I may not have written my last response helpfully…the absolute value of gain doesn’t tell you much about your actual power output, it’s very waveform dependant. It was a hint to go vary this and see what happens. If you drop a QT GUI Frequency Sink after your Cyclic prefixer you’ll get a good feel for where all your signal power should be in an ideal world. If that’s not what you see on a spectrum analyzer then something downstream is causing your problem. If you think about it, your actual OFDM waveform generation part of the flow graph is Nyquist limited by the sample rate it runs at. Any digital interpolation that happens from that sample rate to the ADC clock rate within the FPGA applies appropriate filtering to prevent significant aliasing.
Now I think about it, you previously mentioned you are X310 based….not sure it can interpolate in H/W from as low as 50/100kHz…the limit used to be 512:1 when I originally designed it. Have you checked what UHD reports when the flow graph starts? You may not be running at the sample rate you asked for……(May need flow graph interpolator in S/W for low bandwidths) > On May 11, 2018, at 11:43 AM, Jacob Knoles <knole...@gmail.com> wrote: > > In the DSP prior to sending to USRP I scale everything by a const 0.05, the > TX gain for the USRP is 0.5 normalized. > ----------------------------- > Jacob Knoles > > > > On Fri, May 11, 2018 at 11:40 AM Ian Buckley <i...@ionconcepts.com > <mailto:i...@ionconcepts.com>> wrote: > What TX gain have you got set? Are you sure you are operating the PA in it’s > linear region? OFDM waveforms are notorious for there PAPR requirements. > >> On May 11, 2018, at 11:23 AM, Jacob Knoles <knole...@gmail.com >> <mailto:knole...@gmail.com>> wrote: >> >> Hey guys, >> >> Please pardon my ignorance, I am trying to learn everything I need for DSP / >> OFDM on the fly. But I have noticed something with this OFDM gnuradio >> example. If I take the example and run it directly into the usrp without >> changing any variables, I get a 350 KHz wide signal out. >> Now the sample rate is only 100k, the occupied carriers are -26 to 26 (zero >> omitted) with pm 21 and pm 7 as pilot carriers. The fft length is 64. >> >> If the bandwidth is directly related to the sampling rate how am I getting >> 3x the bandwidth at low sample rates. >> >> For some further comparison I changed only the sample rate to a few other >> values, here is what I observed: >> >> Sample Rate : Observed Bandwidth >> 100k : 350 KHz >> 50k : 350 KHz >> 1M : 840 KHz >> 20M : 17 MHz >> >> The 1M and 20M rates make sense but I don't understand what is happening >> with the 100k and 50k rates. >> >> Thank you for the help. >> ----------------------------- >> Jacob Knoles >> >> >> >> On Wed, May 9, 2018 at 6:22 PM Marcus D. Leech <mle...@ripnet.com >> <mailto:mle...@ripnet.com>> wrote: >> On 05/09/2018 07:57 PM, Jacob Knoles wrote: >>> Thanks for the quick reply guys. >>> >>> Marcus the Re-sampling option makes perfect sense, and I believe, in >>> theory, since I am writing data to a file for later use I could interpolate >>> it just before writing then read it out at the usrp sample rate, right? >> Yes. >> >>> >>> Ian, very interesting suggestion. I will have to give it a try. Thanks for >>> the input. And since I am doing all of the heavy processing prior to tx'ing >>> I don't image this change will create too great of a burden on the CPU. As >>> for reading from the file, I am just creating a small data set which gets >>> loaded into memory and repeated. >>> >>> Thanks!! >>> ----------------------------- >>> Jacob Knoles >>> >>> >>> >>> On Wed, May 9, 2018 at 4:21 PM Ian Buckley via USRP-users >>> <usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com>> wrote: >>> >>> > On May 9, 2018, at 4:07 PM, Marcus D. Leech via USRP-users >>> > <usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com>> wrote: >>> > >>> > On 05/09/2018 06:53 PM, Jacob Knoles via USRP-users wrote: >>> >> Hello All, >>> >> >>> >> I am trying to generate OFDM signals of various bandwidths using the >>> >> X300 (UBX-160), particularly 20/40/80 and 160 MHz bandwidths. >>> >> I have used the gnuradio ofdm_tx.grc example file to generate a data >>> >> file which I then feed into the USRP an monitor on a spectrum analyzer. >>> >> >>> >> To quickly note, I do not care about the data being transferred, it will >>> >> not be received or demodulated in any way and is simply an interfering >>> >> signal. >>> >> >>> >> At this time I can produce a 20 MHz wide OFDM signal as well as a 100 >>> >> MHz wide signal (?) but the 40/80 MHz signals are rounded and look more >>> >> like an 802.11b signal. >>> >> >>> >> I have noted a message from the X300 that the requested sample rates >>> >> (40/80 MS respectively) cannot be achieved due to the 200/x ratio being >>> >> odd. >>> >> >>> >> So my question is this, how do I decouple the USRP's sample rate with >>> >> the bandwidth of the signal I am trying to produce? >>> >> To put it another way, I produce a data file at 40 MS/s rate then run it >>> >> on the X300 at 100 MS/s and I get a 100 MHz wide signal instead of the >>> >> 40 MHz I want. >>> >> >>> >> Thanks for the help. >>> >> ----------------------------- >>> >> Jacob Knoles >>> >> >>> > You would need to interpolate it up to the desired rate. UHD has no way >>> > of knowing that your samples represent data sampled at 40Msps, so when you >>> > pull it out of your file at 100Msps, it will get presented as if it were >>> > 100Msps data. >>> > >>> > You'll need to use some DSP code, or Gnu Radio to up-sample your sample >>> > file. >>> > >>> ….or perhaps generate it off line using a non 2^n Fourier transform size >>> that targets the USRP sample rate…for example instead of 64 bins @ 40MHz, >>> 80 bins @ 50MHz, >>> With zero data in the extra outlying bins (as you would have anyway in >>> other bins). Might get interesting getting high bitrates out of a file, but >>> equally, high bitrate M:N sample rate conversion will also be tricky for CPU >>> _______________________________________________ >>> USRP-users mailing list >>> USRP-users@lists.ettus.com <mailto:USRP-users@lists.ettus.com> >>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >>> <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com> >> >
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