Monday, April 14, 2008
Tuning and Matching an NMR Probe
The circuits in NMR probes are essentially band pass filters. The properties of the band pass filter are determined by the values of capacitance for the capacitors in the circuit and the inductance of the sample coil. One is usually able to vary the capacitance of the tuning and matching capacitors to optimize the probe for the experiment at hand. The adjustments are typically made by turning rods that extend from the bottom of the NMR probe. These rods are often labelled "T" for tune amd "M" for match. Changing the tuning capacitor will shift the band of the bandpass filter and is therefore used to tune the probe from on frequency (or nucleus) to another. This is much like tuning your radio to a particular radio station. The tuning control of a radio is also a variable capacitor. Changing the matching capacitor changes the efficiency of the band pass filter. These two capacitors are not independent and they must be adjusted iteratively to optimize the NMR probe. Proper tuning and matching are essential in getting high quality NMR results. The figure below shows the response on an oscilloscope for an NMR probe connected to a sweep generator. One can think of these plots as frequency on the horizontal axis vs efficiency (or match to 50 Ohms) on the vertical axis. The horizontal line represents a 50 Ohm reference. The first panel shows a probe that is well matched and tuned. The second panel shows a probe that is properly matched but poorly tuned and the third panel shows a probe that is properly tuned but poorly matched.