Multiple-tuned circuit and probe for NMR spectrometer

Details Multiple-tuned circuit and probe for NMR spectrometer Multiple-tuned circuit and probe for NMR spectrometer

2002-04-08

2004-02-03

Gutierrez, Diego (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S318000

Reexamination Certificate

active

06686741

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiple-tuned circuit and a probe for use in a nuclear magnetic resonance (NMR) spectrometer and, more particularly, to a multiple-tuned circuit and a probe used in an NMR spectrometer and which show enhanced resistance to RF voltages by performing a balanced operation.
2. Description of Related Art
In the description given below, a higher NMR frequency and a lower NMR frequency are often designated as HF (higher frequency) and LF (lower frequency), respectively.
FIGS. 1 and 2
show the conventional double-tuned circuit. In
FIG. 2
, the amplitude of an RF voltage developed across the coaxial resonators
2
,
3
when a higher frequency for an HF nucleus is at resonance are shown simultaneously. The circuit shown in
FIGS. 1 and 2
can be tuned to the resonance frequency of an HF nucleus (e.g.,
1
H nucleus). In addition, the circuit can be simultaneously tuned to the resonance frequency of an LF nucleus (e.g.,
13
C nucleus). The circuit has a sample coil
1
consisting of a solenoid coil, saddle coil, or the like. Coaxial resonators
2
and
3
have a length equal to a quarter wavelength of the resonance frequency of the HF nucleus. The outer conductors of the resonators
2
and
3
are grounded during use. The coaxial resonator
2
that is electrically open is connected with one end of the sample coil
1
, while the coaxial resonator
3
that is short-circuited is connected with the other end. A tuning variable capacitor
4
and a matching variable capacitor
5
are connected with the HF input/output side. A tuning variable capacitor
6
and a matching variable capacitor
7
are connected with the LF input/output side. A capacitor
8
acts to compensate for insufficiency of the capacitance of the LF tuning variable capacitor
4
.
The operation is described next. As shown in
FIG. 2
, at the resonance frequency of the HF nucleus, the RF voltage developed across the open coaxial resonator
2
assumes a minimum amplitude of 0 at the upper end (as shown in the drawings) and a maximum amplitude of V
m
at the lower end. The RF voltage at the shorted coaxial resonator
3
assumes a maximum amplitude of V
m
at the upper end and a minimum amplitude of 0 at the lower end. The frequency can be adjusted with the tuning variable capacitor
4
. Since the voltage amplitude is minimal at the upper end of the open coaxial resonator
2
at this time, HF power flowing into the LF side is small. At the resonance frequency of the LF nucleus, the open coaxial resonator
2
is not associated but the shorted coaxial resonator
3
acts as a grounded inductance L. Therefore, the frequency can be adjusted by the tuning variable capacitor
6
connected in parallel with the sample coil
1
and the opened coaxial resonator
2
. In this way, this type of double-tuned circuit can adjust the HF and LF independently.
FIG. 3
shows another conventional double-tuned circuit. Note that like components are indicated by like reference numerals in various figures including
FIGS. 1 and 2
. A sample coil
1
consists of a solenoid coil, saddle coil, or the like. Two conductors
31
and
32
have a length equal to a quarter wavelength of the resonance frequency of an HF nucleus and form a parallel transmission line. The conductors
31
and
32
are grounded via tuning capacitors
10
and
11
for an LF nucleus during use. The sample coil
1
is connected between the two conductors
31
and
32
. The conductors
31
and
32
are surrounded by a conductive outer tube
14
that is grounded. A tuning capacitor
9
for an HF nucleus is connected with the conductor
31
. A tuning variable capacitor
4
and a matching variable capacitor
5
for the HF nucleus are connected with the conductor
32
. At this time, the tuning variable capacitor
4
for the HF nucleus and the tuning capacitor
9
for the HF nucleus are so set up that their capacitances are nearly equal. A tuning variable capacitor
6
and a matching variable capacitor
7
for the LF nucleus are connected with the conductor
31
.
At the resonance frequency of the HF nucleus, the tuning capacitors
10
and
11
for the LF nucleus have large capacitances and so their impedances are small. The conductors
31
and
32
are equivalent to the case where their ends are short-circuited. The conductors
31
and
32
are grounded together with the outer tube
14
. As a result, the conductors
31
,
32
and the outer tube
14
together operate as a quarter wavelength balanced resonant circuit at the resonance frequency of the HF nucleus. In particular, with respect to the conductors
31
and
32
, the capacitance of the tuning variable capacitor
4
for the HF nucleus and the capacitance of the tuning capacitor
9
for the HF nucleus are set to nearly equal values. Therefore, RF voltages V
m
/2 and−V
m
/2 which are substantially equal in amplitude but opposite in polarity are produced at the opposite ends of the sample coil
1
. Electrical currents of opposite polarities flow through the conductors
31
and
32
by the action of a kind of transformer. These RF voltages are halves of the voltage V
m
shown in
FIGS. 1 and 2
. These voltages are applied to the tuning variable capacitors
4
and
5
for the HF nucleus.
Meanwhile, at the resonance frequency of the LF nucleus, the tuning capacitors
10
and
11
for the LF nucleus and the tuning variable capacitor
6
for the LF nucleus together form an LC resonant circuit. The capacitor
10
is connected in series with the sample coil
1
and the conductor
31
. Similarly, the capacitor
11
is connected in series with the sample coil
1
and the conductor
32
. The tuning variable capacitor
6
is connected in parallel with the capacitor
10
. The frequency can be adjusted with the tuning variable capacitor for the LF nucleus. At this time, RF voltages which are almost equal in amplitude but opposite in polarity are produced at the opposite ends of the sample coil
1
by appropriately setting the capacitance of the tuning capacitors
10
and
11
for the LF nucleus. Therefore, the RF voltages applied to the tuning variable capacitors
6
and
7
for the LF nucleus can be held down to halves of the values in the case of
FIGS. 1 and 2
.
In the example of
FIGS. 1 and 2
, one end of the sample coil
1
is at ground potential at HF resonance and is near ground potential at LF resonance. Therefore, at HF resonance, a potential difference corresponding to the maximum amplitude at HF is directly applied across the variable capacitors
4
and
5
. At LF resonance, a potential difference corresponding to the maximum amplitude at LF is directly applied across the variable capacitors
6
,
7
and capacitor
8
. Therefore, when high electric power is applied to the sample coil
1
, electric discharging takes place, thus damaging these electrical parts.
The extraction line from the sample coil
1
is lengthened. This creates loss in the current path at LF resonance. Consequently, it is impossible to increase the resonance frequency of the LF nucleus. In this case, it may be conceivable to increase the resonance frequency by adding a dummy coil in parallel with the sample coil
1
to lower the inductance of the whole coil assembly. If this countermeasure is taken, however, an electrical current also flows through the dummy coil, increasing power loss. In this way, this countermeasure is inappropriate.
The configuration of
FIG. 3
has the advantage that the voltage applied to the tuning variable capacitor
4
and matching variable capacitor
5
at HF resonance and the voltage applied to the tuning variable capacitor
6
and matching variable capacitor
7
at LF resonance are halves of the voltages applied in the case of FIG.
1
. The conductors
31
and
32
are connected in series with the sample coil
1
. Therefore, these conductors act as extraction lines at LF resonance. This increases the inductance of the whole coil assembly. As a result, the LF resonance frequency drops.
In the configuration of
FIG. 3
, it is necessary to connect ca

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