Unexpected low frequency component in received USRP signal


This is a radio problem I can not explain:

Transmitter: USRPN210, BPSK, linkrate = 100kbps, TX-gain=15
Receiver: USRPN210, Rx-gain=15
TX-RX: 1.5m.

Below is what the I/Q samples I get, after timing synchronization. There
no center frequence offset as I share the same clk for tx and rx.

It looks like the if the I or Q amplitude is low, a frequency leakage is
displayed in this picture. I have used the tune_request for both the tx
rx to the same offset of sample rate.

And I have monitored the radio spectrum by measuring the noise, no
interference is found.

Really confused for this observation.

Dreams can come true just believe.