Tunable NMR Receiver Front-End Using Variable Mems Capacitor

Abstract

In this paper, a tunableMicroElectro Mechanical Systems Nuclear Magnetic Resonance (MEMS NMR) receiver front-end, which consists of three main parts, is presented. In the first part, a proper microcoil for detecting [Formula: see text] and [Formula: see text] with complete specifications regarding the sample is optimized to get the better Signal to Noise Ratio (SNR) and quality factor (Q). In the second part, the passive network is discussed and the amounts of the desired capacitor and the desired tunability range are calculated and based on the calculations and the fabrication process, a new MEMS tunable capacitor is presented. Due to the large capacitance value of the required capacitor, the proposed capacitor has a fixed part with a capacitance value of 28[Formula: see text]pf and a variable part with a tunability range of 193% (2.25[Formula: see text]pf–6.6[Formula: see text]pf) with 1.5 A applied to a thermal actuator. A buckling effect due to the weight of the fingers of the capacitor and thermal stress after the release is analyzed and a fabrication process based on the routine processes is proposed. In the third part, a low noise amplifier (LNA) is presented for the proposed receiver with a gain of 47.6[Formula: see text]dB at the bandwidth of 384 MHz and noise figure of 0.5[Formula: see text][Formula: see text]. In this work, modeling and investigation of surface microcoils and also the capacitor are conducted using finite element simulation in COMSOL Multiphysics and post-processing. The method for obtaining signal sensitivity and SNR of the microcoils are based on the MR signal generation by employing the principle of reciprocity.