An Acquisition System Design of NMR Analyzer

We designed an acquisition system which connects a miniaturized probe to measure the NMR signals in gradient field or uniform magnetic field. ^[1] Key issues of the data acquisition system are high power output, weak signal acquisition and short dead time. ^{[2] [3]} Power amplifier circuit which was designed by the combination class E power amplifier transmits excitation pulse, and the peak to peak value is 1200V at the 180V DC power supply. Power amplifier driver was constructed with half-bridge switch. The receiver was constructed from a series of low-noise component. The preamplifier is based on low-noise triode amplifier architecture. The programmable attenuation was employed to widen the dynamic range of the circuit to the maximum gain of 65dB. The dual-MOSFETs were utilized for bleeder circuit to discharge the antenna directly, which quickly release the energy and shorten recovery time of the antenna. Additionally, de-coupler controlled by active MOS was adapted to protect the receiver as well as further decrease Q value and shorten recovery time when the voltage of antenna is less than 1V. Additionally, de-coupler controlled by active MOS was adapted to protect the receiver. Digital Signal Processor (DSP) and Field Programmable Gate Array (FPGA) were embedded in main-controller to generate pulse sequences, control instruments and acquire signals, etc. The amplitudes and phases of echo signals were extracted with the digital phase sensitive detection (DPSD) algorithm. We used this device to test a downhole fluid Analyzer probe, and acquired good CPMG echo trains.