On 11/25/2019 04:32 AM, Julian Ilinca via USRP-users wrote:
Hello all,
This message is a reply to a previous thread which I started. The
contents of which can be found below.
https://www.mail-archive.com/usrp-users@lists.ettus.com/msg09327.html
My question now concerns what data is actually stored in the final I
and Q components that are sent to the host. The question and the
answer are presented below.
Q1
The daughter-board is set so that one channel retrieves the I
component and the other channel the Q. I retrieved this information
with the uhd_usrp_probe command. In the output under RX0 its says
connection type = Q and on RX1 connection type = I. What command do I
have to use to change connection type of RX0 from Q to I, so that I
could have two channels measuring the real component.
A1
That isn't possible. Signals are presented to the host as complex
samples. The TVRX2 *analog hardware* uses a so-called "low-IF"
architecture, and the N210 has a single 2-channel complex ADC. Each
of those two channels is *hard wired* to go to one of the ADC
channels, and the DDC algorithms in the FPGA convert that to a complex
base-band signal that is then filtered, downsampled, and sent to the
host computer.
From this answer, am I to understand that both of the signals coming
from antenna 1 and 2 respectively are first mixed with a signal
provided by the local oscillator (LO) so that they are separately
converted to baseband. After which the information of the baseband
originating from antenna 1 is stored in the Real part of the complex
number and the signal coming from antenna 2 to imaginary part. And
only then then filtering and down sampling occurs. What I want to know
is that are the downsampled and filtered baseband signals from antenna
1 and 2 preserved in the IQ components respectively.
Furthermore, as I and Q usually mean in-phase and quadrature
component, is the signal coming from antenna 2 mixed with a LO-signal
that has been shifted by 90 degrees in phase compared to the LO-signal
that is mixed with channel 1.
The two analog channels (antenna ports) on the TVRX2 are separate--they
can be tuned separately, and go through separate
mixing pathways down to a "low-IF". That low-IF is sampled by the
single two-channel ADC on the motherboard, which creates
two separate I/Q data streams by converting the low-IF to baseband
digitally inside the FPGA. Again, there are TWO channels
that can be created by the TVRX2, and EACH of those channels
appears to the host as a complex-baseband.
Thank you again
PS.
I'm sorry, but I could not reply directly to the thread as I did not
receive a response email to my in-box. If someone can instruct me how
to accomplish the reply procedure with gmail, or provide a link with
the relevant information I would be more than happy to use the list as
it is intended to be used.
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