There is too much power being delivered to the instrument.
The Spectrum Analyser is a Swept Superheterodyne design and does not have preselection filters. Therefore, the mixer is seeing all of the power (full 9kHz-1.5GHz frequency range) regardless of what you have set for the start and stop values.
A fixed attenuator should be added to the input before running any more tests. Perhaps 10dB to start. Then, scan quickly (100kHz RBW) from 9kHz-1.5GHz and look for peaks near 10dB or greater. These spikes are overpowering the mixer and could damage the instrument.