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? 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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