[Discuss-gnuradio] Why I cannot see a TX pulse and the RX pulse in the same oscilloscope in time domain

Hello all,

I did some test when transmitting and receiving a pulse. After seeing the transmitted and received signal on a single oscilloscope (in GRC).

Flowgraph:
Vector source - Oscilloscope A - Rational resampler - USRP2 sink (500 interpolation)
USRP2 source (500 decimation) - Rational resampler - Oscilloscope A - Nule sink

The received pulse signal is not stable (oscilloscope triggered with tx signal). It is like random phase so that I can not measure the delay between transmitted and received pulse.
I expected to have two fix pulsed signals in the oscilloscope at a fixed delay.How can I do it?

Many thanks,
Jorge.