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> 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 > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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