Hello, In my radar research, I use USRP X310 and UBX-160 daughterboard. I observed high spike at all center frequencies the daughterboard operates. I read suggestion made by Marcus Müller <http://dsp.stackexchange.com/users/13320/marcus-m%c3%bcller> [1] to use uhd.tune_request(cener_freq, lo_offset) instead of blocks.set_center_freq(center_freq) to solve the problem. Sure enough the spikes were no more there when I use tune_request() but if only the blocks are of complex type. For example, no spike if the grc flowgraph is: usrp_source-->qt_gui_sink (complex_type) (if tune_request is used). But there are spikes if the grc flowgraph is: usrp_source-->complex_to_mag--> qt_gui_sink (float) (even if tune_request is used) Why is this spike persisting in the latter case? [1] http://dsp.stackexchange.com/questions/30562/large-spike-at-the-center-frequency-when-using-ettus-x310

On 02/17/2017 06:53 AM, Haile Berhe via USRP-users wrote: > Hello, > In my radar research, I use USRP X310 and UBX-160 daughterboard. I > observed high spike at all center frequencies the daughterboard > operates. I read suggestion made by Marcus Müller > <http://dsp.stackexchange.com/users/13320/marcus-m%c3%bcller> [1] to > use uhd.tune_request(cener_freq, lo_offset) instead of > blocks.set_center_freq(center_freq) to solve the problem. Sure enough > the spikes were no more there when I use tune_request() but if only > the blocks are of complex type. For example, no spike if the grc > flowgraph is: > usrp_source-->qt_gui_sink (complex_type) (if tune_request is used). > > But there are spikes if the grc flowgraph is: > usrp_source-->complex_to_mag--> qt_gui_sink (float) (even if > tune_request is used) > > Why is this spike persisting in the latter case? > > [1] > http://dsp.stackexchange.com/questions/30562/large-spike-at-the-center-frequency-when-using-ettus-x310 > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com Because a complex-to-mag or complex-to-mag-squared produces a very strong DC component. Indeed, those operations are very often used in power estimation, so mathematically, they *have* to produce a strong DC component.

Thanks for the reply. I don't want this spike at the DC. Is there any means to avoid them? On Fri, Feb 17, 2017 at 2:53 PM, Haile Berhe <haile1999@gmail.com> wrote: > Hello, > In my radar research, I use USRP X310 and UBX-160 daughterboard. I > observed high spike at all center frequencies the daughterboard operates. I > read suggestion made by Marcus Müller > <http://dsp.stackexchange.com/users/13320/marcus-m%c3%bcller> [1] to use > uhd.tune_request(cener_freq, lo_offset) instead of > blocks.set_center_freq(center_freq) to solve the problem. Sure enough the > spikes were no more there when I use tune_request() but if only the blocks > are of complex type. For example, no spike if the grc flowgraph is: > usrp_source-->qt_gui_sink (complex_type) (if tune_request is used). > > But there are spikes if the grc flowgraph is: > usrp_source-->complex_to_mag--> qt_gui_sink (float) (even if tune_request > is used) > > Why is this spike persisting in the latter case? > > [1] http://dsp.stackexchange.com/questions/30562/large- > spike-at-the-center-frequency-when-using-ettus-x310 >

Maybe you could describe what it is you're trying to achieve? Applying a complex-to-mag to a signal will absolutely, positively, result in a strong DC component. This isn't a quirk of the radio hardware, or Gnu Radio. It's the fundamental mathematics of the situation. ANY time you apply a non-linear transform to a signal, you cannot expect the spectrum of the signal to be the same after the transform. You can remove the DC component downstream by the addition of a high-pass or DC-block operation. But without understand exactly what it is you're trying to achieve, not sure that would be helpful. Also, general DSP and Gnu Radio questions should go to the discuss-gnuradio list, not there. On 2017-02-17 12:27, Haile Berhe via USRP-users wrote: > Thanks for the reply. I don't want this spike at the DC. Is there any means to avoid them? > > On Fri, Feb 17, 2017 at 2:53 PM, Haile Berhe <haile1999@gmail.com> wrote: > >> Hello, >> In my radar research, I use USRP X310 and UBX-160 daughterboard. I observed high spike at all center frequencies the daughterboard operates. I read suggestion made by Marcus Müller [1] [1] to use uhd.tune_request(cener_freq, lo_offset) instead of blocks.set_center_freq(center_freq) to solve the problem. Sure enough the spikes were no more there when I use tune_request() but if only the blocks are of complex type. For example, no spike if the grc flowgraph is: >> usrp_source-->qt_gui_sink (complex_type) (if tune_request is used). >> >> But there are spikes if the grc flowgraph is: >> usrp_source-->complex_to_mag--> qt_gui_sink (float) (even if tune_request is used) >> >> Why is this spike persisting in the latter case? >> >> [1] http://dsp.stackexchange.com/questions/30562/large-spike-at-the-center-frequency-when-using-ettus-x310 [2] > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com Links: ------ [1] http://dsp.stackexchange.com/users/13320/marcus-m%c3%bcller [2] http://dsp.stackexchange.com/questions/30562/large-spike-at-the-center-frequency-when-using-ettus-x310