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[Discuss-gnuradio] UHD:USRP source FE options: recent experience and 2 q

From: Achilleas Anastasopoulos
Subject: [Discuss-gnuradio] UHD:USRP source FE options: recent experience and 2 questions
Date: Wed, 15 May 2019 10:46:17 -0400

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.

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 and UHD
 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...


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