On 15/11/2024 08:19, yibinden...@outlook.com wrote:
Hi Müller and Leech,
Thanks for your help, and sorry for the ambiguity in my description.
The signal I used in my first figure was a rectangular wave. In the
other two figures, I used QAM-modulated data packets filtered with a
root-raised cosine filter. As you mentioned, the tailing issue was
indeed resolved after DSP tuning.
However, I seem to have encountered a new problem. I tried to send a
QAM data packet filtered with a root-raised cosine filter. According
to theoretical analysis, it should be a band-limited signal. However,
even after DSP tuning, the received signal at the receiving end has
its energy distributed across the entire operating band, and it is
difficult to demodulate correctly. Interestingly, this issue does not
occur when I use Pluto to send the same signal. I have attached the
code I used for sending.
Additionally, I have a curiosity-driven question: do I need to
constrain the input within the 0-1 range or any specific range?
Baseband signals using the default floating-point host-side
representation must be constrained to {-1.0,+1.0}, and
generally smaller than that to prevent saturation -- {-0.85,+0.85} is
often what I've used in the past.
You could also try backing your TX gain off by about 10dB -- you may be
getting a bit of clipping on the receive
side -- I think you'd mentioned a direct connection through a 30dB
attenuator.
Check that your signal in your .mat file doesn't exceed the required
baseband magnitude, as described above.
Marcus D. Leech wrote:
On 11/11/2024 10:10, Marcus Müller wrote:
Hello!
Regarding what you see in trailing, my guess is that this is
the step response of the built-in DC offset cancellation
filter; "DC offset cancellation" is high-pass filter
behaviour. This affects only frequencies in your signal that
are very low. It is meant to remove imperfections that happen
on every quadrature mixer&ADC device. So, unless you really
see a problem with the signal itself, this is probably fine!
You say you have an issue with this, but don't explain the
actual issue.
The phase in that trailing part can remain constant, that's
OK. The step response of a real-valued filter is real, and you
should simply see the phase of the last output sample at the
moment of "input switchoff".
Regarding "Amplitude and Signal length": I can't really tell
what you're showing us here. What kind of signal did you feed
into the USRP? Where does it come from? At which frequency?
What is the USRP tuned to? What's its sample rate? Most
importantly: What is it that worries you about this? As far as
I can tell, this might seem normal, and not an issue!
Best regards, Marcus
What type of signal? Narrowband signals can be considerably more
affected by DC-offset correction than wideband signals. One can
use offset-tuning to move the signal outside the "view" of the
DC-offset correction. The second argument to "tune_request"
allows you to specify an offset.
Also, how are these devices connected? "Over the air" or with a
cable. If with a cable, please ensure that there's adequate
attenuation in the cable to prevent overload or even damage to the
B210 front-end.
On 11.11.24 14:18, yibinden...@outlook.com wrote:
Hi everyone,
I'm working on a project where I generate a signal and
simultaneously receive it using both the Pluto SDR and the
USRP B210. However, I’m running into some unexpected
issues with the B210's reception, and I’m hoping for some
guidance.
Here are the main problems I’m encountering:
/Signal Trailing/: As shown in the figures, The signal
received by the Pluto has clear boundaries, while the
signal received by the B210 has noticeable trailing
compared to the Pluto.
/Strange Phase Characteristics/: The phase behavior of the
B210-received signal is unusual. Specifically，during the
trailing phase of the signal, the phase remains constant,
which is unexpected. When there is no signal, the phase
appears to be chaotic.
/Amplitude and Signal Length/: As shown in the figure,
when the signal length is relatively short, both the
maximum and the average amplitude increase as the signal
length grows.
I suspect that each sample might be significantly
broadened in the time domain, but since I am not entirely
familiar with the USRP B210's hardware processing, I am
unsure if this is the root cause. I am wondering if these
issues could potentially be improved by modifying the
hardware configuration, such as adjusting the filter
settings or other parameters. The code I’m using for the
B210 receiver is attached.
Has anyone experienced similar issues or have suggestions
on adjusting the B210's configuration or setup to address
these distortions? Any insights would be greatly appreciated.
Thanks in advance for your help!
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