Switching and high spike in spectrum

GNU Radio
1

USRPN210r4 with SBX

I am observing a strong spike at the center of the receive spectrum when
I
start burst transmission.

My top flowgraph contains following two hierarchical blocks

  1. A transmitter flow graph with (tx_time, tx_sob, tx_eob)
  2. A receiver flow graph

When I run top flowgrpah (without transmitting anything) and observe the
FFT of the received signal the spectrum does not contain high spike in
the
center.

But as soon as I start transmitting in burst mode I see a very high
spike
in center of the received signal FFT spectrum. It looks like LO
(transmitter or receiver ) is being received? Which one is it ? And why
is
it happening? How can I avoid it because it is affecting my packets.

When I apply the offset in digital using DDC/DUC, the spike moves out of
the band.