Dear Felix, I use the B200 to generate DAB signals (OFDM, 2048ksps sample rate, 1.5MHz effective BW, DQPSK) and while I cannot directly answer your questions, maybe there are some pointers that could be useful to you.
I did get some feedback from Ettus about spurious signals that I wrote down at the bottom of this page: http://wiki.opendigitalradio.org/USRP_B200_Measurements What we found empirically is that we need to keep some headroom in our digital signal because OFDM has a high PAPR, and make sure we do not get into digital saturation. (That's the "digital_gain" topic in the linked discussion) Regarding your first question: you can control the master_clock_rate yourself, and the FPGA in the B200 will interpolate from your sample rate to the master clock rate. See https://files.ettus.com/manual/page_usrp_b200.html#b200_mcr I'm also curious about improving spectrum quality, and looking forward to the discussion in this thread. Cheers mpb On 30/04/18 09:18, Felix via USRP-users wrote: > Hello everyone, > > > > currently I am working on a project to transmit and receive OFDM-signals > with USRP B200 Hardware. For this purpose, I am creating my OFDM > waveforms within Matlab, save the created IQ-data as a binary file and > use the UHD example functions ‘tx_samples_from_file’ and > ‘rx_samples_to_file’ to transmit the signals over a channel. The > demodulation with the received samples is the done within Matlab again. > > As I am using different Modulation Schemes (up to 1024-QAM) I noticed > that the USRP B200 Hardware has its limits for these transmissions. > > For my B200 I tried to characterize the TX sources of error by sending > two sine-tones and looking at the spurious elements with a Spectrum > Analyzer. It looks like the main signal distortion is caused by the > AD9346 TX-amplifier (appearing as Third Oder Intermodulation Products in > the spectrum), as DC-Offset and IQ-imbalance are corrected internally > and the phase noise is rather low. I therefore tried to find a TX gain > setting, to minimize this distortion while still sending with rather > high power. I found that a tx gain value of 84dB for my B200 produced > relatively good results so I used this value for my later performance > measurement. > > > > To characterize the B200 performance, I used a test transmission setup, > sending my signals with different Hardware (B200, Signal Generator, > Spectrum Analyzer) over a pure SMA-cable connection with a 30dB > attenuator. I then compared the EVM-values of the different Hardware > sending OFDM-signals with a bandwidth of 20MHz. For my first test it > turned out that the TX:B200-RX:B200 setup produced the lowest > EVM-values, whereas TX:Generator-RX:Analyzer has much lower EVM values. > Also sending with the B200 seemed to be the main distortion as the > B200-Analyzer setup had worse EVM results than Generator-B200. > > For these tests I had set the sample rate of my Hardware to exactly > 20MHz (--rate 20e6 within the UHD function) since from what I read about > the automatic MasterClock rate, I assumed the MasterClock is > automatically set to the highest integer multiple (40 MHz in this case) > within the B200. But from the program output, it seems that the clock is > set to exactly 20MHz. This is why I started using oversampling (e.g. > 28MHz sampling rate for a 20MHz signal). The interpolation and > decimation of the signal was done within Matlab. After this oversampling > the EVM values for my pure B200 connection improved significantly. On > the other hand the mixed hardware results, especially the connection > Generator-B200 got worse using oversampling which I cannot explain. > > > > My first question: Is the internal decimation from Masterclock to > SampleRate done automatically using the UHD functions > ‘tx_samples_from_file’ and ‘rx_samples_to_file’? If yes, why does an > increase of my sample rate (oversampling) produce much better results if > the Masterclock is already at 40MHz for my 20MHz sampling rate tests? If > no, is there a straightforward approach to increase the clock rate using > the UHD functions I mentioned? > > > > My second question: Does anybody have an idea why oversampling my signal > only improves the performance of my B200-B200 connection but does not > improve or even worsens the other HW-connections? > > > > My third question: Might I be missing any part of the B200 where I could > still impove performance and do you think a high performance USRP (X3x0) > with a UBX daughterboard might be capable of achieving higher Modulation > Schemes? I actually looked in the datasheet and found better specs for > the power amplifier of this daugherboard. > > > > Best regards, > > Felix > > > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
