Efficient, balanced, transmission line RF circuits by back propagation of common impedance nodes

Abstract

We present a new, efficient strategy for designing fully balanced transmission line RF circuits for solid state NMR probes based on back propagation of common impedance nodes (BPCIN). In this approach, the impedance node phenomenon is the sole means of achieving mutual RF isolation and balance in all RF channels. BPCIN is illustrated using a custom double resonance 3.2 mm MAS probe operating at 500 MHz ([superscript 1]H) and 125 MHz ([superscript 13]C). When fully optimized, the probe is capable of producing high homogeneity (810°/90° ratios of 86% and 89% for 1H and 13C, respectively) and high efficiency (γB[superscript 1] = 100 kHz for [superscript 1]H and [superscript 13]C at 70 W and 180 W of RF input, respectively; up to 360 kHz for [superscript 1]H). The probe’s performance is illustrated by 2D MAS correlation spectra of microcrystals of the tripeptide N-f-MLF-OH and hydrated amyloid fibrils of the protein PI3-SH3.