Credit-card sized field and benchtop NMR relaxometers using field programmable gate arrays

Abstract

The tools for developing systems for NMR improve each year. Some firmware based designs based on Field Programmable Gate Arrays (FPGAs) have been developed to implement both the core and peripheral apparatus for NMR Relaxometers and NMR Cryoporometers. Hand held in size they are particularly suitable for desktop, mobile and field use. Software tools now enable the writing of firmware that enable the rapid design of digital and digitised R.F. data and control systems. This firmware can then be loaded into and run on hardware that often is only the size of a credit-card. These devices can include a single-chip Linux computer with an FPGA capable of processing digital RF on the same chip. These can generate RF signals digitally, process RF signals for up- and down-conversion, and can provide an on chip NMR pulse sequencer. An extremely well tested NMR digital transmitter has been upgraded to use the latest surface-mount Bipolar and Field Effect Transistors. The circuitry is now the size of a book of matches, and interfaces to credit-card sized FPGA modules for RF generation, phase shifting and drive signals. This NMR transmitter design uses digital logic principles rather than conventional tuned R.F. techniques Offering both active-damping and active-quench, a very square R.F. Pulse with fast turn off is achieved, with Mark 1 giving 2.6 μs π/2 and 4.8 μs π pulses into a 5 mm diameter NMR sample (2.7 mT B₁) at 20 MHz for Protons. T1ρ pulses are stable and consistent.

Keywords: Compact nmr; Desktop nmr; Digital NMR; Digital receiver; Digital transmitter; FPGA field programmable gate array; Low field nmr; Low noise; NMR PowerAmplifier; NMR firmware; NMR instrumentation; NMR pulse sequencer; NMR receiver; NMR relaxation; NMR relaxometer; NMR transmitter.