Tx Rx IQ diagram won't resolve

[Frey66]

Hello,

I'm using two NI USRP-2901s with the Modulation Toolkit Examples Tx PSK and Rx PSK.

When I first used this, I could transmit QPSK with no filter and receive with no problems. I left it for a while and returned to try again and for some reason the received IQ constellation won't resolve and seems to pulse in bizarre patterns or in a circle. I've tried changing the parameters and have checked that the parameters match but to no avail.

Please help!

Thanks

[AAlZibdeh]

Hello, 

Do you still get the problem if you turn off the USRP for a bit and on again?. I suspect that an LO Drift might be causing your issue. 

Regards, 

Abdullah