blocks.vector_sink_c()
self.connect(self.u, self.head, self.vsink)
tb = topblock()
fig, (freqplot, timeplot) = plt.subplots(2, sharex=True)
freqplot.set_title("Frequency domain")
timeplot.set_title("Time domain")
def plot():
tb.run()
data = np.array(tb.vsink.data())
shifted_fft = np.fft.fftshift(np.fft.fft(data))
dBm = 20*np.log10(np.abs(shifted_fft)) - 30
freqplot.plot(dBm)
timeplot.plot(np.abs(data))
tb.head.reset()
tb.vsink.reset()
def run_tb(times=25):
for _ in range(times):
plot()
plt.show(block=False)
Douglas Anderson | Intern
DOC/NTIA/ITS-T | 325 Broadway St., Boulder, CO 80305 | P: 303 497 3582
________________________________
From: Marcus D. Leech [mle...@ripnet.com]
Sent: Thursday, January 15, 2015 11:40 AM
To: Anderson, Douglas J.
Cc: GNURadio Discussion List
Subject: Re: [Discuss-gnuradio] voltage pulse from UHD driver
On 01/15/2015 01:11 PM, Anderson, Douglas J. wrote:
Marcus, thank you.. the "optimized for steady state streaming" comment confirms
some suspensions, although I'd love to understand more deeply why exactly this
happens consistently to acquisitions long after a retune.
I think the code snippet I posted to StackExchange might illustrate the
difference I'm seeing a little better. In that example, I did not use
uhd.finite_acquisition, I used a standard flowgraph and topblock.run().
What if I took that code and tried to just continually plot acquisitions,
almost like a simplified uhd_fft. I would get these "pulses" whereas you don't
see them in uhd_fft. The frontend tune only happens once, and the only
difference I can see about how things are being handled is that one (uhd_fft)
creates one flowgraph and uses a block to plot the data, whereas in my example
I create one flowgraph and plot the data returned after a toblock.run()
I'm trying to figure out if there's some garbage data/samples created each time
the flowgraph is started that doesn't happen in a constantly running flowgraph,
and your comments somewhat confirm that that is the case.
Thanks for the info!
-Doug
Could you post that StackExchange code here, please, so that we can comment on
it?
When a flow-graph is run, then stopped, then run again, gr-uhd will have to go
through starting/stopping streaming. Which means you're starting and
stopping the DSP chain in the USRP. That will, necessarily, involve
non-steady-state data. I don't think there's a mode in USRP which basically
says "run DSP steady-state, but don't send me data until I want to stream
samples". Starting streaming/stopping stream also starts/stops the
DSP engine.
________________________________
From:
discuss-gnuradio-bounces+danderson=its.bldrdoc....@gnu.org<mailto:discuss-gnuradio-bounces+danderson=its.bldrdoc....@gnu.org>
[discuss-gnuradio-bounces+danderson=its.bldrdoc....@gnu.org<mailto:discuss-gnuradio-bounces+danderson=its.bldrdoc....@gnu.org>]
on behalf of Anderson, Douglas J.
[dander...@its.bldrdoc.gov<mailto:dander...@its.bldrdoc.gov>]
Sent: Thursday, January 15, 2015 10:55 AM
To: Nick Foster
Cc: GNURadio Discussion List
Subject: Re: [Discuss-gnuradio] voltage pulse from UHD driver
I don't think it's tuning effects, if I'm understanding that correctly. You
mean that after you retune the USRP, the LO will take some time to settle?
In the script I posted, that shouldn't be a factor, as the UHD instance is
created and tuned when I import the file in the python interpreter, and the
acquisitions are then run later and without retuning the USRP.
I might be misunderstanding the issue, like if there is something that needs to
settle each and every time an acquisition is requested independent of actual
frequency tuning.
... but that's the reason for my question: so that I can better understand the
underlying process. Thank you for the details!
-Doug
________________________________
From: Nick Foster [bistrom...@gmail.com<mailto:bistrom...@gmail.com>]
Sent: Thursday, January 15, 2015 10:49 AM
To: Anderson, Douglas J.
Cc: GNURadio Discussion List
Subject: Re: [Discuss-gnuradio] voltage pulse from UHD driver
Nothing. The timing might be a little different -- if it's tuning effects
you're seeing, there's effectively a race condition between tuning and sample
collection. Gnuradio will never discard samples off the front unless you use a
Skip Head block, which you should probably be doing as evidently you aren't
expecting your samples to be tightly synchronized to any particular point in
time.
--n
On Thu, Jan 15, 2015 at 9:46 AM, Anderson, Douglas J.
<dander...@its.bldrdoc.gov<mailto:dander...@its.bldrdoc.gov>> wrote:
Okay, this makes sense.
What about the version I posted on StackExchange where I am using GNU Radio's
scheduler to request the samples?
What does GNU Radio do when running a constant flowgraph (like uhd_fft) that it
doesn't to when running topblock.run() for each collection, as far as
discarding samples off the front?
-Doug
________________________________
From: Nick Foster [bistrom...@gmail.com<mailto:bistrom...@gmail.com>]
Sent: Thursday, January 15, 2015 10:40 AM
To: Anderson, Douglas J.
Cc: GNURadio Discussion List
Subject: Re: [Discuss-gnuradio] voltage pulse from UHD driver
In general you cannot use the first few samples of output from an acquisition.
There are a couple of reasons:
First, if you begin collecting data immediately (rather than using UHD to
schedule a collection at a given time) and you are using a daughterboard with a
downconverter (anything but BasicRX or LFRX), tuning takes some time and things
will be ugly while PLLs settle, etc.
Second, there are digital halfband and CIC filters in the USRP, and they are
not reset between acquisitions. This means that the first samples will have
some junk left over from the last acquisition.
Unfortunately, the general answer to what you're trying to do is, don't do that.
Best,
Nick
On Thu, Jan 15, 2015 at 9:26 AM, Anderson, Douglas J.
<dander...@its.bldrdoc.gov<mailto:dander...@its.bldrdoc.gov>> wrote:
Hi all,
I've been slowly working to understand/isolate an issue with a strange voltage
pulse at all freqs and on USRP N210 with 50 Ohm load.
I posted about it on StackExchange here, and there are more details at this
link:
http://stackoverflow.com/questions/27968237/semi-consistent-voltage-pulse-from-usrp-when-using-simple-gnu-radio-flowgraph
Since then, I've further isolated it as a UHD issue by completely removing the
GNU Radio scheduler from the equation and simply using the finite_acquisition
function on UHD to pull samples directly into Python.
Here is the code I'm using to produce this output
http://i.imgur.com/c3YWA22.png:
An interesting thing is that when using the UHD driver is used outside a
flowgraph (uhd.finite_acquisition), I get the strange pulse consistently,
whereas when used in a flowgraph it was inconsistent (see the StackExchange
question).
import numpy as np
import matplotlib.pyplot as plt
FREQ = 800e6
RATE = 1e6
NSAMPS = 100
usrp = uhd.usrp_source(device_addr="", stream_args=uhd.stream_args('fc32'))
usrp.set_center_freq(FREQ)
usrp.set_samp_rate(RATE)
fig, (freqplot, timeplot) = plt.subplots(2, sharex=True)
freqplot.set_title("Frequency domain")
timeplot.set_title("Time domain")
def plot():
data = np.array(usrp.finite_acquisition(NSAMPS))
shifted_fft = np.fft.fftshift(np.fft.fft(data))
dBm = 20*np.log10(np.abs(shifted_fft)) - 30
freqplot.plot(dBm)
timeplot.plot(np.abs(data))
def run_tb(times=25):
for _ in range(times):
plot()
plt.show(block=False)
Douglas Anderson | Intern
DOC/NTIA/ITS-T | 325 Broadway St., Boulder, CO 80305 | P: 303 497
3582<tel:303%20497%203582>
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--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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