Continuously digitally adjustable phase actuator

Abstract

A phase actuator for a continuously adjustable phase displacement at a first frequency is provided. The phase actuator has a first inductance with tapping point, a first continuously variable capacitor, and a transformation network. A signal input and a signal output of the phase shifter are connected by the first inductance. The first continuously adjustable capacitor is connected in parallel to the first inductance. The tapping point is connected via a transformation network to a reference mass, where an impedance value of the transformation network corresponds to a quarter wave transform of a capacitance value of the first continuously variable capacitance at the first frequency.

Claims

1. A phase actuator for a continuously adjustable phase displacement at a first frequency, the phase actuator comprising: a signal input; a signal output; a reference mass; a first inductance with a tapping point; a first continuously variable capacitance; and a transformation network, wherein the signal input and the signal output are electrically connected by the first inductance, wherein the first continuously adjustable capacitor is connected in parallel to the first inductance, wherein the tapping point is connected via a transformation network to the reference mass, and wherein an impedance value of the transformation network corresponds to a quarter wave transform of a capacitance value of the first continuously variable capacitance at the first frequency.

2. The phase actuator of claim 1, wherein the tapping point of the inductance is a central tapping point.

3. The phase actuator of claim 1, wherein the first continuously variable capacitance has a capacitance diode.

4. The phase actuator of claim 3, wherein the capacitance diode is a first capacitance diode, and wherein the first continuously variable capacitance has a second capacitance diode, the first capacitance diode and the second capacitance diode being connected antiparallel in series.

5. The phase actuator of claim 4, wherein the transformation network has a Collins filter and a second variable capacitance.

6. The phase actuator of claim 5, wherein the phase actuator is configured to set the first continuously variable capacitance and the second variable capacitance to a capacitance value that is essentially the same.

7. The phase actuator of claim 5, wherein the second variable capacitance has two capacitance diodes connected antiparallel in series.

8. The phase actuator of claim 7, further comprising a control voltage generator configured to control the first capacitance diode and the second capacitance diode of the first continuously variable capacitance and the two capacitance diodes of the second variable capacitance with a same voltage.

9. A magnetic resonance tomograph comprising: a phase actuator for a continuously adjustable phase displacement at a first frequency, the phase actuator comprising: a signal input; a signal output; a reference mass; a first inductance with a tapping point; a first continuously variable capacitance; and a transformation network, wherein the signal input and the signal output are electrically connected by the first inductance, wherein the first continuously adjustable capacitor is connected in parallel to the first inductance, wherein the tapping point is connected via a transformation network to the reference mass, and wherein an impedance value of the transformation network corresponds to a quarter wave transform of a capacitance value of the first continuously variable capacitance at the first frequency.

10. The magnetic resonance tomograph of claim 9, wherein the tapping point of the inductance is a central tapping point.

11. The magnetic resonance tomograph of claim 9, wherein the first continuously variable capacitance has a capacitance diode.

12. The magnetic resonance tomograph of claim 11, wherein the capacitance diode is a first capacitance diode, and wherein the first continuously variable capacitance has a second capacitance diode, the first capacitance diode and the second capacitance diode being connected antiparallel in series.

13. The magnetic resonance tomograph of claim 12, wherein the transformation network has a Collins filter and a second variable capacitance.

14. The magnetic resonance tomograph of claim 13, wherein the phase actuator is configured to set the first continuously variable capacitance and the second variable capacitance to a capacitance value that is essentially the same.

15. The magnetic resonance tomograph of claim 13, wherein the second variable capacitance has two capacitance diodes connected antiparallel in series.

16. The magnetic resonance tomograph of claim 15, further comprising a control voltage generator configured to control the first capacitance diode and the second capacitance diode of the first continuously variable capacitance and the two capacitance diodes of the second variable capacitance with a same voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of a magnetic resonance tomograph with one embodiment of a phase actuator;

(2) FIG. 2 shows a diagram of function blocks of one embodiment of a phase actuator; and

(3) FIG. 3 shows a typical circuit layout for one embodiment of a phase actuator.

DETAILED DESCRIPTION

(4) FIG. 1 shows a schematic diagram of a magnetic resonance tomograph 1 with one embodiment of a phase actuator.

(5) The magnet unit 10 has a field magnet 11 that creates a static magnetic field BO to align the nuclear spin of samples or patients 40 in a recording region. The recording region is arranged in a patient tunnel 16 that extends in a longitudinal direction 2 through the magnet unit 10. A patient couch 30 is able to be moved by the drive unit 36 in the patient tunnel 16. The field magnet 11 may involve a superconducting magnet that may provide magnetic fields with a magnetic flux density of up to 3T, with the latest devices even more than 3T. For lower field strengths, however, permanent magnets or electromagnets with normally-conducting coils may be employed.

(6) The magnet unit 10 also has gradient coils 12 that are configured, for spatial differentiation of the imaging regions in the examination volume acquired, to superimpose variable magnetic fields on the magnetic field B0 in three spatial directions. The gradient coils 12 may be coils made of normally-conducting wires that may create fields orthogonal to one another in the examination volume.

(7) The magnet unit 10 likewise has a body coil 14 that is configured to irradiate a radio-frequency signal supplied via a signal line into the examination volume and to receive resonant signals emitted from the patient 40 and output the resonant signals via a signal line.

(8) A control unit 20 (e.g., a controller) supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14 and evaluates the received signals.

(9) Thus, the control unit 20 has a gradient controller 21 configured to supply the gradient coils 12 via supply lines with variable currents, which provide the desired fields temporally coordinated in the examination volume.

(10) The control unit 20 also has a radio-frequency unit 22 configured to create a radio-frequency pulse with a predetermined timing curve, amplitude, and spectral power distribution for exciting a magnetic resonance of the nuclear spin in the patient 40. In such cases, pulse powers in the Kilowatt range may be reached. The excitation pulses may also be irradiated into the patient 40 via the body coil 14 or also via a local transmit antenna.

(11) In order to achieve a homogeneous excitation of the nuclear spin on transmission of the excitation pulses or also to achieve a selective sensitivity on receipt, antennas such as the body coil 14 or the local coil 50 have individually controllable segments. By a suitable combination of signals with different phase displacements, a desired directional effect may be achieved in such cases. Phase actuators 100 of one or more of the present embodiments may be provided in the receive and/or transmit path in the radio-frequency unit 22.

(12) Shown in FIG. 2 are the major functional blocks of a phase actuator 100 of one or more of the present embodiments. The phase actuator 100 has a signal input 101, at which an input signal is supplied to the phase actuator 100 with reference to the reference mass 103, of which the phase is to be changed. The phase-shifted output signal, which is likewise to be output with reference to the reference mass 103 then lies at a signal output 102.

(13) Arranged between signal input 101 and signal output 102 is a parallel resonant circuit, made up of the inductance 110 and the first capacitance 120. However further capacitances or other components, with which the resonant frequency of the parallel resonant circuit may be displaced (e.g., a parallel capacitance to the variable first capacitance 120) may also be provided. The parallel resonant circuit may, however, as well as a phase displacement of the input signal dependent on the capacitance 120, also lead to a change in amplitude with a frequency f0, since the resonant frequency of the parallel resonant circuit also changes with the capacitance and thus the amplitude at a fixed frequency.

(14) In order to compensate for this effect, a transformation network 130 is arranged at a tapping point 111 of the inductance 110, via which the tapping point 111 is connected to the reference mass 103. When the phase actuator 100 is to be transparent in relation to the amplitude, and signal input 101 and signal output 102 are to have the same surge impedance, the phase actuator is symmetrical to signal input 101 and signal output 102. This provides that the tapping point 111 of the inductance 110 is also to be symmetrical (e.g., the tapping point 111 is a central tapping point of the inductance 110), so that with a coil as inductance, precisely as many windings are located are between signal input 101 and tapping point 111 as between tapping point 111 and signal output 102, provided the geometry is otherwise unchanged on both sides. If however, as well as a phase displacement, an impedance matching by the phase actuator 100 is also desired, the tapping point 111 may also be arranged asymmetrically. Equivalent to a tapping point 11 are also two inductances connected in series with a common core or corresponding coaxial alignment as a common long coil without core.

(15) With an impedance Z.sub.0 at signal input 101 and at signal output 102, the complex resistance X1 of the first capacitance 120 and the complex resistance X2 of the inductance 110, for a pure phase displacement without change in amplitude, the following condition is produced:
X1*X2+Z.sub.0.sup.2=0

(16) With X1=1/(C) and X2=L, the following condition is produced
L=C*Z.sub.0.sup.2

(17) While C may be embodied, for example, by a capacitance diode as a variable, electrically adjustable capacitance, it is, however, significantly more problematic to provide a simple, electrically adjustable inductance.

(18) The inductance is replaced by an easily adjustable capacitance such as, for example, a capacitance diode. In accordance with one or more of the present embodiments, this is realized by a transformation of the complex resistance that maps the inductance to a capacitance and vice versa. This may be achieved, for example, by a quarter wave line (e.g., Lambda Quarter Line). In this case, Lambda is the wavelength of an electromagnetic wave with the frequency f=/(2) on the line. If the capacitance (e.g., a capacitance diode) is connected to the end of the quarter wave line, then the line behaves at the other end, with a surge impedance Z.sub.0 connected, like the required inductance.

(19) Typical frequency ranges for magnetic resonance tomographs are today between 20 MHz and 150 MHz, which corresponds to wavelengths 15 m and 2 m in a vacuum. Even with a shortening by a dielectric quarter wave, lines would thus be too unwieldy for the usual circuit technologies.

(20) FIG. 3 shows an example of a realization of a phase actuator. The same objects are labeled with the same reference numbers as in FIG. 2.

(21) The first capacitance, in the form of embodiment of FIG. 3, is represented by a fixed capacitance 121, and two capacitance diodes 122 (e.g., type KV1560) are connected antiparallel. The capacitance diodes together provide a series capacitance able to be controlled by a voltage, which is connected in parallel to the fixed capacitance 121. The antiparallel capacitance diodes 122 obtain a control voltage from a control voltage generator 140 via a common connection point, which may be embodied by an adjustable resistance 141 that may be embodied as a DA converter, and is provided to a voltage controller 142 and an operational amplifier 143. The two capacitance diodes 122 are connected at anodes via a resistance or additionally the inductance 110 to the reference mass, so that a positive control voltage from the control voltage generator 140 pre-powers the capacitance diodes 122 in the blocking direction.

(22) The transformation network 130 is provided by a PI or Collins filter 131 instead of the space-consuming quarter wave line, which together with an upstream capacitor, represents the adjustable second capacitance 133 behind the Collins filter as a variable inductance at the tapping point 111 before the Collins filter provided by the two capacitance diodes 132. At 10 MHz and a rated impedance of 50 Ohm, the two capacitors of the PI filter have a capacitance of 318 pF, and the inductance may have a value of 796 nH.

(23) The two capacitance diodes 132 are supplied in the same way already described by the control voltage generator 140 with an adjustable voltage for changing the capacitance. A symmetrical behavior of the first capacitance 120 and of the transformation network 130 over the entire adjustment range is provided by the same voltage and identical capacitance diodes 122, 132.

(24) Although the invention has been illustrated and described in greater detail by the exemplary embodiments, the invention is not restricted by the disclosed examples. Other variations may be derived herefrom without departing from the scope of protection of the invention.

(25) The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

(26) While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.