Sorry, that's incorrect. The B210 is completely linear up to +/- 1.0.
Here's a spectrum analyzer trace confirming it.
B210 linearity test
Ron
On 6/6/22 11:37, WarMonkey wrote:
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
