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 > >
