A Nuclear Magnetic Resonance Radiofrequency Pulse Transmitter Based on Digital Modulation Technology

Abstract

Radiofrequency (RF) pulse transmitter is an important part of a nuclear magnetic resonance (NMR) spectrometer. In this paper, a simple NMR RF pulse transmitter was designed based on field programmable gate array (FPGA) and direct digital synthesis (DDS) chip AD9910. Digital modulation of various parameters was achieved. The control accuracy of frequency, phase and amplitude reached 32 bits, 16 bits and 14 bits, respectively. The time accuracy of pulse modulation was 0.01 μs, capable of generating soft/hard pulses with duration longer than 0.1 μs and frequency less than 400 MHz. A "pulse + delay" model was established, and a generic list-type sequence controller was presented, as such the requirements for higher-level control units were reduced. The enveloping waveform of soft pulses was optimized using Hanning window, instead of rectangular window. Simulation and experimental results demonstrated that the use of Hanning window could effectively reduce frequency sidebands of the soft pulses.

Keywords

• nuclear magnetic resonance (NMR)
• radiofrequency pulse
• digital modulation
• sequence control
• Hanning window