Operational amplifiers revisited for low field magnetic resonance relaxation time measurement electronics

Advances in permanent magnet technology has seen many examples in the literature of sensor applications of low field magnetic resonance and these are usually based on the measurement of relation times. Whilst most reports are either in the 10-20MHz range or in the earths field, measurements at below 1MHz are beginning to become more widespread. This range of frequencies is below the need for careful radio frequency electronics design but above the audio domain where extensive commercial hardware is readily available and represents an interesting cross over. Most wideband commercial NMR spectrometers do not include the pulse power amplifier, duplexer and preamplifier as these depend on the frequency range over which they are to be used. In this work we demonstrate that the humble operational amplifier in the most simple form of an inverting design, using only two resistors and decoupling, can effectively provide this ‘front end’ electronics. A Linear Technology LT1363 amplifier provides the pulse power amplifier and a Linear Technology LT1222 the preamplifier. Example data using a CPMG sequence and an olive oil sample is presented from which T2(effective) can be determined and the signal intensity as a function of repetition time to give an estimate for the spin lattice relaxation time T1. The low powers used mean crossed Ge diodes provide an excellent duplexer and it has been found to be ideally suited to battery powered applications.