Spectrum Analyzer Mode -- TX Duty Cycle With Time Domain?
Since the input frequency is fixed you can set the spectrum analyzer to this frequency and then change the X-axis to display amplitude over time instead of over frequency.

This mode is called zero span mode. Here's an example:



The X-axis shows time from 0 (trigger point) to 10 ms

The Y-axis shows the signal amplitude

Note the "Span 0 Hz" in the bottom right corner
Here the signal(s) are present at 3 MHz in a 3 MHz (-3 dB) resolution bandwidth.

So signals between 1.5 MHz and 4.5 MHz show with nearly the correct (less than 3 dB attenuation) amplitude.