Abstract:
An electrically adjustable variable capacitor device comprises a linear motor, at least one stator electrode and a movable electrode, thereby forming a variable capacitor. The linear motor comprises a piezoelectric transducer frictionally coupled to the movable electrode. Application of electrical signals to the piezoelectric transducer of the linear motor produces a motion of the surface of the piezoelectric transducer. The frictional coupling between the piezoelectric transducer surface and the movable electrode transmits a fraction of piezoelectric transducer motion to the movable piston electrode thereby changing the capacity of the variable capacitor. The amount and sign of the capacitance change is selectable by the operator through control of the electrical signals applied to the piezoelectric transducer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a variable capacitor adjustable by a linear motor. The linear motor is electrically adjustable and with circuit driver apparatus provides improved tuning and matching of NMR radio frequency probe coils.  
         [0002]     An NMR spectrometer system is comprised of: a DC magnet which provides a stable, homogeneous static magnet field required for polarizing nuclear spins of a sample to be analyzed; a console containing an RF system which provides a suitable RF excitation source to the nuclear spins, and provides an amplifying and detection system for detecting and recording the NMR response signals from the nuclear spins; and a probe containing RF coils for coupling the RF excitation signals to the nuclear spins and for receiving response signals from the spins, and means for containing and positioned the sample within the probe coils to achieve optimum coupling between the sample spins and the RF probe coils.  
         [0003]     For high resolution NMR studies the sample compound under investigation is usually dissolved in or mixed with a suitable solvent, is in liquid form and contained in a sample tube which is typically 5 mm in diameter. Solid samples may be a powder or crystal, and is some cases the sample may be contained in a magic angle spinning (MAS) probe for rapidly spinning the sample with the spinning axis tilted at an angle of approximately 54 degrees from the magnetic field axis. In either case the probe holds the sample tube and is positioned in the magnet so the sample is in the most homogeneous region of the magnetic field. The RF probe coil or coils for coupling the RF excitation to the sample and for detecting the NMR response signal must be tuned to the excitation frequencies and matched to the cable impedance leading to the preamplifier which may be located in the console or in the probe. The tuning and matching is typically done by variable capacitors that can be adjusted for optimum tuning and matching before running experiments with each sample.  
         [0004]     Modern NMR spectrometer systems employ superconducting magnets typically consisting vertically mounted superconducting solenoid coils that are mounted in a Dewar structure with a central reentrant section extending up through the center of the superconducting solenoid coils. Typically the probe structure comprises a long cylindrical section that fits within the reentrant section of the magnet Dewar and a lower section that remains below the magnet Dewar that may contain a preamplifier and other parts. The sample and the transmit/receive RF probe coils are located in the cylindrical region of the probe. The probe is positioned in the magnet and Dewar structure so that the sample is centered on the center axis of the superconducting coils. This arrangement provides the most uniform magnetic field in the sample region. The space about the sample containing the RF coils and the tuning and matching capacitors is rather limited. Tuning and matching variable capacitors in this region have shafts extending to the lower region of the probe where they may be turned either manually or by motors located in this region or by more distant motors coupled by flexible cables.  
         [0005]     A multi-frequency NMR probe has two or more RF probe coils with tune and match capacitors for each frequency. For example a triple resonance probe is capable of simultaneously operating at three different frequencies to excite three species of nuclear spins plus a “lock” signal. The “lock” signal typically is from deuterium nuclei in the solvent that may be deuterium oxide or deutero chloroform. To obtain optimum results with minimum excitation power, each of the four frequencies must be tuned and matched, requiring a total of eight variable capacitors. Because of limited space often compromises are made, and variable match capacitors may be provided for only one or two of the nuclei thereby reducing the variable capacitor count to 6 or less. In cryogenically cooled probes the RF probe coils are cooled to a low temperature. The coils may be either constructed of normal metals or high temperature superconducting materials. In these probes space is even more limited, with the further problem of heat transfer along the coupling shafts between the cold variable capacitors and the warm region at the bottom of the probe containing motors or knobs for manual adjustment. Heat transfer along these shafts puts an additional heat load upon the system used to provide the cooling.  
         [0006]     Controlling the tune and match capacitors electrically provides the capability of remotely adjusting the tune and match capacitors thereby enabling the operator to remain at the console while tuning and matching the probe for optimum signal to noise ratio (S/N). To achieve it, the operator typically applies the desired excitation frequency and adjusts the tune and match capacitors to obtain a minimum of reflected power. Sometimes a small “dither” is applied to this frequency while the operator observes the reflected power from the probe. This enables the operator to readily determine whether the tune or match capacitor needs adjustment to further optimize the S/N.  
         [0007]     It is also possible to use special software to control electrical signals applied to motors that are used to adjust the tune and match variable capacitors. Most prior art systems required a separate motor for each variable capacitor. As mentioned above, a shaft is run from the variable capacitor which is very close to the RF probe coil, to the motor at the bottom of the probe outside the magnet where space is limited. In all the prior art systems using superconducting magnets, one or more rotatable shafts were required to transmit the rotary motion of the motor located in the base of the probe to the sample region where the probe coils and tune and match capacitors are located. In cryogenic probes, the probe coils and tune and match capacitors are cooled to a low temperature. Problems with these systems include heat loss arising from heat being conducted up the rotatable shaft from the motor region which is near room temperature to the sample region which typically is at a very low temperature, usually at 25 K or less. To avoid a temperature rise due to this heat loss, additional cooling power is required. An another problem is maintaining alignment of the various parts in the cooled region. Forces are generated and transmitted along the rotatable shafts during the cooling phase causing misalignment of the NMR probe coils with each other and with the external magnet and gradient coils.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the present invention, each electrically adjustable variable capacitor device comprises a variable capacitor coupled to a linear electrical motor. The variable capacitor comprises a stator electrode and a movable electrode and the linear motor comprises a stator transducer component and a movable piston. A piston electrode is a common component that serves as both the movable electrode of the capacitor and the movable piston of the linear motor. A dielectric housing supports the stator electrode of the capacitor and the stator part of the linear motor. The electrically adjustable variable capacitor device has an internal mechanism for adjusting the capacitance of the device in response to electrical signals applied to the device. No shafts are required to control the devices.  
         [0009]     The adjustable variable capacitor assembly is supplied by an operating control unit, which provides electrical control current to the variable capacitor device. Electrical control currents are supplied over electrical conductors that have very low thermal conductivity. The internal drive mechanism and capacitance generating regions are combined to form a compact device. One electrically adjustable variable capacitor device can be used for each tune and one for each match variable capacitor in a NMR probe. In many cases sufficient space will be available so that all tune and match variable capacitors may be controllable by electrically adjustable variable capacitor devices resulting in better performance. Applying an appropriate electrical signal to it changes the capacity of this device. When the electrical signal is removed the electrical adjustable variable capacitor device retains the capacity value it had just prior to the removal of the electrical signal.  
         [0010]     In order that the electrically adjustable variable capacitor device not perturb the homogeneity of the magnetic field, it is made of materials with a low magnetic susceptibility. No ferromagnetic or highly paramagnetic materials are used in construction. The device uses piezoelectric crystals or ceramic materials to convert the electrical drive to a mechanical motion. These materials are also capable of operating at the low temperature required for cryogenically cooled probes. They are also compatible with the vacuum requirements of cryogenically cooled probes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The preferred embodiments of the present invention is given by the way of non-limiting examples that will be described below with the reference to the accompanying drawings in which:  
         [0012]      FIG. 1  is a cutaway isometric view of a section of an electrically adjustable variable capacitor device.  
         [0013]      FIG. 2  is a diagram of the construction of a piezoelectric transducer from a plurality of piezoelectric actuators.  
         [0014]      FIG. 3A  is a sawtooth waveform with a slowly rising component and a rapidly falling component.  
         [0015]      FIG. 3B  is a sawtooth waveform with a rapidly rising component and a slowly falling component.  
         [0016]      FIG. 4  is a cutaway isometric view of an electrically adjustable variable capacitor device showing a fully extended movable piston electrode.  
         [0017]      FIG. 5A  is an operating control unit, which shows a connection diagram of a sawtooth wave generator and switch apparatus for applying and selecting the polarity of the sawtooth output to drive the piezoelectric transducers of an electrically adjustable variable capacitor device.  
         [0018]      FIG. 5B  is an operating control unit, which shows a connection diagram of a sawtooth wave generator and software controlled relay apparatus that select and control the sawtooth signals applied to the piezoelectric transducer thereby providing automatic adjustment capability of a electrically adjustable variable capacitor device.  
         [0019]      FIG. 6  illustrates the motion of a progressive surface acoustic wave on a piezoelectric substrate.  
         [0020]      FIG. 7  illustrates the interdigital electrodes arrangement used to launch surface waves in either direction along the length of a piezoelectric substrate.  
         [0021]      FIG. 8  is a cutaway isometric view of electrically adjustable variable capacitor device using surface acoustic waves to change the capacitance.  
         [0022]      FIG. 9A  is a variable capacitor assembly comprising an operating control unit to enable manual adjustment of an electrically adjustable variable capacitor device employing surface acoustic waves.  
         [0023]      FIG. 9B  is a diagram of a variable capacitor assembly comprising an operating control unit to enable automatic adjustment of an electrically adjustable variable capacitor employing surface acoustic waves. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 1  is a cutaway isometric view showing the major elements of the invention. Dielectric housing  11  supports stator electrode  12  placed on the exterior of dielectric housing  11 . On the interior of dielectric housing, a piston, with conductive surface  13 , forms movable piston electrode  8 . Stator electrode  12  and movable piston electrode  8  comprise a first variable capacitor. Conductive surface  13  extends around the entire periphery of piston electrode  8 , making electrical contact with contact finger  6 . The piston may be comprised of an insulator with a conductive metallic coating to provide conductive surface  13 , or alternatively it may be made of a metal that provides conducting surface  13  and forms piston electrode  8 . Electrical feed-through  7  provides an electrical connection between contact finger  6  and terminal  4 . Stator electrode  12  and terminal  4  provide electrical connections to the first variable capacitor.  
         [0025]     Piezoelectric transducer  15  has its proximal end  16  fixed to dielectric housing  11 . Friction plate  17  is fixed to the distal end  20  of piezoelectric transducer  15 . Friction plate  17  makes frictional contact with piston electrode  8  thereby providing a frictional coupling between piezoelectric transducer  15  and movable piston electrode  8 . Voltage may be applied to piezoelectric transducer  15  by lead  18  and return lead  19  causing it to lengthen or shorten in the z-direction of coordinate axes  10 .  
         [0026]     In one embodiment the effective friction (or positional accuracy and repeatability) is enhanced between friction plate  17  and movable piston electrode  8  by forming a set of matching microgrooves on each member thereby restricting the relative static positions of piston electrode  8  to those in which the two microgrooves partially interlock.  
         [0027]     As shown in  FIG. 2 , piezoelectric transducer  15  comprises of a stack of actuators (piezoelectric elements)  30 . Thin conductive electrodes  32  between each piezoelectric element  30  permits the application of a voltage across each piezoelectric element  30 . Alternate conductive electrodes  32  are connected to lead  18  and return lead  19 . Each adjacent piezoelectric element  30  is poled in the opposite direction, or for spontaneously piezoelectric crystals such as quartz, the alternate elements have reversed orientation as indicated by the arrows on the piezoelectric elements  30 . Applying a positive voltage on lead  18  and a negative voltage on return lead  19  causes each element to expand. As shown in  FIG. 1 , the proximal end  16  of piezoelectric transducer  15  is fixed to dielectric housing  11 , this expansion causes the distal end  20  of piezoelectric transducer  15  and friction plate  17  to move along the plus z-axis of coordinate axis  10 . A negative voltage on lead  18  and a positive voltage on return lead  19  causes piezoelectric transducer  15  to contract and its distal end and friction plate  17  moves in the negative direction of the z-axis.  
         [0028]     The principal of the linear motor drive might be called “stick and slip” motion, the combined effect of friction and inertia. Referring to  FIG. 1 , friction plate  17  presses against piston electrode  8 , which is free to move along the z-direction of coordinate system  10 . For slow motion of piezoelectric transducer  15 , non-sliding contact is maintained between friction plate  17  and conductive surface  13  of piston electrode  8 , moving piston electrode along with the motion of friction plate  17  and distal end  20  of piezoelectric transducer  15 . For very rapid motions of piezoelectric transducer  15 , friction between friction plate  17  and conductive surface  13  of piston electrode  8  is insufficient to overcome the inertia due to the mass of the piston and thereby preventing any substantial motion of piston electrode  8 . By applying a saw-tooth drive voltage between lead  18  and return lead  19  that slowly rises and quickly falls, as shown in  FIG. 3A , moves piston electrode  8  along the positive z-axis thereby increasing the capacitance between stator electrode  12  and conductive surface  13  of piston electrode  8 .  
         [0029]      FIG. 4  is a cutaway view showing the position of piston electrode  8  that is nearly fully extended resulting in a nearly maximum capacity between stator electrode  12  and piston electrode  8 . The labeling of parts of  FIG. 4  is the same for the same parts as shown in  FIG. 1 .  
         [0030]     Applying a sawtooth drive voltage that rises quickly and falls slowly between lead  18  and return lead  19  as sketched in  FIG. 3B , moves piston electrode  8  along the negative z-axis thereby decreasing the capacitance between stator electrode  12  and conductive surface  13  of piston electrode  8 .  FIG. 1  is a cutaway view of the electrically variable capacitor with piston electrode  8  fully retracted resulting in a minimum capacity between stator electrode  12  and piston electrode  8 .  
         [0031]     The electrically adjustable variable capacitor device of  FIGS. 1 and 4  are symmetric about the Y=0 plane of coordinate system  10 , so a second piezoelectric transducer  25  with its friction plate is identical to piezoelectric transducer  15 , but has been rotated by 180 degrees about the z-axis. Piezoelectric transducers  15  and  25  facing each other, and both fixed to dielectric housing  11  at their proximal ends  16 . Friction plates  17  and  27  (of  FIG. 4 ) are mounted on the distal ends  20  of piezoelectric transducers  15  and  25  respectively. Stator electrode  22  is indicated by the dotted line is symmetric with stator electrode  12 . Piezoelectric transducer  25  is identical in construction to piezoelectric transducer  15 , being made up of individual piezoelectric elements and connected in the same way. The individual elements of transducer  25  are connected to lead  18  and return lead  19 . In operation lead  18  and return lead  19  of piezoelectric transducer  25  are connected to lead  18  and return lead  19  of transducer  15 . By applying a sawtooth voltage that rises slowly and falls quickly, friction plate  27  is moved along the positive z-axis in synchronism with the motion of friction plate  17 . The friction coupling of friction plates  17  and  27  to piston electrode  8  causes it to move along the positive z-axis of coordinate system  10 . This motion increases the capacitance of the first capacitor formed by stator electrode  12  and conductive surface  13  of piston electrode  8 , and also increases the capacitance of the second capacitor formed by stator electrode  22  and conductive surface  23  of piston electrode  8 . The conductive surfaces  13  and  23  of piston electrode  8  are electrically connected thereby connecting the first and second capacitors in series, so capacitance between stator electrodes  12  and  22  also increases corresponding to the series capacitance of said first and second capacitors. A sawtooth voltage drive that rises quickly and falls slowly applied to both piezoelectric transducers via leads  18  and return leads  19  causes a decrease in capacitance between stator electrodes  12  and  22 .  
         [0032]     Piston electrode  8  is common to said first and second capacitors. Electrically connecting stator electrode  12  to stator electrode  22  forms a common connection to the stator electrodes of the first and second capacitors. Electrical connection to terminal  4  and the common connection of the stator electrodes  12  and  22  provide external parallel connections of the two capacitors.  
         [0033]     The piezoelectric transducers  15  and  25  are held in place by tension block  41 , providing the correct spacing of transducers  15  and  25 . Tension band  42  presses the transducers against tension block  41  and insures that friction plates  17  and  27  maintain contact with conductive surfaces  13  and  23  of the piston.  
         [0034]     Manual adjustment of an electrically adjustable capacitor device is achieved through the circuit of  FIG. 5A . Sawtooth Voltage Generator  50  supplies a slowly rising voltage followed by a rapid fall. Typically the voltage slowly rises from −100 volts to +100 volts, and then quickly falls back to −100 volts. The voltage appears on output lead  54  and the return lead  55 . The time taken for the voltage to rise typically is 10 or more times longer than the time for the voltage to fall. The output leads  54  and return lead  55  are coupled to double pole, 3-position switch  52 .  FIG. 5A  shows switch  52  in position A with output lead  54  coupled to lead  58  and return lead  55  is coupled to lead  59 . Lead  58  is coupled to leads  18  of  FIGS. 1, 2  and  4 , and lead  59  is coupled to return leads  19  of  FIGS. 1, 2  and  4 . With this connection arrangement, switch  52  enables the operator to adjust the capacitance of the electrically adjustable variable capacitor device of  FIGS. 1 and 4 . With switch  52  in position A as shown in  FIG. 5A , the voltage will slowly rise and quickly fall on leads  18  of  FIGS. 1, 2  and  4  causing the capacity to increase. With the switch  52  in position B, no voltage is applied, and the capacitance value remains at the value it had just before the switch was changed. With switch  52  in position C, the voltage on leads  18  will rise quickly and fall slowly causing the capacitance of the electrically adjustable variable capacitor device of  FIGS. 1 and 4  to decrease capacity. The switch  52  of  FIG. 5A  can be placed at the operator&#39;s console, or any other place that is convenient for the operator. One switch is used for each electrically adjustable variable capacitor.  
         [0035]     Automatic adjustment of the electrically adjustable variable capacitor device is achieved through the circuit of  FIG. 5B . Sawtooth Voltage Generator  50  supplies a slowly rising voltage followed by a rapid fall on output lead  54  and return lead  55 . Relay  62  is a double pole single throw relay that when activated connects the output of sawtooth voltage generator  50  leads  54  and  55  to leads  64  and  65  respectively. Relay  66  is a double pole double throw relay that when activated couples leads  64  to lead  69  and  65  to lead  68 . When not activated, relay  66  connects lead  64  to lead  68  and lead  65  to lead  69 . Lead  68  is coupled to leads  18  of  FIGS. 1, 2  and  4 , and lead  69  to leads  19  of  FIGS. 1, 2  and  4 . Relay coils  63  and  67  of  FIG. 5B  are coupled to software controller and driver  74 . The software is designed to adjust tune and match capacitors of a probe using signals reflected from the probe. A single sawtooth generator may be used to supply all electrically adjustable variable capacitor devices, but separate relay circuits and relays are used for each electrically adjustable capacitor device.  
         [0036]     A second embodiment of a variable capacitor driven by a linear motor employs surface acoustic waves known as Rayleigh waves. The Rayleigh waves are frictionally coupled to the piston head. In response to a propagating Rayleigh wave, surface molecules of the propagating medium undergo an elliptical motion. Frictional contact of the piston head with these surface molecules causes the piston head to move in response to the motion of the surface molecules.  
         [0037]      FIG. 6  illustrates a surface acoustic wave  210  propagating along the surface of piezoelectric substrate  215  from distal end  20  toward proximal end  16  as indicated by arrow  218 . The piezoelectric substrate  215  may be composed of a piezoelectric material such as Y-cut lithium niobate or Y-cut crystalline quartz. Molecules on surface of piezoelectric substrate  215  undergo an elliptical motion as illustrated by ellipse  219 . Piston head  230  makes frictional contact with the molecule at the wave crests  213  that are moving in the direction of arrow  220 . The piston head  230  loses contact with these molecules as the wave moves on and the molecules move below the normal active surface  212  to form a trough. Friction between piston head  230  and molecules at the wave crests  213  causes the piston head to move in the same direction as the wave crests  213 .  
         [0038]     Applying a radio-frequency voltage between interdigital electrodes  216  and  217  excites the surface acoustic wave  210 , which propagates along active surface  212  of piezoelectric substrate  215  in the direction of arrow  218 . As the wave propagates it is somewhat attenuated by the absorption of acoustic energy in the piezoelectric substrate. It may be further attenuated by coupling of some of its energy out through the interdigital electrodes  316  and  317  located at proximal end  16  of piezoelectric substrate  215 . Coupling electrodes  316  and  317  to load resistors (shown in  FIG. 9A ) may dissipate this energy.  
         [0039]     Referring to  FIG. 7 , a set of interdigital electrodes  216  and  217  located near the distal end  20  of piezoelectric substrate  215  and are electrically coupled to leads  310  and  311  respectively. A second set of interdigital electrodes  316  and  317  are located near the proximal end  16  of piezoelectric substrate  215  and are electrically coupled to leads  313  and  312  respectively. The two sets of interdigital electrodes are fixed to the active surface  212  of piezoelectric substrate  215 .  
         [0040]      FIG. 8  is a cutaway view of a complete electrically tunable variable capacitor device using two piezoelectric substrates  215 A and  215 B that are identical with piezoelectric substrate  215  of  FIG. 7 . Each piezoelectric substrate  215 A and  215 B has the same interdigital and lead connections as piezoelectric substrate  215  and is connected as shown in  FIG. 7 . The piezoelectric substrates  215 A and  215 B only differ in that one has been rotated by 180 degrees about the z-axis of coordinate system  10  of  FIG. 8 . Piston head  230  makes friction contact with the active surface  212  of piezoelectric substrates  215 A and  215 B. Tension block  141  at proximal end  16  provides spacing between piezoelectric substrates  215 A and  215 B at a distance equal to the y-dimensions of piston head  230  thereby enabling movement of piston electrode  228 . Tension band  142  provides pressure to preventing slippage of piston head  230  when the piezoelectric substrates  215 A and  215 B are not electrically activated.  
         [0041]     Piston head  230  is fixed to the end of piston electrode  228 . Both sides of piston electrode  228  have conductive surfaces  113  and  123 , and the two sides are electrically connected. Dielectric housing  111  supports stator electrodes  112  and  122  on the exterior of the housing. Stator electrode  112  and conductive surface  113  of the moveable piston electrode  228  form a first variable capacitor and stator electrode  122  and conductive surface  123  of the movable piston electrode  228  form a second variable capacitor. Since these two variable capacitors are connected in series, stator electrode  112  and  122  also form the terminals of an electrical adjustable variable capacitor. By providing a terminal arrangement with a sliding electrical contact similar to that of  FIGS. 1 and 4  with terminal  4  connected to the movable piston electrode by feed-through  7  and sliding contact  6 , parallel operation of the two capacitors is achieved.  
         [0042]     The operating control unit of  FIG. 9A  provides manual adjustment means of the surface wave electrically adjustable variable capacitor device. For clarity of the wiring, both piezoelectric substrates  215 A and  215 B are shown in the same orientation, however when installed in the dielectric housing  111  of  FIG. 8 , piezoelectric substrate  215 A must be rotated by 180 degrees about the z-axis. The 4-pole, 3-position switch  340  provides proper connections of leads  310 - 313  to the RF generator  330  and loading resistors  332  and  333 . When switch  340  is in position A, power from RF generator  330  is applied to leads  310  and  311  producing an acoustic wave on piezoelectric substrate  215 A. The acoustic wave is propagating from the distal end  20  toward the proximal end  16  thereby increasing the capacity of the electrically adjustable variable capacitor of  FIG. 8 . Leads  312  and  313  are connected to loading resistor  332  that absorbs energy induced into the interdigital electrodes at the proximal end  16 , and thereby minimizing any surface waves reflected at the proximal ends of piezoelectric substrates  215 A and  215 B. With switch  340  in position B, no RF energy is applied to piezoelectric substrates  215 A and B. Loading resistors  332  and  333  will attenuate any remaining surface acoustic waves. With switch  340  in position C, radio frequency power from RF generator is applied to leads  312  and  313  producing an acoustic wave propagating from proximal end  16  toward distal end  20  of piezoelectric substrates  215 A and B thereby decreasing the capacity of the electrically adjustable variable capacitor. Loading resistor  333  absorbs energy induced into the interdigital electrodes at the distal end  20  of piezoelectric substrates  215 A and B minimizing waves that might be reflected at the distal end.  
         [0043]     Automatic adjustment of the electrical adjustable capacitor is acheved through the operating control unit of  FIG. 9B . Relays  350  and  360  are electrically controlled by software controller and driver  374 . The software is designed to adjust electrically adjustable variable capacitor device shown in  FIG. 8 . This capacitor, for example may tune or match variable capacitors of an NMR probe by using signals reflected from the probe. Each relay of  FIG. 9B  is a double pole, double throw type. With no power applied the contacts of each relay are in the A-position and with activation the contacts are in the B-position. The software controller and driver  374  controls relays  350  and  360  by controlling the driving power applied to relay coils  352  and  362  respectively. With power removed from both coils  352  and  362 , the contacts of both relays are in the A-position and no RF power from the RF generator  330  is applied to piezoelectric substrates  215 A and B, and the capacitor is in a quiescent state. To increase the capacity of the voltage adjustable variable capacitor device software controller and driver  374  applies power only to relay coil  352  causing the contacts of relay  350  to move to the B-position while and the contacts of relay  360  remain in the A-position. With this arrangement radio frequency oscillator  330  is connected to the interdigital electrodes at the distal end  20  of transducer substrates  215 A and B, causing surface waves to propagate from the distal end  20  toward the proximal end  16 . The reflected power is minimized as relay  360  is in the A-position so that energy arriving at the proximal end  16  of piezoelectric substrate  215  is partially adsorbed by loading resistor  332 . With this switch arrangement the capacity of the electrical adjustable variable capacitor of  FIG. 8  increases.  
         [0044]     Conversely to decrease the capacity of the voltage adjustable variable capacitor device software controller and driver  374  applies power only to relay coil  362  causing the contacts of relay  360  to move to the B-position while and the contacts of relay  350  remain in the A-position. With this arrangement radio frequency oscillator  330  is connected to the interdigital electrodes at the proximal end  16  of transducer substrates  215 A and B, causing surface waves to propagate from the proximal end  16  toward the distal end  20 . The reflected power is minimized as relay  350  is in the A-position so that energy arriving at the proximal end  16  of piezoelectric substrate  215  is partially adsorbed by loading resistor  333 . With this switch arrangement the capacity of the electrical adjustable variable capacitor of  FIG. 8  decreases. When power is removed from coils  352  and  362  the capacitor maintains the capacitance value it had just before the power was removed.  
         [0045]     Additionally, driving the set of electrodes on the end of the transducer receiving the acoustic wave with an electrical signal phased to absorb the energy of this wave would eliminate the need for resistors  332  and  333  and their associated relay connections.  
         [0046]     Although the invention has been described in its preferred embodiments, those skilled in the art will recognize many variations may be made thereto without departing from the spirit and scope of the invention. A progressive surface acoustic waves could be produced a number of ways, for example by two piezoelectric transducers spaced a quarter wavelength apart and driven by an RF frequency that has a 90 degree phase lead or lag to one of the transducers. By interchanging the drives to the two transducers, the direction of the progressive is reversed. A progressive bending wave could be used inplace of a surface acoustic wave to achieve similar results.  
         [0047]     The invention has been illustrated as part of magnetic resonance spectrometer, and indeed it has great utility in this application. Those skilled in the art will recognize the invention has utility in many other applications as well, such as in tuning and matching of electrical circuits in radio, television, radar and many other electrical and electronic devices.  
         [0048]     The invention of applying electrical signals to a linear motor to adjusting a variable capacitor has been illustrated using piezoelectric transducers to convert the electrical energy to linear mechanical motion. It will be obvious to those skilled in the art there are many other ways to convert electrical energy to linear mechanical motion including using electromagnetic and electrostatic forces.  
                                         PART NUMBERS                                 4   electrical terminal        6   contacting finger        7   electrical feed-through        8   piston electrode       10   coordinate system       11   dielectric housing       12   stator electrode       13   conductive surface       15   piezoelectric transducer       16   proximal end       17   friction plate       18   lead       19   return lead       20   distal end       22   stator electrode       23   conductive surface       25   piezoelectric transducer       27   friction plate       30   actuators       32   conductive electrodes       41   tension block       42   tension band       50   sawtooth voltage generator       52   double pole, 3 position switch       54   output lead       55   return lead       58, 59   output leads       62   relay (double pole single throw)       63   relay coil       64, 65   leads       66   relay (double pole double throw)       67   relay coil       68, 69   leads       74   software controller and driver       111    dielectric housing       112    stator electrodes       113    conductive surface       122    stator electrode       123    conductive surface       141    tension block       142    tension band       210    surface acoustic wave       212    active surface (of piezoelectric substrate 215)       213    wave crest       215    piezoelectric substrate       215A, 215B   piezoelectric substrate       216, 217   interdigital electrodes       218    arrow (direction of wave propagation)       219    ellipse (representing the motion of surface molecules)       220    arrow (direction of motion of piston head)       228    piston       230    piston head       310, 311   electrical leads       312, 313   electrical leads       316, 317   interdigital electrodes       330    RF generator       332, 333   loading resistors       340    4-pole, 3-position switch       350    double pole, double throw relay       352    relay coil       360    double pole, double throw relay       362    relay coil       374    software controller and driver