Markhasin, E., et al., Efficient, balanced, transmission line RF circuits by back propagation of common impedance nodes. J. Magn. Reson., 2013. 231(0): p. 32-38.
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 (1H) and 125 MHz (13C). 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 (γB1 = 100 kHz for 1H and 13C at 70 W and 180 W of RF input, respectively; up to 360 kHz for 1H). 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.