DAC nonlinearity is not a problem. The problem is the datapath.
In USRP B210, the path is PC -> FPGA -> AD9361 datapath-> AD9361 DAC.
They are not saturated to {-1.0, +1.0}, >=0.5 will cause internal logic
overflow and wraps around.能书能言 <2127629...@qq.com> 于2022年6月6日周一 19:51写道： > Hi,WarMonkey > Thank you for your answer, do you mean that the baseband amplitude > exceeds 0.5 and the DAC nonlinearity is reached? This is different from > what I understand, my knowledge is so scattered that I can't find the > answer I want from your answer, can you explain my question in more detail? > Best, > Regards > > > > linge93 > > > ------------------ 原始邮件 ------------------ > *发件人:* "WarMonkey" <luoshumym...@gmail.com>; > *发送时间:* 2022年6月6日(星期一) 下午3:18 > *收件人:* "能书能言"<2127629...@qq.com>; > *抄送:* "discuss-gnuradio"<discuss-gnuradio@gnu.org>; > *主题:* Re: Nonlinear distortion of USRP > > The problem is worse than what you think. > There's some digital processing logic from UHD data stream to USRP > internal ADC. DAC is the same. These logics are implemented in FPGA using > fixed point math. Wrap around happens if I/Q amplitude >=0.5, outputs are > totally messed up. > After DAC it's in analog domain, they are suspicious to saturation. > Normally DAC can be driven to its maximum output, but the following stages > (amplifiers, mixers) have P1dB limit, higher output result to lower gain > and higher intermodulation products. > > > 能书能言 <2127629...@qq.com> 于2022年6月6日周一 11:43写道： > >> Hi, >> Recently, I am confused about the process from baseband signal to RF >> transmission. I know that baseband signal is sent to USRP through UHD. If >> the signal amplitude exceeds 1, what will happen? I looked up some mailing >> lists and mentioned nonlinearity. I wonder what this nonlinearity means? >> There are many nonlinear devices in RF, such as ADC, amplifier, etc. which >> is the specific one? >> As I understood it earlier, suppose a baseband signal sample >> (0.8+0.8i) becomes Udacmax* (0.8+0.8i) when it enters the USRP. Udacmax is >> the maximum voltage. When a signal sample is (2+2i), it becomes udacmax* >> (1+1i)?Amplitudes exceeding 1 are forced to be limited to 1 and multiplied >> by the maximum voltage? >> When the baseband signal has a high PAPR, it will affect the RF >> operation. A high amplitude will bring nonlinear distortion to the power >> amplifier. Is this nonlinear distortion the same as the nonlinearity >> brought by the amplitude exceeding 1 mentioned above? If it is the same, >> can I evaluate the impact of high PAPR by changing the value of the >> "multiply const" module (the scaling factor before the baseband signal >> enters the USRP sink) to make the signal enter nonlinearity? >> Finally, I would like to know the whole process from baseband signal >> to RF electromagnetic wave (on USRP). Is there any website you can >> recommend? I have some fragmentary knowledge of signal processing, but I >> can't combine them to figure out the whole process. >> Looking forward to your reply, thanks in advance！ >> Sincerely, >> Regards >> >> >> >> linge93 >> >>
