TSG operation in LimesuiteGUI

I could be missing something in the concept or use of the TSG feature in LimesuiteGUI with the LMS7002M and thought to check if someone has a comment after a glance at the screen shot. The transmitter output is connected to the receiver input externally. Should loading the DCI or DCQ of the DC register with zero set the respective I or Q output to zero? Setting either one to zero causes both Q and I to drop by 3 dB. Monitoring the Tx output with another SDR (rtl-sdr with gnuradio) agrees with the FFT viewer display.

Confusion on my part. I didn’t have the rxTSP mixer set to down convert so that dc on the tx side should appear as a tone and the tone appear at d.c. This seems to be the most reliable way of extracting phase shift through a channel.