Spectrum analyzer affcted by external trigger signal

[Guangtao Zhu]

I use an electrical spectrum analyzer (ESA) to observe a 2.5-GHz signal. Then I observe the Y-output of the ESA in a oscilloscope (OSC) (Channel 1, CH1), as shown in Fig. 1. This is the desired spectrum.

Then, I need to send a trigger signal (100 KHz) from a function generator (FG) to the OSC. However, if I connect the trigger signal to either CH2 or EXT TRIGGER port, the spectrum displayed in the ESA completely changes, as shown in Fig. 2. It looks like several narrow and unnatural fast-sweeping peaks. It is completely not the spectrum I need. How can the ESA be affected by a signal connected to the OSC?

Same thing happens when I connect the trigger signal to the external trigger port of the ESA, as shown in Fig. 3.

In all three cases, the ESA runs in the free runnning mode without being controlled by the trigger signal.

What is the possible cause of this problem and how can I solve it? I want to observe the correct spectrum while using the trigger signal.