I came across this sentence in this paper.
Alternatively, we modulate the frequency of the microwave excitation voltage at 971 Hz with a square wave and a bandwidth of 32MHz.
In my limited FM knowledge, this sounds like the carrier frequency is 971Hz, meaning the signal is modulated at 971 Hz. The bandwidth is the width of the signal in the frequency space. So the signal within the frequency range 16 MHz below and above the carrier frequency would be transmitted.
Given that the modulation frequency is 971 Hz, the picture I had makes little sense to me.