FYI,
this is a fft scope WIHTOU TX (uhd_fft.py -f 450e6 -A TX/RX)
and this is the fft scope WITH TX (benchmark_tx.py -f 450e6 -A TX/RX)
as you can see, gain increased when TX is turned on.
Songsong,
That signal looks horrible. I'm pretty sure you're clipping on either the transmitter or receiver. Notice that the signal level is -10 dBm! The skirts only go down to -60 dBm, too, so you have some serious nonlinearities occurring there. Try backing off the power on the transmitted signal (--tx-amp=) and see how that does.
Tom