This tip was taken from the October 14th, 2002 edition of Varian VNMR News


    Tuning a probe consists of adjusting the probe's resonant frequency (the actual frequency tuning) and adjusting its impedance (matching the impedance, i.e., making the impedance match the impedance of the cable and the preamp). Traditionally this is achieved by changing tuning and matching while observing the reflected power from the probe on a tune meter, via directional coupler: the smaller the reflected power, the better the tuning (for details on the tuning procedure see the installation manual for your probe, such as "AutoSwitchable NMR Probes Installation", Pub. No. 01-999121-00).

    Adjusting tuning and matching sounds like a simple task - but unfortunately, the two adjustments are heavily interactive, i.e., by changing the tuning you also change the matching and vice versa. Many beginners start with optimizing one adjustment, then they optimize the other one, switch back and optimize the first adjustment, etc.; with every iteration, the changes will become smaller and smaller, down to infinitesimal adjustments (you need to increase the gain at the tune meter to see the changes) - this can be a tedious and lengthy task!

    You can shorten this procedure considerably by "over-adjusting" in the first iterations, i.e., look for the minimum reading, then turn the tuning (or matching) rod PAST THE MINIMUM, in the direction of the initial adjustment, then do the same thing with the other adjustment. This way you "anticipate" some of the tuning changes due to the matching adjustment, and vice versa, and you approach the optimum much faster. Many experienced spectroscopists may never have been taught this trick, but they probably apply it by intuition!

    UNITYplus and UNITY INOVA users can use "qtune" to directly observe the frequency-dependent response of the probe: the current tuning "position" (frequency) is indicated by a "dip" in the reflected power, the matching quality is indicated by the depth of the "dip" in the response. "qtune" simplifies the task of tuning a probe: especially when the adjustments are far off, "qtune" makes it obvious whether the tuning and matching rods are turned in the correct direction, while with the tune meter alone one may need to find the correct direction empirically.

    The above principles actually also apply to tuning and matching amplifiers (UNITY INOVA, UNITYplus, UNITY, Infinity and InfinityPlus systems with high power r.f. amplifiers for solids NMR), as well as to manual / interactive shimming in some cases of non-orthogonal shim gradients that interact with each other.

Print this page.

Now stop crying and get back to work!