Researchers tend to use the units of kHz to represent the power of decoupling or shaped pulses in research papers. The reason for the use of this unit is to easily transfer the pulse-widths & power-levels used in the experiment from one spectrometer to another, as one can back calculate the pulse-widths & power-levels as described below. The pulse frequency that is described here (in Hz) is the precession frequency about the magnetic field experienced due to the pulse in the rotating frame. This is not the frequency of pulse (B0), so please don’t confuse with this value.
The flip angle of any given pulse is given by
Where τα is the duration of the pulse to cause the flip angle α, with B1 being the magnitude of the magnetic field caused in the rotating frame. But the precession frequency(Hz) is defined as
Solving for B1 will result in
And for a 90° flip angle we can substitute α=90 or π/2, we get
For example, a 25 kHz decoupling pulse would have a 90° flip angle of 10 µs.
Now that we know how long the pulse need to be applied, we still need to figure out the power level for this pulse. Assuming a linear amplifier, we use the following equation for determining the unknown power level,
So if a calibrated pulse of 7 µs at -9.6 dB is known, a 25 kHz (ie 10 µs pulse) would require -6.5 dB power level to flip desired magnetization by 90°.
