On 02/09/2021 03:48 PM, Luca Lodigiani via USRP-users wrote:
Hello,
My team and I are doing some experiments with the USRP B210 and NB-IoT
waveforms.
During tests we have observed a few issues with the RF output, that we
would like to understand better, and /hopefully/ fix or mitigate. I
must admit that we are not yet very familiar with USRP SDRs, so I
apologize in advance for any stupid question! There are so many
options to tweak the USRP :-) !
This is what we have observed:
1. Apparent RF leakage from RX to TX when idle (i.e. not transmitting
the 180 kHz NB-IoT carrier). Our B210 was connected to an RF
matrix where other signals in other channels not immediately
adjacent to our center frequency also exist, and, every time we
were not transmitting, some of those signals (a MHz away) were
picked up from the RX and “leaked” into the TX – this could be
observed from a spectrum analyzer connected to the B210 TX and it
appears to be picking up quite a wide bandwidth (more than 5-10 MHz).
1. Is this a known issue with the B210 RF frontend?
2. Does anyone have any suggestions on how to mitigate it (if
even possible)?
3. We tried limiting RX bandwidth as described here
https://files.ettus.com/manual/page_usrp_b200.html#b200_fe_bw , but
it seems not change anything at least when idle. Any ideas?
Given that the leakage path that you're (I think) talking about would be
VERY VERY early in the RF chain inside the AD9361 chip, setting
analog bandwidth would have ZERO effect, since that sets the *IF*
bandwidth--well after the bits and pieces you're likely dealing with.
I haven't looked at the TX/RX isolation spec in the AD9361
data-sheet, but it would surprise me very much if it were better than 50dB.
It's pretty hard inside such a tiny tiny piece of silicon to isolate
RF signal pathways more than that. What power levels is the RX2 port
seeing in your application--your "RF matrix" really needs to not be
presenting more than about -15dBm to the RX2 port(s), or there's
potential for damage.
Also, when you talk about 180kHz carrier, you're talking *bandwidth* or
center frequency? The center frequency on the B210 cannot go
below about 50MHz.
1.
1. Out-of-band emissions and spectral regrowth: The output NB-IoT
carrier shows quite large out-of-band emissions (~-30 dBc @200 kHz
from center frequency, -50 dBc @400 kHz).
1. Could this be due to non-linear distortion caused by the B210
frontend (AD9361) ?
2. Would limiting TX analog bandwidth with set_tx_bandwidth(bw)
help here?
It might Help to lower TX analog bandwidth, but UHD automatically sets
it based on your sample rate. Also try reducing baseband magnitude
a little bit, as well as reducing TX gain a little bit.
1. Any other suggestions?
I also have some other general questions...:
1. Even after reading this
(https://files.ettus.com/manual/page_power.html ) I am still not
super clear about if and how the set_tx_power_reference() (and
rx) affect the gain level. I think understand TX gain setting
will obviously affect the output power though.
The tx_power_reference() API is new, so I cannot comment on how it works
out in practice. The B2xx have about 80dB of TX gain setting
range, but with peak power output of about +7dBm over most of its
frequency range, I'd say only the top 30dB is really useful.
1. When talking about multiple "gain stages", how does this translate
to the B210? Different stages of the AD9361 ?
Yes, different stages within the AD9361 in the case of B2xx.
1. How are set_tx_bandwidth and set_rx_bandwidth implemented in
practice in the B210?
By using features inside the AD9361 to set the effective analog
bandwidth in front of/behind the ADCs/DACs. I'm not sure whether
the implementation inside the AD9361 is purely-analog (like
switched-capacitor filters, and changing clocks), or a hybrid approach
that uses the DSP logic inside the AD9361. The ADCs and DACs in the
AD936x family operate at very high sample rates internally,
and use DSP logic to provide the actual
delivered-to-digital-interface sample rates, so there's room to do both
a digital and analog
approach in providing the necessary effective analog bandwidth. But
UHD normally picks an analog bandwidth setting that is
appropriate to the desired sample rate. The only time this may not
be appropriate is if your actual signal is very much narrower than
is implied by the selected sample rate.
Many thanks
Luca
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