A frequency-selective limiter using nuclear magnetic resonance

A new device is reported which promises to be of significant value in combating in-band interference. By the application of an inhomogeneous magnetic field to a specially shaped volume of nuclear spins, frequency-selective limiting is obtained over a band of frequencies much larger than the unbroadened NMR (nuclear magnetic resonance) linewidth. The NMR limiter is a passive, self-adaptive-filter with the capability of selectively attenuating signals that are separated in frequency by as little as a few cycles per second. The theory of this device is developed, and experimental results are given for a model operating at 30 MHz with a bandwidth of 1.2 kHz. Suppression by 30 dB of CW interference placed within the sidebands of an AM voice signal has been observed.