Probe Tuning Techniques

Why Tune The Probe?

In an NMR experiment, the pulse transmitter is connected to the probes' detection circuits during the rf pulse. The receiver's preamplifier is connected to the probe's detection circuit during the detection of the NMR signal. In addition, the decoupler transmitter, if used, is connected to the decoupler coil circuits all the time.

With perfect matching, we obtain the shortest possible pulse width, the best possible signal to noise and, if decoupling is used, the most efficient decoupling.
Mismatch conditions lead to longer 90 pulses (not a big problem), loss of signal (thus resulting in lower S/N - definitely a problem) and inefficient Broad Band decoupling, leading to increased residual line broadening.

The impedance of the pulse and decoupler transmitters, and the connecting cables of an NMR spectrometer is usually 50 Ohm . The probe must therefore present a 50 Ohm load impedance to avoid the above problems.

Basic Probe Circuit

The basic NMR probe circuit consists of a coil and two capacitors. The coil and the capacitors, C1 and C2, form a resonant circuit. The inductance of the coil, L, and the (approximate) sum of C1 + C2 determine the resonance frequency. C2 provides transformation of the high impedance parallel resonant circuit to a lower impedance (generally 50 ohms) to match the output impedance of a transmitter and the input impedance of a receiver.

Probe Tuning

Tuning a probe is obviously necessary when a different nucleus is to be observed with a so-called broad-band probe. Not so obviously, it is often necessary after changing the sample! This is due to the sample's rf-properties. Its dielectric constant affects the probe tuning by (simplistically speaking) adding a capacitance to C2. The sample's conductivity results in "loading" the circuit, the effect is similar to placing a resistor across the coil. This means that the impedance transformation is affected. C2 must be increased, and C1 must be decreased.

To tune a probe, we need a frequency source, a display device and some form of "impedance sensor." Two simple "sensors" are generally used, an impedance bridge or a directional coupler. The output can be displayed on a meter or an rf-oscilloscope.

Tuning For Different Samples

The arrangement of the probe coils matters as well. The proximity of the sample to the coil is an important factor - the closer the sample is to the coil, the more it will affect the tuning. A probe configured for direct detection of X-nuclei has the X(low)-frequency coil close to the sample and the proton (higher frequency) coil on the outside. Most samples will not noticeably change the proton tuning or the carbon tuning (except perhaps for aqueous solutions as noted above). With an Inverse Detection probe, the coil arrangement is reversed and different samples will more likely change the probe's proton tuning. For example, if a regular probe is tuned to 20 MHz, changing a 5 mm sample will hardly change the probe's 20 MHz tuning. With an Inverse Detection probe at 600 MHz, using 10 mm samples, the proton circuit will most certainly need tuning for each sample.

Using the Bruker Reflection Bridge

The original Bruker unit is meant to replace the preamplifier in the preamplifier housing while tuning the probe. We have modified the units to avoid wearing out the fragile connectors by frequent swapping of the bridge and preamplifiers. To connect the modified units, remove the BNC cable at the appropriate probe connector 1H for proton, X for other nuclei) and plug it into the matching cable on the bridge. Connect the other cable of the bridge to the probe.

With X-nuclei, a reading of 1 on the x 5 range is probably the lowest one can expect. For protons (and fluorine), it should be possible to tune for a reading of 1 on the x 10 range.

Tuning Procedure

Once the ratio of the maximum reading with the probe de-tuned and the reading with the probe tuned is better than 20:1, no greater performance improvement will be obtained from further tuning.

Tuning Probes on the AM and AMX Spectrometers

The H-C Inverse probe (AM-500) and the Dual C-H probe (AMX-300) use rotary capacitors for proton and carbon.

The QNP probes are more complex: Users must not attempt to tune carbon etc., if tuning is needed, let the facility staff take care of it! At most, proton tuning might be advisable!

AM-400/500

For proton tuning, the decoupler can be used, BUT: first, the range must be switched so the system is set up for X-nuclei observe! Otherwise components in the pre-amp housing will be destroyed!

1) Turn the spinner off. Type 'PR H 2' followed by II (or, easier, RJ CARBONx.x00, II) then DP = 8H (decoupler level in high power mode). Type 'CW' to turn it on. Set Console switch to DEC REFL to display reflected decoupler power.

2) Minimize reflected power with the YELLOW Tune control, perhaps together with the Match control.

3) Type 'PO' to turn decoupler off. If a parameter file exists, read in the proper parameters - otherwise, type 'PR' = 1 1. Type 'II' to set the interface to the new PR parameters.

1) To tune the observe coil, use RJ with the appropriate parameter file. Install the reflectance bridge between probe connector marked "BB" or "C" and the cable normally in place. With the Broad Band probes, set the controls Tune and Match to previously established numbers. Type 'PRBT' (or 'EXE PROBETUN') to start rapid pulsing. Minimize meter reading with the Tune and Match controls. Switch the meter range as needed. At the "5" position, a reading of 1 is about the best you can hope for. A reading of >3 in the "4" position is marginal. Press CTRL/H when done. Remove the reflectance bridge and re-connect the regular probe cable.

2) Tune the proton coil as described above.

The reflection bridge is normally used for tuning. Select a parameter set appropriate for the nucleus. Type (xau) "PROBETUNE" to start the rapid pulsing. Tune the probe as described above. Click on Task -> Kill, select "go" to stop pulsing or press CTRL/U. The previously used parameters are restored after the xau program has finished.

Bruker Console Indicators

The pulse power amplifier's reflected power indicator should only be used when there is no external attenuator connected between the output and the probe (as might be required to prevent arcing of probe circuits as a result of too high an rf power). The attenuator also reduces the reflected power, thus providing a low reading even if the probe is not well tuned! In such a case, the Bruker reflectance bridge should be used.

The decoupler contains a directional coupler as well. Using the decoupler for (proton) tuning, the reflected power is minimized by adjusting C1 and C2.

Changing Probes, Tuning for Other Nuclei

After Restoring Normal Probe and/or Tuning

Relaxation times for benzene-d6 are very long! Wait at least 300 seconds between pulses if you want to check S/N again!

If you change or re-tune probes, it your responsibility to assure that the system is back to normal after you are finished. The regular standard sample must be in the probe on all instruments. It must locked, spinning, and auto-shimming on Z1-Z3.

AMX-300

VBAMX-400, AM-400

To assure proper operation, facility staff must be consulted before changing probes. Users may, on occasion, be permitted to install a different probe but the facility staff must perform the testing and tuning (if required) after the QNP probe is re-installed, Please allow for up to three hours when reserving instrument time!

AM-500

AMX-300

For verifying the proton performance, determine the 90 pulse width and, with the 0.1% ethylbenzene sample, run use the xau program "protsn". S/N should be > 150:1. Note 90 pulse widths and S/N in the log book and give us the resulting plots.

VBAMX-400

For verifying the proton performance, determine the 90 pulse width and, with the 0.1% ethylbenzene sample, run use the xau program "protsn". S/N should be > 130:1.

Note 90 pulse widths and S/N in the log book and give us the resulting plots.

AM-400

To verify proton performance, measure the 90 pulse width, with the 0.1% ethylbenzene sample, run 'EXE PROTSN.400' . This EXE will run a single scan and plot the result. S/N should be > 130:1.

Give us the plots from the QNPTEST and this test. We can quickly tell if something is amiss. Note the 90 pulse widths and the S/N in the log book.

AM-500

To verify proton performance, measure the 90 pulse width, with the 0.1% ethylbenzene sample, run 'EXE PROTSN.500' . This EXE will run a single scan and plot the result. S/N should be > 320:1.

Give us the plots from the ASTM and this test. We can quickly tell if something is amiss.

Other Tuning Methods

An impedance bridge or a directional coupler can conveniently be used for probe tuning. A frequency synthesizer (or stable generator) is needed for an rf source or the spectrometer can be used to supply either pulses or low power CW signals. An (rf) oscilloscope is usually needed to display the output.

One of the simplest methods is the use of a sweep generator (a further option, the bi-directional Watt-meter, will be discussed later).

Using a Sweep Generator

Impedance Bridge

These devices can be very broad band but above 200 MHz, some units may be of limited usefulness.

Directional Coupler

The directional coupler has rf-properties which make it useful for tuning circuits. It has a low-loss path between IN and OUT. A large loss (or small coupling) occurs between the COUPLED port and the OUT port. A smaller loss (or closer coupling) occurs between the COUPLED port and the IN port. The loss is expressed in decibels (dB).

Typical properties are:
        IN - OUT:        -0.1 dB
        OUT - COUPLED:   -40 dB
        IN - COUPLED     -10 to 20 dB

If an rf-source is applied to OUT port, practically the full level will appear at IN port. A small signal (-40 dB) will appear at the COUPLED port. If the IN port is terminated into 50 W , no power will be reflected. If mismatched, the reflection will not only return to the OUT port but also, at 10 or -20 dB, at the COUPLED port. Thus, if a scope is connected, we can observe the reflection. Directional couplers can be very broad band. If in doubt, consult manufacturers specs.

Using an In-Line Bi-directional RF-Watt Meter

An in-line rf Watt meter can be used together with a spectrometer's amplifiers. Normally, only the decoupler is used; the observe power amplifier output is often too high to be turned on for any length of time without burning out the probe circuits, or it may not be able to operate in continuos wave mode (CW).

The meters use a one (or two) plug-in(s) of appropriate frequency and power range. Rotating the plug-in selects forward or reflected power display.

The biggest advantage is simplicity of operation. The drawbacks include the need for different plug-ins. Only the more elaborate models are capable of operating in pulsed mode. This means that the simple variety cannot be used on many of the Bruker spectrometers for tuning nuclei other than Protons.

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Questions, comments to: Rudi Nunlist rnunlist@bloch.cchem.berkeley.edu

Last Update: 4/18/95.