Tuning (15):

The ¹H RF coil needs tuned each time a sample is placed in the spectrometer.  Tuning of the ¹H RF coil should be repeated  if the same sample is removed and then returned to the spectrometer.  The ¹³C and ¹⁵N coils are less sensitive to the specifics of the sample, but still require occasional tuning.  There is some risk that a previous user tuned the  ¹³C and ¹⁵N coils and did it badly.  Tuning for the avance 500 and 700 is done entirely from the console using the atmm command.  Tuning of the Avance600 is done by adjusting pots on the bottom of the magnet after using the wobb command (1).  Tuning of the ¹⁵N coil of the Avance600 only requires changing a cable.  You should consider rechecking ¹H tuning just prior to starting an experiment with a lengthy set up (13).
 

Tuning with atmm command.

• If you do not have a copy under your own ownership of the relevant tuning dataset (tune1h, tune13c, tune15n), obtain it from the ref user using edc (6).
• Type atmm (5).  When the message line indicates that the display is ready in the acquisition window, type acqu.
• Click the <wobb-SW> button (mid left), agree with the two popup boxes,  and wait for the curve to redisplay (2,7,8).
• Use the tuning adjustments in the popup window to center the tip of the V-shaped portion of the Wobb curve (4,11) on the solid vertical line, and then use the matching adjustments to lower the tip of the V as far as it will go (10).
• You can use the /8 or /2 buttons (upper left) or equivalent commands to see the tip of the V better (9).
• Press and hold the left mouse button down on the adjustment buttons, particularly the fine adjustment (indicated by single arrows).
• The tuning and matching adjustments interact, so you may have to go back and forth between them (12).
• Exit the atmm routine by <file> <exit> in the tuning adjustment window; then click <return> in the xwinnmr window (3).

Tuning with wobb and manual adjustment of the capacitors.

• If you do not have a copy under your own ownership of the relevant tuning dataset (tun1h, tune13c, tune15n), obtain it from the ref user using edc (6).
• Type wobb (5).  Wait for the message line to announce that the wobb curve is in the acquisition window, then type acqu.
• If the wobb curve already centered on the solid vertical line and touching the bottom, tuning is not needed; type halt.
• To tune the probe, you will have to go to the magnet. Since you can not see the console from there, there is an LCD display to guide the operation on the preamp, which is located on the floor behind the magnet.
• Locate two capacitors for the coil to be tuned on the probe located at the underside of the magnet.  The color code for each nucleus is indicated where the cables attach to the probehead unit. The tuning and matching capacitors are distinguished by a T or M on the capacitor shank.
• Using the tool connected by a chain, turn the pot marked T until the lit up LCDs in the horizontal row are centered. The capacitor should turn easily.  Do not force it. If it doesn't turn, you have reached the endpoint (10).  Never force the capacitor past this point, else you will damage it and the probe will have to be sent back to Bruker for repair.
• Then turn the pot marked M to reduce the height of the vertical row of lit LCDs.
• The two pots interact, so go back and forth between them optimizing the tuning (12).

When optimally tuned, there should be a single LCD lit in the center of the horizontal row and two or three lit at the bottom of the vertical row.
Return to the terminal and type "halt" to stop the acquisition ("halt" stops the acquisition after that pulse is finished.)
• Click <return> on the acqu menu to exit to the main xwinnmr window (3).

Tuning the ¹⁵N coil on the Avance600

• Locate where a cable from the probehead passes through a small box labeled "¹³C bandpass" which screws into the second unit from the top in the preamplifier stack.  Unscrew the ¹³C bandpass filter from the preamp unit.
• Locate where a cable from the probehead passes through a small box labeled "15N bandpass" and connects to another cable that bypasses the preamp unit.  Unscrew the side of the ¹⁵N bandpass filter away from the probe and connect it to the preamp unit where the ¹³C connection had been.  Be very gentle with the ¹⁵N and ¹³C bandpass filters; they contain inductors (essentially small coils of copper wire) that are sensitive to mechanical distortion which in turn will alter their filtering properties.
• Complete the ¹⁵N tuning operation in this configuration.
• Return the cables to their original configuration before attempting any other operation (4a).

Comments and Troubleshooting.

• (1) More exactly, it is the probehead installed that determines if atmm or wobb is used.  See the instrument status page for information on the probeheads available for each spectrometer.  If servo motors for tuning by the atmm process are installed on the probehead, then there are no adjustment pots and manual tuning at the underside of the magnet is not possible.  The gradshim window gives the name of the installed probehead.
• (2) The <wobb-SW> button (or wbchan command) reinitializes the sweep width from a parameter in the dataset.  The wobb function (in manual tuning) can tolerate different sweep widths, but the atmm function requires a particular sweep width for each nucleus.  At present, atmm is sometimes starting with an inappropriate sweep width, or at least an inaccurate display suggesting an inaccurate sweep width.  A future fix may make this step unnecessary.  The displayed frequency range should equal the sweep width, which should be 40 MHz for ¹H and 1 MHz for ¹³C or ¹⁵N.  We have observed the ¹H display to be far enough obscured by the errant sweep width setting to place the resonance position off screen.
• (3) If atmm  or wobb is not properly exited the process continues in the background without  display window.  Then you will  get a "device in use by other process" error when you subsequently try to do another acquisition.  If so, use the kill command to kill the persisting atmm process.
• (4) The cable change for tuning ¹⁵N on the Avance500 is required because its preamp can only handle only one X channel at a time (i.e. ¹⁵N or ¹³C).  The system is usually set up to route the ¹³C channel through the pre amplifier.  Thus, for ¹⁵N tuning, the 15N signal has to be rerouted through one of the pre amplifier channel usually used for ¹³C.  This is partly accomplished by the cable switch.  Additionally, there are parameters in the tune15n data set which reroute the RF so that ¹⁵N RF enters the rear of the preamplifier instead of ¹³C RF..  To see how signals are routed by dataset parameters use the edasp command and compare the tune15n data set to other data sets. The Avance 500 and 700 spectrometers are equipped with two X-nucleus preamplifier units each, thus no cable change is necessary.
• (4a) Do the ¹⁵N tuning before the ¹³C tuning.  That way, if you forget to change the cables back, you will immediately notice it.
• (5) A message to the effect that a nucleus can not be wobbled means that you probably haven't loaded a tuning dataset.

• (6) An error message to the effect that write permission is denied may result if you have not established ownership of the tuning dataset. There was at one time a bug in the setup of the avance 500 such that edc immediately gave a "permission denied" error when acquiring a ref dataset, even though there was in fact no error.  This problem should now be fixed.  There is a "ref" user on both drives /u and /avance6001 on the Avance600, but only the one on /avance6001 has the reference library.
• (7) If in doubt about the shape of the curve, click <wobb-SW> and agree to the two popup boxes.. The keyboard equivalent is wbchan.  It does not help to type wbchan prior to running atmm.
• (8) If you may have inadvertently changed parameters in the tuning dataset by typing something in the popup boxes generated by <wobb-SW>, then the best thing to do is to reobtain the tuning dataset from the ref library.
• (9) If vertical rescaling has confused the issue, restore default vertical scaling by clicking the up arrow pointing to the horizontal line in the upper left button panel.  The keyboard equivalent is vreset.
• (10) If tuning and matching are both far off to start with, and tuning is not working well, then try improving the matching first.
• (11) The 40 MHz sweep width used by atmm for the ¹H probe includes secondary minima at the high and low end of the displayed frequency range.  This is normal.  Different sweep widths are set up for the atmm function than for the wobb function where atmm is not implemented.  Therefore it is normal for the curves to appear to be different.
• (12) The ¹H wobb curve on both the avance500 and avance600 has a distortion that places two closely spaced minima right at the resonance frequency when the coils are tuned.  The distortion also makes the tip of the curve jump around a little bit making it hard to decide on optimal settings.  This does not indicate an error in your technique.  If you are that close, your are close enough.
• (13) If ¹H tuning drifts over the course of the experiment, it may because of an inhomogeneity in the salt concentration in the sample tube.  For example, if you top off a sample with water and don't mix it up, this behavior may result.  Bubble formation is another possibility.
• (14) Resonance frequencies currently in the Avance500 tuning datasets are 500.13385041, 125.757789, and 50.677733.  For the Avance600, they are 600.084620616, 150.8946364, and 60.820685787851.  These don't have to be fine adjusted to the posted reference frequencies.

Theory

• (15) Tuning actually refers to two processes, called tuning and matching.  Tuning is adjusting the resonance frequency of the RF coil to more closely match the frequency range of the nucleus under observation.  The tuning dataset should have the relevant frequency set  as an sfon parameter within it (14).  Matching refers to adjusting the impedance of the RF coil to that of a reference resistor embedded in the electronics to represent an optimal operating condition.  The resonance frequency arises as follows:  As the transmitter passes and alternating current through the coil, the associated electromagnetic fields interact most constructively at a particular frequency based on the geometry of the coil.  Impedance arises because the generated coil fields resist passage of alternating current.  Impedance is lowest at the resonance frequency, and the intensity of the emitted RF field is highest in the center of the coil at the resonance frequency.  Working in conjunction with the receiver, the coil is most sensitive at generating an alternating current from radiation emitted by the sample if the radiation is near the resonance frequency.  The impedance of the RF coil includes a component of inductance contributed by salt ions in the sample itself.     The resonance frequency of the coil can be somewhat modified by a variable capacitor placed between the leads of the coil.  This is the tuning capacitor.  The impedance can be modified by a second capacitor within one of the leads of the coil.  This is the matching capacitor.  During the tuning operation, the transmitter synthesizes a sweep through the frequency range by repeatedly putting out a series of closely spaced pulses.  The wobb window display acts as an oscilloscope in in the circuit in sync with the transmitter.  The dip in the wobb curve represents the resonance frequency as the point of lowest impedance.  A device called a bridge balances the current through the coil with a shunt through a reference resistor.  The lowest overall impedance occurs when the impedance of the coil matches that of the reference resistor.  This allows the effect of adjusting the matching capacitor to be visualized as vertical motion of the dip in the wobb curve.
• Further discussion can be found the Bruker acquall.pdf file in /u/prog/docu/english/xwinproc/pdf/ or through the xwinnmr help function.
• In certain circumstances, the ¹H coil is intentionally detuned (frequency adjusted to one of the highest points on the Wobb curve) to eliminate radiation damping.