Intermodulation in systems obeying Bloch's equations

Intermodulation effects in spin systems are analyzed using Bloch's equations with two sinusoidal driving signals. Circular polarization is assumed, and the results are given in terms of complex Fourier components of magnetization. Both homogeneously and inhomogeneously broadened lines are considered. It is found that maximum intermodulation occurs not when the two signals are nearly coincident in frequency, but when they are spaced by
\gammaH_{1}
rad/s, where H
1
is the amplitude of the larger signal. As a corollary, it is shown that in mixer applications, the local oscillator field should be chosen such that
\gammaH_{1}
is equal to the intermediate frequency. Experimental results are given for inhomogeneously-broadened nuclear magnetic resonance (NMR) lines and are shown to confirm theoretical predictions. These predictions are also found to agree with published data on intermodulation in nuclear-and electron-magnetic resonance limiters.