I can't comment on issue #1, but I do have experience with issue #2. To
get a clean output spectrum, you need to do two things.
First, the IQ values generated by your application can never exceed
+1.0/-1.0 in value. You have to account for the PAPR of your waveform.
If you're using GNU Radio, I have a block to dump peak IQ levels.
https://github.com/drmpeg/gr-iqlevels
Second, you can't set the TX gain to maximum. Something less than or
equal to 78 will typically provide the best performance (although the
exact maximum value will depend on the PAPR of your waveform). Here's a
B210 transmitted signal at a gain of 78.
Ron
On 2/9/21 12:48, 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?
2. 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?
3. 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.
2. When talking about multiple "gain stages", how does this translate
to the B210? Different stages of the AD9361 ?
3. How are set_tx_bandwidth and set_rx_bandwidth implemented in
practice in the B210?
Many thanks
Luca
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