A Digital rf Pulse Burst Generator for NMR Relaxation Experiments

Abstract

This paper describes a pulse burst generator designed to provide coherent rf pulse sequences with lengths applicable to NMR relaxation experiments. The burst generator is based on a principle which provides an inherently high on/off ratio. The frequency of interest is generated by mixing two higher frequencies. Bursts of arbitrary length are generated by gating these two higher frequencies so that in the off intervals there is no signal at the frequency of interest in the entire generator system. Burst lengths are determined digitally to within one cycle by a preset counter. A number of different burst lengths can be selected by several sets of switches to generate a Carr‐Purcell sequence of bursts. Bursts with 90° rf phase shift can be generated in this sequence as is required for the Meiboom‐Gill method. Phase and frequency relations in the generator are determined digitally, so that no tuning adjustments are required for operation of the generator over the whole frequency range. The described generator employs MECL II integrated circuits which restrict the highest output frequency of the generator to about 10 MHz.