NMR tuning procedure

The RF probe of a quadrature sensitive FT-NMR spectrometer is tuned and matched to its electrical environment by utilizing the phase quadrature sensitivity of said spectrometer to function as an impedance vector analyzer for the probe in situ. Phase and amplitude measurements of the in situ probe are compared to an occasionally measured and stored amplitude and phase characteristic observed when the probe is replaced by a 50 ohm terminator.

The present invention relates to Fourier transform NMR spectroscopy and, in 
particular, to the tuning and impedance matching of the sample probe of 
such apparatus. 
The sample probe of a modern Fourier transform nuclear magnetic resonance 
spectrometer couples an RF transmitter to a sample and also couples said 
sample to an RF receiver. The probe must be matched to the coupling 
network by careful tuning procedures. Owing to the crucial role of the 
probe for excitation and detection of nuclear resonance, probe performance 
is often the limiting factor of spectrometer performance. The procedure of 
the present invention is best illustrated with the aid of FIGS. 1 and 2 
wherein probe 10 communicates with the receiver channel and transmitter 
channel through duplexer 20. In one common arrangement, duplexer 20 
comprises back to back diode pairs 22 and 24 and quarter wavelength 
transmission line 26. Ordinarily, the transmission line of the quarter 
wavelength strip exhibits a characteristic impedance of 50 ohms at the 
operating frequency. Preamplifier 28 is designed to exhibit a minimum 
noise figure for this input impedance. The problem of matching the probe 
to the correct impedance has, in prior art, been accomplished with 
reconfiguration of the physical connection to the probe. Very accurate 
procedures employ a vector impedance analyzer, or using another approach, 
a directional coupler or standing wave bridge is inserted in the system to 
monitor reflected power while tuning the probe to minimize same. Still 
another approach relies upon injection of a signal onto the decoupler coil 
of the probe. The probe is tuned to maximize the signal induced on the 
signal coil within the probe structure, the latter disposed in proximity 
to the decoupler coil. This approach does not maximize signal-to-noise 
performance of the system because the signal output can be maximized with 
the preamplifier while the preamplifier remains unmatched to its input. 
These tuning methods require certain changes to the probe connections which 
are inconvenient, at best, and certainly do not correspond to normal 
operation. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the present invention, tuning of the probe is carried out in 
situ by reference to the phase and amplitude of a signal transmitted to 
the network of FIGS. 1 and 2. Phase and amplitude are each functions of 
the impedance which are readily determined from conventional quadrature 
resolving FT instruments. In this way, the FT-NMR spectrometer functions 
as a whole in the role of a vector impedance analyzer. Tuning procedure is 
calibrated occasionally by substitution of a 50 ohm termination for the 
probe and comparison of the phase and amplitude response therewith to the 
phase and amplitude response when the probe is replaced.

DETAILED DESCRIPTION OF THE INVENTION 
A typical block diagram of pertinent portions of an FT-NMR spectrometer 
appears in FIG. 3. An RF oscillator 30 is modulated by modulator 32 and 
the signal is directed through duplexer 34 to the probe 36. The signal 
output of the probe presented to duplexer 34 is directed to RF amplifier 
38. Phase sensitive detector 40 operates on a reference phase derived from 
the RF oscillator 30 and the RF amplifier output signal to resolve the RF 
amplifier signal into phase quadrature components by incorporating two 
phase detectors fed with reference signals at the same frequency but 
differing in phase by 90.degree.. For a signal frequency exactly equal to 
the reference frequency, the quadrature detection method provides 
independent determination of two quadrature components of the signal. 
Analog to digital conversion 42 operates on the phase sensitive detector 
outputs and the digital signals are available for further processing by 
processor apparatus 44. 
The present timing procedure measures two parameters, phase and amplitude, 
rather than the single parameter techniques of prior art such as signal 
peaking, reflected wave nulling and the like. The two channels of the 
quadrature resolving receiver are ideally suited for processing of the 
relevant information and storage in the spectrometer memory. 
Note that during operation according to the present invention the modulator 
32 may be considered bypassed in the operational sense. The RF carrier is 
applied through the duplexer to the probe 10 or a probe substitute in the 
preferred from of a standard 50 ohm termination. 
First consider the substitution of the 50 ohm termination for the probe. 
There is then derived a signal represented by a vector A defined by the 
quadrature components from the outputs of the quadrature phase detector. 
With the termination removed and the probe in place, the same procedure 
generally yields a signal B. The vector A is preferably retained and 
vector difference A-B is then observed while adjustment of the probe is 
carried out through the tune and match capacitances of the probe. If the 
magnitude of the vector difference, .vertline.A-B.vertline., is displayed 
by the spectrometer display 46, the tuning procedure proceeds by a 
minimization process much as in the case of reducing reflected power by 
minimizing the difference signal. This very simple technique is suitable 
for manual operation referenced to a null meter. 
The method described is also capable of implementation under control of the 
spectrometer control processor. Here it is noted that a single parameter 
minimization, as indicated above, is but one precedure effective for the 
case of a computer controlled matching process; a two parameter 
minimization of the vector difference A-B can yield a more rapid 
convergence as in FIG. 4. In a simple technique, one vector (A) is 
arbitrarily assigned a phase .phi.A and the magnitude is taken from the 50 
ohm termination data store by the processor. The magnitude 
.vertline.B.vertline.and phase difference .phi.B-.phi.A is measured. The 
magnitude of the vector difference .vertline.A-B.vertline.is then 
minimized either manually by operator interaction via the display 46, or 
automatically by a suitable search procedure for function minimization. 
It will be appreciated that the above described procedure is not limited to 
the context of a specific NMR Fourier transform spectrometer, buy may be 
employed wherever the elements of transmitter, receiver and RF signal 
probe are operable in conjunction with a quadrature sensitive processing 
system akin to the present subject matter. While the above description 
represents a particular embodiment and has been so illustrated and 
described, modifications and changes will become apparent to those skilled 
in the art and it is intended to cover in the appended claims all such 
modifications and changes as come within the spirit and scope of the 
invention.