Qtune

H.Zhou updated Feb 2011

Qtune is an alternative, graphic interface for probe tuning in vnmr software. It's been available since the early versions of the software. Due to the very different tuning sensitivity of the repaired SWP probe, it becomes necessary sometimes to use qtune to quickly tune the probe. The only issue is that good eyesight is needed to watch the displayed curve on the monitor from a distance. We'll try to possibly add a second monitor closer to the magnet legs just for this purpose. Before that, you may have to walk back and forth a bit. It is helpful to turn the computer minotor towards you for better viewing. The qtune procedure should work with other probes and on NMR400 as well.

Procedure

The cable connections are the same as in the normal tuning method using the tuning interface in between the broadband and 1H/19F preamp boxes. The only difference is that you do not change the tune channel and attenuator setting from the interface (see below).

• Install, if necessary, a capacitor (or inductor) stick in the probe. Make sure the stick is all the way in. You may have to overcome small resisitance as you screw the stick in, but do not overtighten it. Turn the tune knob (top section of X-channel tuning knob) so that the displayed value in a small window on the side of the probe matches the labeled value.
• Install a 1/4-wavelength cable on the broadband preamp for the desired nucleus
• Disconnect the X-channel cable from the filters on the broadband preamp (left) and connect it to the PROBE port on the tuning interface.
• Disconnect the cable from the broadband preamp OUTPUT port and connect it to the OUTPUT port on the tuning interface.
• Leave channel at 0 and attenuation at 9.
• In vnmrJ, type qtune.
• Select the desired nucleus from the "Select Center Frequency" pulldown menu.
• Click "Start Probe Tune"
• Change Power to 20dB. Press Enter key.
• Click "Autoscale". Unless the probe is way out of tune, it should see a curve with a dip.
• If you do not see a dip, first try to increase the frequency span. Enter a larger value (i.e. 20) in the "Span" field and press Enter. Once tuning is improved, you need to decrease this span back to ~ 10 MHz so that you can center the dip better.
• The displayed tune number in the window in the probe body should be within ~2-3 units of the designated value. If turning the bottom match knob does not cause a response in the tuning curve, try to turn the knob all the way from one end to the other end.
• Turn the match knob to bring down the dip, to touch the baseline if possible.
• Turn the tune knob to center the dip at the vertical, center line.

The following is what you see if the P31 channel is way-off tune and match. Tuning the probe actually has two different aspects: tune and match.

• Tune is to bring the response frequency of the probe channel to the desired resonance frequency (dictated by the detection nucleus).
• Match is to reduce the impedance at a particular frequency.
• The end result needed is to center the dip and make the dip as deep as possible.

Tune and Match

C13 as an example

Initial look of C13 tuning graph. Match is way off. Tune isn't as bad.
First, turn the match knob (bottom section of the tune control) to bring down the dip as deep as possible. This may temporarily shift the dip off center.
Next, turn the tune knob (top control) to bring the center of the dip to match the center line (C13 frequency).
Next, fine adjust tune/match knobs to have the dip centered and deep,ideally touch the horizontal axis (this may not be possible for some nuclei).
Click "Stop Probe Tune", then click "Quit Probe Tune". Probe tuning is complete.

• After tuning, connect the X-channel cable back to the filter. Connect the tune output port cable back to the OUTPUT port on X-channel preamp
• After you are done with the experiments, reverse the changes you have made to the default for C13 detection
• Remove tuning stick from probe
• Change the 1/4-wavelength cable to that for C13
• Tune probe back for C13 with a sample with CDCl3 as solvent.