Problem when using USRP sink and USRP source in the same flowgraph

[address@hidden]

Hello everyone

I want to use the received signals from USRP source to control the transmission 
signals into USRP sink but it doesn't works well. 
(As shown in the attached flowgraph)

The USRP sink doesn’t transmit at all while flowgraph is running without any 
error and alarm, or just transmits in an intermittent manner (as observed in an 
external spectrum analyzer). 
It also outputs lots of “L” in command window thus I can only terminate it by 
terminating the gnuradio-companion.

But when I replace the USRP source with a random source, USRP sink works and I 
can see the continuous transmit signals in spectrum analyzer. 

In order to emit the effect from unstable control output from received signal, 
I also test to process it through a Or block with a constant source 1. I expect 
that the USRP sink can transmit in a continuous manner in this case, but it 
still doesn’t not work.  

I also tried to set USRP source and sink with different clocks (one with PPS 
and one with PC-clock), or different frequencies, but all failed.

The USRP source and USRP sink use TX/RX and RX2 in the same daughter board of 
X310, respectively.
(There is an attenuator in the loopback part between TX/RX and RX2) 

My OS is Ubuntu 18.04, with gnuradio 3.8, gcc 7.4.0, Python 3.6 and uhd 3.15.0. 

Is there anyone can help me? Thanks in advance

Yun

flowgraph.png
Description: flowgraph.png