Cable Length and Probe Tuning
 

Cable Length and Probe Tuning

NMR probes can be tuned and matched on the bench or while in the magnet using a sweep generator and oscilloscope or a specialized tuning box such as the one available through Morris Instruments. More typically, probe tuning and matching are monitored using the electronics in the NMR console and preamplifier. In either case, it is important to realize that any filter or cable between the preamplifier and the probe is part of the rf circuit being tuned and should therefore be present while adjusting the tuning and matching capacitors of the probe. This is illustrated in the figure below. An NMR probe was tuned and matched using the wobble function of a Bruker AVANCE spectrometer. There was a bandpass filter and a short cable between the preamplifier and the probe. The tuning curve is shown in the top of the figure. The 90° pulse was measured at 11.25 ?sec. The short cable was then replaced with a long cable and the probe tuning and matching capacitors were left unchanged. The tuning curve is shown in the bottom of the figure below. It is clear that the overall circuit is no longer tuned and matched. The 90° pulse was measured at 13.75 ?sec using the long cable.

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Source: University of Ottawa NMR Facility Blog