Abstract: An apparatus for tuning and/or matching a RF coil in a NMR probe comprising a first variable capacitor electrically connected to the coil and a first motor capable of coupling to the first variable capacitor. The first motor when coupled to the first capacitor adjusts a capacitance associated with the first capacitor. The first motor is capable of operating in a strong magnetic field and is capable of not disturbing homogeneity of said magnetic field when said motor is not operating. The apparatus may further include a second variable capacitor and a second motor. The first motor is further capable of coupling to the second capacitor for adjusting a capacitance associated with the second capacitor. The second motor moves the first motor between coupling with the first capacitor and coupling with the second capacitor. The mechanism may be employed to manipulate variable inductive loads for tuning and/or matching.

Abstract: Parasitic excitation of an NMR sample is minimized by providing shielding of the sample against RF pick-up from coil leads or like proximate conductive structure in the interior region of the RF coil.

Abstract: A correction for NMR spin echo spectral data is obtained by adjusting the time origin of an extended density data set and transforming the adjusted origin data set of original density to the frequency domain.

Abstract: An air bearing sample spinner for use in an NMR probe has a first air bearing established between frusto-conical mating surfaces of rotor and stator to provide levitation and rotation. A cylindrical portion projects from the rotor into a similar coaxial cylindrical portion of the stator where a journal bearing is provided for centering of the rotor in the stator housing.

Abstract: The field-frequency lock system of an impulse NMR spectrometer operates in an automatic bidirectional search mode characterized by relatively low pulse repetition rate, and relatively high power excitation. Searching is accomplished by monitoring the frequency of an absorption mode lock signal with a bidirectional counter. The sense input of the bidirectional counter is derived from a phase comparison of the absorption mode signal with the concomitant dispersion mode signal. The resulting counter output is converted to analog form yielding a search mode control signal. Transitions in the sign of the phase comparison signal initiate a transition to the relatively low power, high repetition rate excitation of the lock mode. In the search mode, facility is provided to maintain the control signal output although certain transient conditions may reduce the amplitude of the lock signal below minimum levels for operation.

Abstract: In a nuclear magnetic resonance spectrometer a gate is employed for time share multiplexing of the resonance and transmitter signals. An improved gate circuit is connected in series between the receiver and the resonance detecting circuit employed for picking up resonance of the sample. The improved gate circuit includes a pair of parallel connected branches of PIN diodes connected between the input and output terminals of the gate. The diodes in each parallel branch are poled toward an intermediate control node in such a manner so that a positive control potential applied to the control of one branch node turns that branch on and conversely a negative control potential applied to the other control node turns on the other branch. Equal magnitude and opposite sign control pulses are applied to the control nodes so that transient switching signals, tending to leak through the gate, cancel on the output line to the RF receiver.

Abstract: In a nuclear magnetic resonance spectrometer, the probe structure includes an analysis channel RF magnetic coil disposed in RF magnetic field exchanging relation with a sample of gyromagnetic resonators under analysis for detecting nuclear magnetic resonance of the sample. A control group of nuclear magnetic resonators is also disposed in the sample region and nuclear magnetic resonance of the control group is excited and detected by a second RF magnetic coil. The control RF coil is formed and arranged relative to the sample RF magnetic coil to minimize cross coupling between the sample and control channels. In a preferred embodiment, the control RF magnetic coil is coaxially disposed of the analysis RF coil and is further segmented into a pair of axially spaced portions disposed at opposite ends of the analysis coil and connected in magnetic field bucking relation relative to each other so that the RF magnetic field generated by the control RF coil cancels in the region of the analysis RF coil.

Abstract: An analytical instrument employing ensemble averaging to improve the signal-to-noise ratio including means for determining the most efficient scaling factor and for controlling the instrument to maintain assurance of efficient averaging.

Abstract: RF excitation for a spectrometer is obtained by pulse width modulating an RF carrier to produce the desired broadband RF exciting spectrum. The RF excitation includes a train of composite RF pulses, each composite pulse having a primary pulse portion of a first RF phase and a second pulse portion of a second RF phase opposite that of the first. In this manner, the finite rise and fall times of the primary pulse portion are compensated for by the corresponding rise and fall times of the secondary pulse portion. The primary pulse portion is lengthened by an amount equal to the secondary pulse portion so that the secondary pulse portion cancels the added primary pulse portion. In a spectrometer the compensating second pulse component removes certain undesired side bands of the RF excitation caused by the finite rise and fall times of the applied RF pulses.

Abstract: Broadband RF excitation for a Fourier transform spectrometer is obtained by synthesizing the time domain excitation from the desired frequency spectrum of RF excitation. Means are provided to Fourier transform the desired spectral data from the frequency domain into the time domain to obtain a modulation output for modulating an RF carrier signal to generate the desired sideband of RF excitation. In a preferred embodiment, the desired frequency domain data, as tabled in the computer, has a pseudorandom phase shift added to each component for scrambling the phases of the RF excitation. The excited RF resonance signals in the time domain are detected, time-averaged, Fourier transformed to the frequency domain and, in the case of phase scrambling, unscrambled by the pseudorandom phase angle sequence, and displayed.