This sinc function represents the B_{1} field as a function of frequency being sent into the sample.
Only magnetization vectors in the sample with resonant frequencies that are also found in the sinc function will experience a rotation.
The NMR signal is related to the amount of Z magnetization (M_{Z}) rotated into the XY plane (M_{XY}).
For a rotation angle q, the signal is

For M_{XY} to be greater than or equal to 0.9, θ must be between 64.15 and 115.85 degrees.
The rotation angle is proportional to B_{1} through the rotation equation

B_{1} is proportional to the magnitude (sqrt(RE^{2} + IM^{2})) of the sinc function.
Adopting a normalized B_{1} (B_{1}=1 for a 116 degree rotation), B_{1}=0.553 for a 64.15 degree rotation.
Define ν as the frequency in the laboratory frame of reference.
The RE and IM parts of the function are:

Solving this equation numerically we get the range of frequencies, Δν = 22.6 kHz, or 359 ppm.

This result is significant because it tells us that an 359 ppm wide spectrum would easily be rotated by approximately 64 to 116 degrees (giving 90% of the possible signal) with a 50 microsecond wide pulse.