Thank you for the examples and the explanation. I believe I understand. Sticking with a square wave or anything that involves a sinc envelop will always raise the spectrum. Is there something that I can transmit that won't let this occur? Perhaps something that will peak in a specific area in the FFT. Sorry if the question might seem elementary, my understanding of signals does not go very far. To be clear, I want to know if I can display a graph that will show a peak a certain region due to a detection of signal.
On Wed, Jul 22, 2015 at 4:55 PM, Marcus Müller <marcus.muel...@ettus.com> wrote: > Hi Ashraf, > > I don't know what frequency f_squarewave your square wave has, but > rectangular signals have sinc-shaped envelope, with peaks every > f_squarewave. > Having a sinc envelope especially means it exists over the whole nyquist > band -- so that explains why you see your complete spectrum rise! > > I think what you should do is have a look at the spectrum of your > *transmit* signal. This will make it easier to understand what you see at > the receiver. > Try this: http://i.imgur.com/EARpJLx.png > > [image: flowgraph with squarewave of f_sample/32] > <http://imgur.com/EARpJLx> > > so, a squarewave with frequency of f_sample/32 (remember, there's no real > frequencies in DSP -- this really just tells GNU Radio that a period is 32 > samples long). > Important: the QT sink is set to have an FFT length of 1024 points. > You should see this: http://i.imgur.com/33xrCl6.png > > Now, if you have a look at this spectrum, you'll notice deep "wells" > between the peaks, which aren't there in the receive PSD, right? > > There's a simple reason for that: In the simulation / the TX spectrum, > your f_squarewave is probably an integer factor of f_sample. This means > that the period is a whole number of samples, and that whole number of > samples also happens to be an integer factor of your FFT length! Therefore, > your signal is perfectly periodic (as far as the observer can tell), and > hence, has line spectrum characteristics. > > Now, we'll play around with the frequency of the square wave: > http://imgur.com/Kduv5fL > set it to f_sample/32/2**0.5 ; the root of 2 is not a rational number, so > no FFT window in this world could transform this without leakage. So you > get this: > http://imgur.com/1Fkl8f8 > Looking familiar? > > Now, these are extremes. But the problem here is that frequency > synchronization between your N210 and B200 will not be perfect -- hence, on > the receiver side, the signal period might not be as exactly the integer > factor that you have on the TX side, and you see the inter-peak leakage. > > I hope that has explained the most of this phenomenom. > > Best regards, > Marcus > > On 22.07.2015 20:40, Ashraf Younis wrote: > > > ---------- Forwarded message ---------- > From: Ashraf Younis <shraff...@gmail.com> > Date: Wed, Jul 22, 2015 at 2:39 PM > Subject: Re: [Discuss-gnuradio] Creating a FFT plot like the one in this > youtube variable > To: Marcus Müller <marcus.muel...@ettus.com> > > > I apologize for the way it seems I'm relaying information. I will attempt > to give you all new information with your perspective in mind. > > I have two screenshots now, they have a fixed y-axis from 0 to -100 dB. > The first is the graph that is displayed when there is no signal > transmitted and the second is when there is one being transmitted. From the > screenshots it is evident that the signal raises all of the graph's values, > instead of where the signal is being transmitted. The receiving gain is set > to 1. I deleted the bandwidth QT GUI Entry since my I did not know what it > was doing. The receiver is is a USRP B200 > > The signal I am transmitting on the USRP 2921 is a simply square wave. > the IQ rate is 100k, the antenna is a vert 2450 dual-band, it is set to 10 > gain. It was transmitting in the same channel the B200 is receiving. > > I just want to make sure we're headed in the right direction. I want to > use a wide band spectrum sense graph > > On Wed, Jul 22, 2015 at 1:55 PM, Marcus Müller < > <marcus.muel...@ettus.com>marcus.muel...@ettus.com> wrote: > >> sure :) >> >> On 22.07.2015 19:45, Ashraf Younis wrote: >> >> Excuse me, it was my mistake. >> I sent it as an attachment, is that okay? >> >> On Wed, Jul 22, 2015 at 1:29 PM, Marcus Müller <marcus.muel...@ettus.com> >> wrote: >> >>> Hi Ashraf, >>> >>> your mail is only 8.5kB large -- did you possibly forgot to include >>> links to the images? >>> >>> Best regards, >>> Marcus >>> >>> >>> On 22.07.2015 19:28, Ashraf Younis wrote: >>> >>> Thank you for replying. >>> I included some screenshots of the program. One is of the block diagram >>> and the other is of the graph itself. >>> I am generating a signal using a different device and program. having >>> only one B200 has prompted me to use this method. I am using a NI USRP-2920 >>> with lab view to generate a signal >>> >>> >>> On Wed, Jul 22, 2015 at 12:01 PM, Marcus Müller < >>> <marcus.muel...@ettus.com>marcus.muel...@ettus.com> wrote: >>> >>>> Hi Ashraf, >>>> >>>> If you've configured the USRP source correctly, you're very likely >>>> actually displaying the spectrum your digital receiver sees -- depending on >>>> the signal, you could a) actually be rising the power level in that whole >>>> band, or b) maybe you're observing something like saturation and hence >>>> intermodulation of additional signals. >>>> >>>> You migth want to share what exactly you are observing, and what >>>> exactly the signal is you're generating. Screenshots are easy to make and >>>> to upload [1], so please illustrate a little better! >>>> >>>> Best regards, >>>> Marcus >>>> >>>> >>>> [1] <http://www.imgur.com>www.imgur.com >>>> >>>> On 22.07.2015 17:56, Ashraf Younis wrote: >>>> >>>> Hello, the issue I am having is I cannot display a graph that shows a >>>> wide range of frequencies and their power. When I attempt it with the QT >>>> GUI Frequency in GRC, I get something similar to the one in this video (FFT >>>> plot <https://youtu.be/cygDXeZaiOM?t=3m49s>) but then I transmit a >>>> signal in the range I am currently looking at and the whole line moves up. >>>> This leads me to believe that I am no displaying the whole range I desire, >>>> but in fact I am displaying the center frequency and a small bandwidth >>>> around it. I want to, for example, scan the 2.4 GHz range and see all of >>>> the channels and their power. And when I transmit at a certain frequency, I >>>> see a spike at the spot in the graph. >>>> >>>> How do I create that graph? >>>> >>>> >>>> _______________________________________________ >>>> Discuss-gnuradio mailing >>>> listDiscuss-gnuradio@gnu.orghttps://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>>> >>>> >>>> _______________________________________________ >>>> Discuss-gnuradio mailing list >>>> Discuss-gnuradio@gnu.org >>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>>> >>> >>> >> >> > > > > _______________________________________________ > Discuss-gnuradio mailing > listDiscuss-gnuradio@gnu.orghttps://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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