How to measure a fast-varying RF power in a circuit?
If you can set the RF signal to a fixed frequency, for example 200 MHz then set the SA's center frequency to that frequency (200 MHz), then use the "zero span" mode.

This will plot power over time (at the 200 MHz we set earlier) instead of the "normal" power over frequency.

Then you can adjust the sweep time so include a couple of 100 Hz periods, 50 ms would be a good choice.

It can be that you have to adjust the Resolution Bandwith (RBW) (and maybe Video bandwidth) to be able to select that 50 ms. Start with high RBW values (a couple of MHz).

If you cannot fix the RF frequency then the signal could be outside the SA's "window". At Fcenter = 200 MHz and RBW = 10 MHz that "window" would be 195 - 205 MHz. Most Spectrum Analyzers unfortunately do not have a higher RBW.