Patent Publication Number: US-8110993-B2

Title: Methods for inductively-coupled RF power source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of and claims the priority benefit under 35 U.S.C. §121 of U.S. patent application Ser. No. 12/265,870 entitled “Inductively Coupled RF Power Source,” filed Nov. 6, 2008, now U.S. Pat. No. 7,940,008, which is a continuation of and claims the priority benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/285,530, now U.S. Pat. No. 7,459,899, entitled “Inductively Coupled RF Power Source,” filed Nov. 21, 2005, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The disclosed embodiments of the present invention relate generally to techniques for implementing a power source, and relate more particularly to a system and method for implementing an inductively-coupled plasma radio-frequency (RF) power source. 
     BACKGROUND OF THE INVENTION 
     Implementing effective methods for implementing analytical instrumentation is a significant consideration for designers and manufacturers of contemporary electronic analytical devices. However, effectively performing analysis procedures with electronic devices may create substantial challenges for system designers. For example, enhanced demands for increased device functionality and performance may require more system functionality and require additional hardware resources. An increase in functionality or hardware requirements may also result in a corresponding detrimental economic impact due to increased production costs and operational inefficiencies. 
     Furthermore, enhanced system capability to perform various advanced operations may provide additional benefits to a system user, but may also place increased demands on the control and management of various device components. For example, an enhanced electronic system that analyzes certain organic substances may benefit from an efficient implementation because of the complexity and precision of the analysis involved. 
     Due to growing demands on system resources and increasing complexity of analysis requirements, it is apparent that developing new techniques for implementing analytical instrumentation is a matter of concern for related electronic technologies. Therefore, for all the foregoing reasons, developing effective techniques for implementing analytical instrumentation remains a significant consideration for designers, manufacturers, and users of contemporary analytical instruments. 
     SUMMARY 
     In accordance with the present invention, a system and method are disclosed for effectively implementing an RF power source. In one embodiment, an RF amplifier of the RF power source provides a variable-frequency RF power signal to a fixed closely-coupled impedance match that is implemented in a balanced manner. The impedance match then transfers the RF power signal to a plasma coil that is positioned adjacent to a plasma torch containing a test sample for analysis. The RF power signal is also returned through a low-pass filter to a phase-locked loop device as a reference phase signal. In addition, a phase probe is positioned near the plasma coil to sample a current operating frequency of the plasma coil. The output of the phase probe is returned through a low-pass filter to the phase-locked loop as a coil phase signal. 
     The phase-locked loop device then employs an RF phase comparison technique to track a peak resonance condition at the plasma coil. In practice, a phase detector of the phase-locked loop device compares the reference phase signal with the sampled coil phase signal to generate an error voltage that represents where the current operating frequency is with respect to peak resonance. A voltage-controller oscillator of the phase-locked loop device then utilizes the error voltage to generate a corresponding RF drive signal to the RF amplifier for adjusting the frequency of the RF power signal. The adjusted frequency of the RF power signal operates to drive the current operating frequency of the plasma coil in a direction towards peak resonance. At peak resonance the error voltage becomes zero volts. 
     Therefore, if the impedance at the plasma coil changes as a result of a varying load from the test sample in the plasma torch, an error voltage is produced with a polarity that drives the operating frequency of the plasma coil in a direction towards resonance. The loop response of the phase-locked loop is only tens of cycles of the operating frequency. The RF power source may therefore rapidly track a peak resonance condition at the plasma coil to effectively provide stable RF power and maintain a plasma state under rapid changes in load impedance. For at least the foregoing reasons, the present invention provides an improved system and method for effectively implementing an inductively-coupled plasma RF power source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a plasma creation system, in accordance with one embodiment of the present invention; 
         FIG. 2  is a block diagram for one embodiment of the RF power source of  FIG. 1 , in accordance with the present invention; 
         FIG. 3  is a schematic diagram for one embodiment of the impedance match and RF amp of  FIG. 2 , in accordance with the present invention; 
         FIG. 4  is a block diagram for one embodiment of the phase-locked loop of  FIG. 2 , in accordance with the present invention; 
         FIG. 5  is a graph illustrating a phase shift-error voltage relationship, in accordance with one embodiment of the present invention; 
         FIG. 6  is a graph illustrating a technique for operating on a resonance slope, in accordance with one embodiment of the present invention; 
         FIG. 7  is a flowchart of method steps for tracking a resonant condition during a plasma creation process, in accordance with one embodiment of the present invention; 
         FIG. 8  is a flowchart of method steps for generating an error voltage, in accordance with one embodiment of the present invention; and 
         FIG. 9  is a flowchart of method steps for adjusting an RF operating frequency, in accordance with one embodiment of the present invention. 
     
    
    
     Like reference numerals refer to corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The present invention relates to an improvement in analytical instrumentation techniques. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     The present invention comprises a system and method for implementing a power source, and includes a power amplifier that generates a radio-frequency power signal with an adjustable operating frequency. The power amplifier also generates a reference phase signal that is derived from the radio-frequency power signal. An impedance match provides the radio-frequency power signal to a plasma coil that has a variable resonance condition. A phase probe is positioned adjacent to the plasma coil to generate a coil phase signal corresponding to the adjustable operating frequency. A phase-locked loop then generates an RF drive signal that is based upon a phase relationship between the reference phase signal and the coil phase signal. The phase-locked loop provides the RF drive signal to the power amplifier to control the adjustable operating frequency, so that the adjustable operating frequency then tracks the variable resonance condition. 
     Referring now to  FIG. 1 , a block diagram of a plasma creation system  112  is shown, in accordance with one embodiment of the present invention. In the  FIG. 1  embodiment, plasma creation system  112  includes, but is not limited to, a radio-frequency (RF) power source  116 , a plasma coil  120 , and a plasma torch  124 . In alternate embodiments, plasma creation system  112  may be implemented using components and configurations in addition to, or instead of, certain of those components and configurations discussed in conjunction with the  FIG. 1  embodiment. 
     In the  FIG. 1  embodiment, plasma creation system  112  operates to initiate and sustain a test sample in a plasma state with improved power delivery and efficiency characteristics. In the  FIG. 1  embodiment, plasma creation system  112  may be utilized for any appropriate applications. For example, in certain embodiments, plasma creation system  112  may be utilized in conjunction with Inductively-Coupled Plasma Optical Emission (ICPOE) systems or with Inductively-Coupled Plasma Mass Spectrometry (ICPMS) systems. 
     Plasma is known as the fourth state of matter, and is composed of an ionized gas that is electrically conductive. Plasma emits electro-magnetic waves that may be analyzed for identifying corresponding atomic elements in the plasma. Each element has a unique set of wavelengths, and the characteristics of a given wave set may be utilized to identify a corresponding element. A ratio of wavelength intensities may be utilized to identify the concentration of each element in a test sample that is being analyzed. The accuracy and dynamic response of the analysis measurements depend on the stability and method of delivering power to initiate and sustain the test sample in a plasma state. 
     In the  FIG. 1  embodiment, the RF power source  116  provides RF power to a plasma coil  120 . A cylindrical plasma torch  124  is typically placed adjacent to the plasma coil  120 . The plasma torch  124  conducts a gas, such as argon, axially through the center of the plasma coil  120 . An impedance match in RF power source  116  is employed to couple the RF power from the RF power source  116  to the plasma coil  120  to efficiently transfer RF power to the gas flowing through plasma torch  124 . 
     Next, a high ignition voltage is discharged through a gas in plasma torch  124 , and the gas releases free electrons. A test sample to be analyzed is injected into the gas stream within the plasma torch  124 . The test sample is then in a conductive state to partially couple the applied RF power from the RF power source  116 . A cascade process ensues to gradually increase the coupling and transfer of RF power from RF power source  116  until a full plasma state is established. During the initial ignition phase, RF power source  116  is required to supply a high level of RF power to initiate the cascade process towards a full plasma state. 
     As a full plasma state is being established, electrical properties of plasma coil  120  transition to a significantly different impedance. The lower impedance reduces the RF power requirement needed to sustain the plasma. The resonance frequency of plasma coil  120  varies depending on the particular test sample in plasma torch  124 . In addition, the transition to a full plasma state produces significant changes in electrical properties of plasma coil  120  and plasma torch  124 . RF power source  116  must therefore effectively support the dynamics of this transition. The implementation and functionality of RF power source  116  are further discussed below in conjunction with  FIGS. 2 through 9 . 
     Referring now to  FIG. 2 , a block diagram for one embodiment of the  FIG. 1  RF power source  116  is shown, in accordance with the present invention. In alternate embodiments, RF power source  116  may include components and configurations in addition to, or instead of, certain of those components and configurations discussed in conjunction with the  FIG. 2  embodiment. In addition, RF power source  116  is discussed below in the context of initiating and sustaining various types of plasma. However, in certain alternate embodiments, the principles and techniques of the present invention may be applied to other appropriate contexts and applications. 
     In the  FIG. 2  embodiment, RF power source  116  is implemented to facilitate greater power stability and an extended range of operation for plasma creation system  112  ( FIG. 1 ). Power stability in RF power source  116  permits more accurate measurement of various different types of test samples. An extended range of operation facilitates analyzing certain test samples, such as organics, that exhibit significant impedance changes at plasma coil  120  and plasma torch  124 . Increased response range to impedance changes permits testing of higher concentrations of test sample solutions. 
     In the  FIG. 2  embodiment, an RF amplifier (RF amp)  216  provides a variable-frequency RF power signal  220  to a fixed closely-coupled impedance match  224  that then transfers the RF power signal to plasma coil  120  via path  228 . The RF power signal  220  is also returned as an unfiltered reference phase signal  244 ( a ) through a low-pass filter (LPF)  280  to a phase-locked loop (PLL)  240  as a filtered reference phase signal  244 ( b ). In the  FIG. 2  embodiment, LPF  280  functions to remove certain harmonic content that may be present in reference phase signal  244 ( a ). In addition, a phase probe  232  is positioned near plasma coil  120  to sample the current operating state of the resonant condition at plasma coil  120 . In alternate embodiments, RF power source  116  may utilize any other appropriate techniques for sampling the resonant condition at plasma coil  120 . For example, phase probe  232  may be located in any effective location with respect to plasma coil  120 . The output of phase probe  232  is returned as an unfiltered coil phase signal  236 ( a ) through a low-pass filter (LPF)  284  to phase-locked loop  240  as a filtered coil phase signal  236 ( b ). In the  FIG. 2  embodiment, LPF  284  is identical to LPF  280  and functions to provide the same time/phase shift as LPF  280  to maintain a ninety-degree phase relationship at resonance. 
     In the  FIG. 2  embodiment, PLL  240  may then employ an RF phase comparison technique to track a peak resonance condition at plasma coil  120 . In practice, PLL  240  compares filtered reference phase signal  244 ( b ) with the filtered coil phase signal  236 ( b ) to generate an error voltage that represents where the current operating frequency at plasma coil  120  is with respect to peak resonance. PLL  240  then utilizes the error voltage to generate a corresponding RF drive signal  248  to RF amp  216  for adjusting the frequency of RF power  220 . The adjusted frequency of RF power  220  operates to drive the operating frequency of plasma coil  120  in a direction towards peak resonance. At peak resonance the error voltage becomes zero volts. 
     Therefore, if the impedance at plasma coil  120  changes as a result of a varying load from plasma torch  124  ( FIG. 1 ), an error voltage is produced with a polarity that drives the operating frequency in a direction towards resonance. The loop response of PLL  240  is only tens of cycles of the operating frequency, which may nominally be set at approximately 27 MHz in certain embodiments. RF power source  116  therefore rapidly tracks a peak resonance condition at plasma coil  120  to effectively provides stable RF power and achieve a plasma state under rapid changes in load impedance. 
     In the  FIG. 2  embodiment, a controller  252  monitors and controls certain functions of RF power source  116 . For example, controller  252  may monitor various operating parameters of plasma creation system  112  ( FIG. 1 ), such as argon pressure, coolant water flow, power loss, plasma status, plasma door interlock, maximum current, and maximum temperature. Controller  252  may receive parameter information from any appropriate source. For example, in the  FIG. 2  embodiment, a plasma sensor  272  provides plasma information to controller via path  276 , and one or more temperature sensors may provide temperature information to controller  252  via path  268 . If any improper operating conditions are detected, controller  252  may initiate a safe shutdown procedure. If AC power is lost, controller  252  is implemented with sufficient operating power to allow controller  252  to complete the shutdown procedure. RF power source  116  may bi-directionally communicate various types of relevant information with a host system (such as a host analytical instrument) through a host interface  264 . 
     In the  FIG. 2  embodiment, a variable power supply  260  may be utilized to select a desired operating power for RF amp  216 . The overall design of RF power source  116  allows for an integrated compact enclosure, where all the components, including RF amp  216 , impedance match  224 , controller  252 , variable power supply  260 , and other circuits, are housed into one modular enclosure. This stand-alone configuration enables RF power source  116  to be incorporated into various analytical instruments without modification. All the components of the RF power source  116  are housed in a common enclosure to make shielding radio-frequency emissions more effective. The implementation and utilization of RF power source  116  is further discussed below in conjunction with  FIGS. 3-9 . 
     Referring now to  FIG. 3 , a schematic diagram for one embodiment of the  FIG. 2  RF amp  216  and the  FIG. 2  impedance match  224  is shown, in accordance with the present invention. In alternate embodiments, RF amp  216  and impedance match  224  may include components and configurations in addition to, or instead of, certain of those components and configurations discussed in conjunction with the  FIG. 3  embodiment. 
     In the  FIG. 3  embodiment, a preamplifier stage (preamp)  330  of RF amp  216  receives an RF drive signal  248  from PLL  240  ( FIG. 2 ) at a given adjustable frequency that is determined by PLL  240 . Preamp  330  then passes the RF drive signal  248  through transformer  1  (T 1 )  336 , transistors Q 1  and Q 2 , and transformer  2  (T 2 )  324  to final stage  328  of RF amp  216 . A first transistor bank of transistors Q 5 , Q 6 , and Q 7 , and a second transistor bank of transistors Q 8 , Q 9 , and Q 10  are arranged in a push-pull amplifier configuration to receive the RF signal from T 2   324 , and generate a balanced RF power signal to impedance match  224  through connections  220 ( a ) and  220 ( b ). Impedance match  224  then passes the RF Power signal to plasma coil  120  through connections  228 ( a ) and  228 ( b ). In addition, the RF power output signal of RF amp  216  is sampled at connection  220 ( a ), and is provided in a feedback loop to PLL  240  ( FIG. 2 ) as a reference phase signal  244 ( a ). 
     In the  FIG. 3  embodiment, RF amp  216  has a power amplifier bias to operate in a class E mode for improved efficiency by completely saturating Q 5 , Q 6 , Q 7 , Q 8 , Q 9 , and Q 10 . The power amplifier may also be configured to operate with Q 5 , Q 6 , Q 7 , Q 8 , Q 9  in a more linear or unsaturated class B mode of operation for reduced efficiency so as to sustain a plasma with a power level lower than can be achieved in the saturated mode. This mode is advantageous for certain applications of mass spectrometry. The design of RF amp  216  exhibits a wide bandwidth with a flat response that delivers constant power over the range of operating frequencies of RF power source  116  ( FIG. 2 ). Power amp  216  is directly close-coupled to impedance match  224 , thus eliminating the need for a coaxial feed cable. Close coupling permits operating at impedances other than the characteristic impedance of a system that utilizes a 50 Ohm coaxial cable. Power amp  216  may therefore operate with dynamic impedance to allow for a greater range of operating impedances at plasma coil  120  and plasma torch  124 . Close coupling also avoids limited dynamic range and radiation of unwanted RF often associated with coaxial cables. 
     Impedance match  224  is fixed (without variable components) to eliminate the need for variable capacitors and servo systems, which are often slow, cumbersome, and costly. The delivery of RF power from RF amp  216  through impedance match  224  to plasma coil  120  utilizes a balanced configuration with a grounded center tap. In alternate embodiments, an unbalanced configuration may be utilized. In the  FIG. 3  embodiment, T 3   320  is implemented as an RF ferrite transformer that operates at an impedance of 5 Ohms or less, depending on RF power requirements. In an alternate embodiment, the transformer T 3  may be replaced by a center-tapped inductor L 2 . This functions to de-couple the RF component from the variable power supply. The load presented to the power amplifier at  220 ( a ) and  220 ( b ) is 5 ohms or less, depending on RF power requirements. The RF power signal from RF amp  216  at connections  220 ( a ) and  220 ( b ) is configured to provide a balanced RF power signal into impedance match  224 . Similarly, impedance match  224  is configured to drive plasma coil  120  in a balanced manner at connections  228 ( a ) and  228 ( b ). The result is an RF field that is balanced around a ground potential. Therefore, the highest voltage required in RF power source  116  is reduced by one half. 
     Variations in the operating conditions of RF amp  216  and impedance match  224  may produce unwanted resonance shifts that result in power delivery variations. To maintain stable operating conditions, impedance match  224  is held at a constant temperature using a water cooling means. In certain embodiments, impedance match  224  is therefore maintained at a constant temperature to reduce changes in component values. In addition, RF power source  116  includes a heat sensor  316  that provides temperature information to controller  252  of  FIG. 2 . In the  FIG. 3  embodiment, a variable power supply  260  ( FIG. 2 ) provides center-tapped operating power to impedance match  224  for powering final stage  328  of RF amp  216 . In the  FIG. 3  embodiment, an RF filter  332  prevents unwanted radio-frequencies from leaking into the variable power supply  260 . The utilization of RF power amp  216  and impedance match  224  is further discussed below in conjunction with  FIGS. 7-9 . 
     Referring now to  FIG. 4 , a block diagram for one embodiment of the  FIG. 2  phase-locked loop (PLL)  240  is shown, in accordance with the present invention. In the  FIG. 4  embodiment, PLL  240  includes, but is not limited to, a phase detector  416 , an integrator  428 , and a voltage-controller oscillator (VCO)  432 . In alternate embodiments, PLL  240  may include components and configurations in addition to, or instead of, certain of those components and configurations discussed in conjunction with the  FIG. 4  embodiment. 
     In order for RF power source  116  ( FIG. 1 ) to deliver full RF power to plasma torch  124  ( FIG. 1 ), the operating frequency of the RF power  220  ( FIG. 2 ) should preferably correspond with the natural peak resonance frequency of impedance match  224  ( FIG. 3 ). In the  FIG. 4  embodiment, a phase-lock control feedback loop with phase detector  416  and voltage-controlled oscillator (VCO)  432  is utilized to control the phase relationship (and hence the frequency) between reference phase signal  244 ( b ) derived from the output of RF amp  216  ( FIG. 2 ) and coil phase signal  236 ( b ) derived from the output of phase probe  232  ( FIG. 2 ). 
     To track a peak resonance condition at plasma coil  120 , phase detector  416  must produce a zero error voltage  424 . Because of certain operational characteristics of phase detector  416 , the reference phase signal  244 ( b ) and the coil phase signal  236 ( b ) must be 90 degrees out-of-phase with respect to each other in order to generate a zero error voltage  424 . If the phase relationship should differ from ninety degrees, then the error voltage  424  from phase detector  416  would be either positive or negative, depending on whether the phase difference was greater or less than 90 degrees. In the  FIG. 4  embodiment, reference phase signal  244 ( b ) is derived from the RF power signal  220  that is output from RF amp  216 , and coil phase signal  236 ( b ) is derived from the output of phase probe  232 , because there exists an inherent 90 degree phase shift relationship between reference phase signal  244 ( b ) and coil phase signal  236 ( b ) as derived from those locations. 
     In the  FIG. 4  embodiment, error voltage  424  is provided to an integrator  428  that amplifies error voltage  424  and removes any unwanted radio-frequency components in error voltage  424 . Integrator  428  then provides the integrated error voltage  424  to VCO  432  via path  436 . VCO  432  responsively generates an RF drive signal  248  that has a RF drive frequency which is determined by the amplitude and polarity of the error signal  424  received from integrator  428 . PLL  240  then provides the RF drive signal  248  to RF amp  216  ( FIG. 2 ) to adjust the operating frequency of the RF power signal  220  that is provided to impedance match  224  ( FIG. 3 ). In the  FIG. 4  embodiment, PLL  240  may be implemented to include a phase offset  436  that causes phase detector  416  to responsively adjust the frequency of RF drive signal  248  so that plasma coil  120  operates on the slope of resonance, rather than at peak resonance. For example, in certain embodiments, phase offset  436  may be implemented by altering the length of the path of reference phase  244 ( b ). One example for operating on the slope of resonance is discussed below in conjunction with  FIG. 6 . The utilization of PLL  240  is further discussed below in conjunction with  FIGS. 5-9 . 
     Referring now to  FIG. 5 , a graph illustrating a phase shift-error voltage relationship is shown, in accordance with one embodiment of the present invention. The  FIG. 5  graph is presented for purposes of illustration, and in alternate embodiments, the present invention may utilize phase shift-error voltage relationships with properties and characteristics in addition to, or instead of, certain of those properties and characteristics discussed in conjunction with the  FIG. 5  embodiment. 
     In the  FIG. 5  embodiment, phase shift values between reference phase signals  244  and coil phase values  236  ( FIG. 2 ) are shown on a horizontal axis  520 . In addition, error voltages  424  ( FIG. 4 ) from phase detector  416  of PLL  240  are shown on a vertical axis  516 . A line  524  is plotted to represent exemplary phase-shift-error voltage relationships. For purposes of illustration, the  FIG. 5  phase-shift-error voltage relationship is shown as being linear. However, in alternate embodiments, various types of non-linear relationships are equally contemplated. As discussed above in conjunction with  FIG. 4 , at a phase shift of ninety degrees, a peak resonance condition  528  is shown on the  FIG. 5  graph with an error voltage of zero volts. In the  FIG. 5  example, as the phase shift increases above ninety degrees, the error voltage increases, and as the phase shift decreases below ninety degrees, the error voltage decreases. 
     Referring now to  FIG. 6 , a graph illustrating a technique for operating on a resonance slope is shown, in accordance with one embodiment of the present invention. The  FIG. 6  graph is presented for purposes of illustration, and in alternate embodiments, the present invention may operating on a resonance slope using values and techniques in addition to, or instead of, certain of those values and techniques discussed in conjunction with the  FIG. 6  embodiment. 
     In the  FIG. 6  embodiment, operating frequency values for plasma coil  120  ( FIG. 2 ) are shown on a horizontal axis  620 . In addition, amplitudes of RF operating power at plasma coil  120  are shown on a vertical axis  616 . A bell-shaped curve is plotted to represent values from exemplary resonance conditions at plasma coil  120 . In the  FIG. 6  embodiment, a peak resonance condition  628  is shown at peak resonance frequency  624 . RF power source  116  may be operated on the slope of resonance so that the operating frequency of plasma coil  120  is selectively chosen at a location that is not directly at peak resonance  628 . In the  FIG. 6  embodiment, RF power source  116  is being operated at slope point  636  at frequency  632 . Operating RF power source  116  on the slope of resonance may be desirable under various types of analysis conditions, and may provide the ability to tailor response characteristics of RF power source  116  for improved performance in certain operating environments. 
     Referring now to  FIG. 7 , a flowchart of method steps for tracking a resonant condition during a plasma creation process is shown, in accordance with one embodiment of the present invention. The  FIG. 7  example is presented for purposes of illustration, and in alternate embodiments, the present invention may utilize steps and sequences other than certain of those steps and sequences discussed in conjunction with the  FIG. 7  embodiment. 
     In the  FIG. 7  embodiment, in step  712 , RF power source  116  ( FIG. 2 ) initiates a plasma creation process by utilizing any appropriate techniques. For example, in certain embodiments, RF power source  116  may initially provide a high-voltage ignition charge to a gas in plasma torch  124  ( FIG. 1 ). In step  714 , RF power source  116  applies an RF power signal  220  from plasma coil  120  to a test sample in a partial plasma state in plasma torch  124 . In step  716 , RF power source  116  tracks a resonant condition at plasma coil  120  through a changing plasma cascade process by adjusting the current operating frequency of the RF power signal  220  provided to plasma coil  120 . In step  718 , the test sample in plasma torch  124  achieves a full plasma state. Finally, in step  720 , RF power source  116  maintains the full plasma state achieved in foregoing step  718  to facilitate various analysis procedures for the test sample. 
     Referring now to  FIG. 8 , a flowchart of method steps for generating an error voltage  424  ( FIG. 4 ) is shown, in accordance with one embodiment of the present invention. The  FIG. 8  example is presented for purposes of illustration, and, in alternate embodiments, the present invention may utilize steps and sequences other than certain of those steps and sequences discussed in conjunction with the  FIG. 8  embodiment. 
     In the  FIG. 8  embodiment, in step  812 , RF power source  116  initially samples a reference phase signal  244  for generating an error voltage  424 . In certain embodiments, reference phase signal  244  may be derived from an RF power signal  220  from RF amp  216  ( FIG. 2 ). Then, in step  814 , RF power source  116  samples a coil phase signal  236  that is generated by a phase probe  232  adjacent to plasma coil  120  ( FIG. 2 ). In step  816 , RF power source  116  provides the reference phase signal  244  and the coil phase signal  236  to a phase detector  416  of a phase-locked loop  240  ( FIG. 2 ). Next, in step  818 , phase detector  416  compares the reference phase signal  244  and the coil phase signal  236  by utilizing any appropriate means. Finally, in step  820 , phase detector  416  generates error voltage  424  to represent the direction and the magnitude of phase shift between reference phase signal  244  and coil phase signal  236 . 
     Referring now to  FIG. 9 , a flowchart of method steps for adjusting an RF operating frequency is shown, in accordance with one embodiment of the present invention. The  FIG. 9  example is presented for purposes of illustration, and, in alternate embodiments, the present invention may utilize steps and sequences other than certain of those steps and sequences discussed in conjunction with the  FIG. 9  embodiment. 
     In the  FIG. 9  embodiment, in step  912 , integrator  428  of PLL  240  ( FIG. 4 ) integrates the error voltage  424 , generated by phase detector  416  ( FIG. 4 ) during step  820  of  FIG. 8 , to remove certain unwanted radio-frequency components. Then, in step  914 , integrator  428  provides the integrated error voltage to a voltage-controlled oscillator (VCO)  432  of PLL  240 . In step  916 , VCO  432  generates an RF drive signal that has a drive frequency which corresponds to the amplitude and polarity of error voltage  424 . In step  918 , PLL  240  provides the RF drive signal  248  to the RF power amp  216  of RF power source  116 . 
     Finally, in step  920 , in response to the drive frequency of RF drive signal  248 , RF power amp  216  generates RF power signal  220  with a frequency that tracks the current peak resonant frequency of plasma coil  120 . The foregoing  FIG. 8  error voltage generation procedure and  FIG. 9  RF operating-frequency adjustment procedure are typically repeated on an ongoing basis to allow RF power source  116  to track and maintain operating parameters at current resonant conditions. For at least the foregoing reasons, the present invention provides an improved system and method for implementing an inductively-coupled RF power source. 
     The invention has been explained above with reference to certain embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. For example, the present invention may be implemented using configurations and techniques other than certain of those configurations and techniques described in the embodiments above. Additionally, the present invention may effectively be used in conjunction with systems other than those described above. Therefore, these and other variations upon the discussed embodiments are intended to be covered by the present invention, which is limited only by the appended claims.