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?".
