On 26/12/2023 06:43, Francisco Gallardo lópez wrote:
Hi all,
I am seeing a quite strange pattern in an USRP B210, and I am trying
to figure out what it is.
Right now this is more like an academic investigation, as this is not
causing malfunctions, but it got my attention.
So, from time to time I am seeing a tone (20dB above noise)
appearing just in the middle of the band, to be precise ±2KHz around
the central frequency that I configured in the USRP.
I was not paying attention at all, until I noticed that it appears
every day seven times a day, and always (more or less) at the same
times. Hence, this is not random.
Gut feeling is that this is caused by some sort of signal induced into
the cables or locally radiated interference, not intended, of course.
I was trying to further investigate, and for that I was trying to get
a detail blocks diagram of the B210, as I saw the following
information in the B210 website: "If you, however, happen to have a
very strong interferer within half the master clock rate of your RX LO
frequency, you might want to reduce this analog bandwidth. You can do
so by calling uhd::usrp::multi_usrp::set_rx_bandwidth(bw)."
B200/B210/B200mini/B205mini - Ettus Knowledge Base
<https://kb.ettus.com/B200/B210/B200mini/B205mini#FAQ>
Now the thing is, I realized I don't really know what the internal
settings of the USRP are, also in terms of the Analog Devices AD9363.
I know that the clock rate is 32 MHz, but from there I don't clearly
know what is the LO frequency that the USRP set.
Also, I would like to know the setting that the USRP is using for the
Analog bandwidth.
In short, I would like to:
a) Get a diagram like this one, but for the detailed case of B210. Is
there something like this? (File:2920 simplified system diagram.gif -
Ettus Knowledge Base
<https://kb.ettus.com/File:2920_simplified_system_diagram.gif>)
b) Know what is the LO freq that the USRP is setting (I assumed based
on the 32MHz clock rate)
c) Know where I can get the information on the setting the USRP is
putting for the analog bandwidth filter.
Thanks!
Fran
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Francisco:
There's the schematics here:
https://files.ettus.com/schematics/b200/
Nearly all SDRs use direct-conversion, which means the LO is the same as
the desired RF frequency. The B2xx series works like this.
Bandwidth can be set with a set_rx_bandwidth call, formally described here:
https://files.ettus.com/manual/classuhd_1_1usrp_1_1multi__usrp.html#a4e026819f286e69c48c2e1956d95c6fb
Normally, UHD (the driver) will set the analog bandwidth to something
"appropriate" given your sample rate.
If you change to a slightly different frequency, does this occasional
RFI stay at the same frequency?
If you lower the RF gain, does it go away?
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