On 05/15/2019 10:46 AM, Achilleas Anastasopoulos wrote:
Hi all,
I have recently run an old experiment that used to work fine:
One UHD:USRP Tx is sending a constant tone at 1GHz
One UHD:USRP Rx is receiving at 1GHz and displays in X-Y mode.
Observations:
1) When the 2 USRPs have default (internal) clock I see a circle (due
to frequency instability).
2) When they have external clock (eg either from octoclock, or one
feeds the other in a master/slave configuration) I see a nice dot (off
center) that is pretty stable thus showing that the Tx/Rx are
time/frequency synchronized.
When I run this experiment recently
(with GNURADIO 3.7.13.5 and UHD 3.14.0.0)
I got observation 1) but NOT 2).
In fact any time I connected an external clock and did 2) the Rx
didn't seem to receive anything.
After several hours of agony (thinking I have burnt the REF IN port) I
realized what the culprit was:
The switch in the UHD:USRP source block in the FE Correction tab was
"Enable DC offset correction: True"
Once I turned off the DC offset correction everything worked as before.
In retrospect I realize that this is the intended behavior, since the
USRP is receiving a DC signal and tries to eliminate it.
Clearly one way to avoid this is to receive at 1GHz+df and then
correct for df in software.
I am writing this note in case someone else comes up with a similar
situation and wonders what is going on...
Another issue I saw is that no matter what the switch is for the IQ
imbalance correction (True/False) I always get a warning when I run
the program:
[WARNING] [MULTI_USRP] Setting IQ imbalance compensation is not
possible on this device.
Q1) Is this a bug or a feature?
It depends on what kind of device/device configuration.
Finally one last question (probably off topic since it is USRP related):
Q2) Is it OK to connect the REF OUT of one USRP to the REF IN of another?
The REF OUT says 3.3V and the REF IN says +15dBm MAX.
I used to do that all the time without burning anything but now I am
skeptical...
thanks
Achilleas
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