Circuit for a digital pulse programmer

Abstract

A versatile digital pulse probrammer circuit is described which is capable of generating all of the major sequences for pulsed NMR spectroscopy involving up to four independently phased rf channels. The system provides control over the applied transverse magnetic field, H1, with a sufficiently varied range of timing intervals that it can be, and has been, applied to both liquid and solid systems. While the programmer has been designed specifically to meet the needs of pulsed NMR, the design could be applied to other systems requiring flexibile control of experimental parameters. Modular in format, the programmer consists of a main clock unit and four identical channel units. The main clock module controls the time between repetitions of the sequence, while the channel modules specify the time placement and duration of the rf bursts through each gating channel within a sequence. Complex pulse sequences are constructed with control pulses sent from module to module down a sequential hierarchy: the ability of each channel to loop a predetermined number of repetitions at a set rate, while producing resetting and initiating pulses to the next channel allows the production of repetitive sequences, such as the Carr–Purcell and the saturation burst. Automatic advance of delay times is also achieved through this sequential initiating technique. The output of trigger pulses from each channel module provides additional flexibility when using peripheral equipment such as signal averagers. Small and medium scale TTL integrated circuits are used throughout as the circuit elements for the device. A 10-MHz crystal clock oscillator provides highly stable timing signals for the entire programmer.