Device for highly precise synchronization of the NMR transmission frequency to the resonance frequency of an NMR line while taking into consideration a non-constant RF phase

摘要

A device that comprises an RF generator (29), an NMR transmission (20) and reception system (21) and a first control loop (28), with which the frequency f_RF of the RF generator is synchronized with the resonance frequency f₀ of an NMR line, wherein, from the signal of the RF generator, a train of excitation pulses (EX) of the repetition frequency f_m is generated, with which nuclear spins of a certain resonance frequency of an associated NMR line are excited quasi-continuously (CW) and, in the times between the excitation pulses, the NMR signal is received (AQ), wherein the period time 1/f_m is chosen to be much smaller than the relaxation time of the NMR line, preferably shorter than 1/10 of the relaxation time, and the NMR signal U_D mixed down into the low-frequency range is used, with the help of the first control loop, to closed-loop-control the value of the transmission frequency (=frequency lock) or the value of the B₀ field (=field lock) in such a way that the frequency and phase of the RF generator and the NMR line match as precisely as possible, characterized in that a phase shifter (22) that rotates the radio-frequency phase of the NMR transmission and reception system in the first control loop by the value Δφ is controlled by a second control loop (27) whose input signal comes from a signal extraction stage (25, 26) that extracts the saw-tooth AC component of the signal U_D and maintains this input signal at zero by closed-loop control. In this way, the influence of the RF phase on the synchronization process can be taken into consideration to improve the precision with which the resonance frequency of an NMR line is synchronized.