10^-32 Really? That is small.
Hold on :
Fundamental is clearly 1MHz (1us period).
First breakpoint is at 1/PiTw = 1/(3.1 * 10ns) = ~32MHz.
Second breakpoint is at 1/PiTr = 1/(3.1 *100ps) = ~ 3200MHz.
Thus the 101st harmonic is on the 1/f slope.
Low frequency amplitude is (from figure 4) 2ATw/T, Tw is 10ns, T is 1us, so below the first breakpoint amplitude is 2% of whatever the peak amplitude of the 'square wave' is. And your 101st harmonic is only about 3 times the first breakpoint, so I find 10^-32V highly unlikely.
Of course you might have a combination of numbers that happens to give a deep null at that particular harmonic, and I really cannot be arsed to do the sums.
Regards, Dan.