On 2021-09-11 3:18 a.m., zhou wrote:
Hi Marcus,
I have a basic question: how to calibrate USRP? I have N321 and X310
models.
For X310, we have been calibrating them with loopback connection, that
is, connecting Tx to Rx with a 30dB attenuator between them. This was
suggested by an ettus engineer a few years back.
I am wondering if I should do the same to N321? or, the new UHD
doesn't need loopback? if no loopback is required, can I leave USRP
connected to other RF device which is our DUT, or I need to disconnect
USRP from it when calibrating?
What type of calibration? I/Q balance calibration?
I'm not sure that the N321 requires such calibration--unlike the systems
with changeable daughtercards, but I could be
wrong. I don't have an N321 in my collection of USRPs, so I'm
"blind" a little bit.
BTW, I find some information on gain range in N321 datasheet spec
obtained from
https://www.ettus.com/wp-content/uploads/2019/03/USRP-N321-Datasheet-5.pdf
<https://www.ettus.com/wp-content/uploads/2019/03/USRP-N321-Datasheet-5.pdf>
On the 3rd page, the Rx gain range is -16dB to 34dB, and Tx gain range
is -30 to 25dB. How can the gains be negative? does this mean
attenuation of signal in device? The small text on the bottom says RX
and TX path gain does not correlate to UHD gain settings. How to
understand this?
In page 5 and 6, the Tx gain and Rx gain ranges are from 0 to 60dB.
The uhd_usrp_probe command gives 0 - 60dB gain range.
All gain control is implemented in RF chains with variable attenuators.
In most cases, there's usually a low-noise gain
stage "up front" followed by a variable attenuator, followed by
another gain stage, or two. Sometimes, it's more
complex than that. I assume that the "-16dB" means taking total gain
below the gain of the first stage. At the end of
the day, what you need to worry about is that your gain setting
provides adequate SNR (without clipping) for the signals
of interest.
One should NOT confuse "gain control range" with "total system gain".
In most RF chains, there are many stages of
both gain and loss, and it's important not to confuse the *control
range* with *total system gain*.
Thanks.
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