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?