Tuning and Matching

Figure 2.2.8 shows a typical tuning and matching circuit for an rf coil. If the selfinductance of the rf coil is too large for the tuning and matching, the rf coil wire... 

The precise measurement frequency varies slightly with solvent, temperature, concentration, sample volume and solute or solvent polarity, so that exact adjustment must be carried out before each measurement. This process, known as tuning and matching, involves variation of the capacity of the circuit. Modern spectrometers carry out such processes under computer control. 

An automatic probe tuning and matching (ATM) accessory allows one to automatically tune the NMR probe to the desired nuclei s resonant frequency and match the resistance of the probe circuit to 50 Q [7]. Traditional NMR instruments are designed so that one must perform these adjustments manually prior to data acquisition on a new sample. The advent of the ATM accessory allows the sampling of many different NMR samples without the need for human intervention. The ATM in conjunction with a sample changer enables NMR experiments to be conducted under complete automation. The sample changers are designed so that once the samples are prepared, they are placed into the instrument s sample holders. Data are then acquired under software control of both the mechanical sample delivery system as well as the electronics of the spectrometer. 

Tune and match the probe head s H and 13C channels, respectively. 

Tuning and matching of the 13 C channel is only necessary for experiments involving 13C (e.g., HSQC, HMBC). 

The probe head is separately tuned and matched (25) for each sample and experiment. The magnetic field... 

The introduction of gradient shimming (that makes this procedure very easy to automate) together with the possibility of performing automatic tuning and matching (via the use of motors mounted on the probe body) has further improved the whole procedure. 

By carefully tuning and matching the probe and the various stages in the transmitter amplifier chain and by allowing for a wide rf bandwidth, it is possible to reduce phase transients (for a definition, see Haeberlen, 1976, Appendix D) to a level where their effect upon the m.p. spectrum becomes insignificant. We therefore confine ourselves here to study by simulations phase errors, nonuniform pulsewidths, and a power droop of the transmitter. 

Figure 3.44. Tuning and matching an NMR probehead. The dark line represents the probe response seen for (a) a mis-tuned and (b) a correctly tuned probe head.

The method for probe tuning on older spectrometers that are unable to produce the frequency sweep display is to place a directional coupler between the transmitter/receiver and the probe and to apply rf as a series of very rapid pulses. The directional coupler provides some form of display, usually a simple meter, which represents the total power being reflected back from the probe. The aim is to minimise this response by the tuning and matching process so that the maximum power is able to enter the sample. Unfortunately with this process, unlike the method described above, there is no display showing errors in tune and match separately, and there is no indication of the direction in which changes need be made, one simply has an indication of the overall response of the system. This method is clearly the inferior of the two, but may be the only option available. 

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