Re: Nonlinear distortion of USRP

[Marcus D. Leech]

On 2022-06-05 23:33, 能书能言 wrote:
> 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
There is a convention established in the SDR world that floating-point 
baseband signals are *scaled* into {-1.0,+1.0}.  The hardware then 
scales this internally into the ranges that
  are appropriate for the DAC and ADCs involved, after possibly 
performing DUC (Digital Upconversion) or DDC (digital downconversion) 
operations inside the FPGA.  If your
  baseband signals exceed {-1.0,+1.0}, they are simply clipped into 
that range.

The hardware designers try to match the largest output of the DAC to the 
largest IF port signal requirements of the mixers and amplifiers, but 
that "match" is necessarily
  somewhat fluid due to the vagaries of analog hardware operating over 
large frequency ranges and other operational parameters.  So, the 
recommendation is to
  have your baseband signals not exceed about {-0.9,+0.9} or perhaps 
somewhat smaller.

Ettus/NI don't provide a "structured walk-through" of their hardware.  
But they all follow a similar approach that would be very familiar to 
any engineer that is somewhat familiar
  with RF system design, most SDRs follow a very similar design 
pattern, regardless of manufacturer.

On the TX path:


computer-software---->interface-to-SDR--->DUC(FPGA)--->DAC--->MIXER--->RF-AMPs---->Antenna-Port
^
|+LO

On the RX path:

Antenna-port-->RF-AMPs--MIXER--->ADC--->DDC---interface-to-SDR---->computer-software
                                              ^
                                               |+LO




You'll find diagrams like that scattered all over Google when you search 
on terms like "what is an SDR?".