Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-09-27
2004-09-21
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
Reexamination Certificate
active
06794874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiple tuning circuit for use in a nuclear magnetic resonance (NMR) spectrometer and, more particularly, to such a multiple tuning circuit having improved resistance to RF voltages.
2. Description of Related Art
FIG. 1
shows a conventional multiple tuning circuit designed to irradiate two species of low-frequency resonating nuclei having resonant frequencies of LF1 and LF2, respectively, with RF signals simultaneously and to detect resulting NMR signals. The RF frequency LF1 is higher than the LF2. For example, LF1 corresponds to the resonant frequency of
13
C nucleus, while LF2 corresponds to the resonant frequency of
17
O nucleus.
In
FIG. 1
, a sample coil
1
consists of a solenoid coil or saddle coil. Inductors
2
and
3
are directly connected to the opposite ends of the sample coil
1
. One end of the inductor
2
is connected with the sample coil
1
, the other end being grounded via a capacitor
6
or directly. Similarly, one end of the inductor
3
is connected with the sample coil
1
, the other end being grounded via a capacitor
9
or directly. In the illustrated example, the two inductors are grounded indirectly via capacitors.
The opposite ends of the sample coil
1
are grounded via two tuning capacitors
4
and
7
, respectively, for LF1. A tuning variable capacitor
10
for LF1 is connected with one end of the sample coil
1
in a parallel relation to the tuning capacitor
7
. The capacitance of the tuning capacitor
4
for LF1 is set almost equal to the sum of the capacitance of the tuning capacitor
7
for LF1 and the capacitance of the tuning variable capacitor
10
for LF1.
A tuning variable capacitor
12
for LF2 and a matching capacitor
13
for LF2 are connected with the grounded end of one of the inductors
2
and
3
. For example, in the example of
FIG. 1
, the tuning variable capacitor
12
for LF2 and the matching capacitor
13
are connected with the grounded end of the inductor
3
. The capacitance of the tuning capacitor
6
for LF2 is set almost equal to the sum of the capacitance of the tuning capacitor
9
and the tuning variable capacitor
12
for LF2.
Under this state, the sample coil
1
, inductors
2
,
3
, tuning capacitors
6
,
9
,
4
, and
7
together form a balanced resonant circuit where the voltage amplitude becomes zero near the center point of the sample coil
1
. Each of the inductors
2
and
3
may be a lumped inductor, such as a helical coil fabricated by winding wire like a coil, or may be a distributed inductor, such as a conducting rod. That is, each of the inductors
2
and
3
may be any inductive component element.
The operation of this multiple tuning circuit is next described.
FIG. 2
shows an equivalent circuit of the multiple tuning circuit when resonating at LF1. The tuning capacitors
6
and
9
corresponding to the resonant frequency LF2 have large capacitances and show sufficiently low impedances at the frequency of LF1. Therefore, the two inductors
2
and
3
are regarded to be connected in series as shown. Since the tuning capacitor
9
has a large capacitance, the contribution of the tuning variable capacitor
12
and matching variable capacitor
13
on the side of LF2 is small.
The series combination of the two inductors
2
and
3
is connected in parallel with the sample coil
1
, thus forming a first resultant inductance. This resultant inductance and the tuning capacitors
4
and
7
corresponding to the resonant frequency LF1 together form a first LC resonant circuit. The tuning variable capacitor
10
and matching variable capacitor
11
on the side of the resonant frequency LF1 together act to tune and match the LC resonant circuit for the resonant frequency LF1. At some instant of time, tuning and matching are made at the resonant frequency LF1, and the resonant current maximizes. One example of the orientation of RF currents under this condition is indicated by the arrows
21
and
22
.
An equivalent circuit of the multiple tuning circuit resonating at LF2 is shown in FIG.
3
. The tuning capacitors
4
and
7
for the resonant frequency LF1 have small capacitances and show sufficiently large impedances at the frequency of LF2. Therefore, the contribution of the two tuning capacitors
4
and
7
is small. Furthermore, the contribution of the tuning variable capacitor
10
and matching variable capacitor
11
on the side of the resonant frequency LF1 is small, because their capacitances are small.
The inductor
2
, sample coil
1
, and inductor
3
are connected in turn and in series to thereby form a second resultant inductance. This second resultant inductance and the tuning capacitors
6
,
9
for the resonant frequency LF2 together form a second LC resonant circuit. The tuning variable capacitor
12
and matching variable capacitor
13
on the side of the resonant frequency LF2 make tuning and matching at the resonant frequency LF2.
At some instant of time, tuning and matching are made at the resonant frequency LF2, and the resonant current maximizes. One example of the orientation of an RF current under this condition is indicated by the arrow
31
.
FIG. 3
indicates an equivalent circuit of the multiple tuning circuit when resonating at the frequency LF2. In this equivalent circuit configuration, the inductor
2
, sample coil
1
, and inductor
3
are connected in series to form the resultant inductance. Therefore, when an RF current
31
flows, an RF voltage developed across the inductor
2
, an RF voltage developed across the sample coil
1
, and an RF voltage developed across the inductor
3
are added up. Consequently, a quite high RF voltage relative to ground potential is generated at positions
32
and
33
. As a result, electric discharge tends to be produced across each of the positions
32
and
33
and ground. Once such electric discharge occurs, electronic parts, such as capacitors, are burned out. Hence, expensive repair cost is necessary.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide a multiple tuning circuit which is for use in an NMR spectrometer and has improved voltage resistance and thus is less likely to produce electric discharge even if high RF power is injected.
To achieve this object, the present invention provides a multiple tuning circuit for use in an NMR spectrometer, the tuning circuit comprising: a sample coil having ends A and B, the end A being grounded via a first capacitive element, the end B being grounded via a second capacitive element; a first inductor having one end connected with the end A of the sample coil via a third capacitive element, the other end being grounded; a second inductor having one end connected with the end B of the sample coil via a fourth capacitive element, the other end being grounded; a first set of matching circuit and tuning circuit for supplying first RF to the sample coil; and a second set of matching circuit and tuning circuit for supplying second RF to the sample coil. This multiple tuning circuit is characterized in that the sample coil, first inductor, second inductor, first capacitive element, second capacitive element, third capacitive element, and fourth capacitive element together form a balanced resonant circuit where the amplitude voltage becomes zero near the center point of the sample coil.
In one feature of the multiple tuning circuit, the grounded end of the first inductor is grounded via a fifth capacitive element.
In another feature of the multiple tuning circuit, the grounded end of the second inductor is grounded via a sixth capacitive element.
In a further feature of the multiple tuning circuit, the first set of matching circuit and tuning circuit is directly connected with one end of the sample coil.
In still another feature of the multiple tuning circuit, the second set of matching circuit and tuning circuit is connected between the first inductor and the fifth capacitive element or between the second inductor and the sixth capacitive element.
In
Gutierrez Diego
Jeol Ltd.
Vargas Dixomara
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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