Field of the Invention The present invention relates generally to electrical systems that use transformers to supply power to a load circuit. More particularly, the invention relates to a self tuning electrical system that automatically provides compensation for variations in a transformer's inductance that occur during operation of the system.

Background of the Invention The voltage gain characteristic of transformers is well known, and many electrical systems exploit this characteristic and use transformers to generate relatively high voltage signals from relatively low voltage signals. For example, many electrical systems receive power from a relatively low voltage power supply and use transformers to internally generate the relatively high voltage signals required for operation of various electrical components. In such a system, a low voltage drive circuit may apply a low voltage periodic drive signal to the primary of each transformer, and the secondary of each transformer may be coupled to a particular load circuit for providing the high voltage periodic signals required for operation of the load circuit. In addition to voltage gain, transformers are characterized by several other parameters including physical size, inductance, hysteresis, frequency range (i.e., the range of frequencies over which a transformer may be operated), and power range (i.e. , the range of power levels over which a transformer may be operated). In general, these parameters may not be selected independently, and therefore in any given application the choice of a particular transformer often represents a compromise between competing design considerations. The inductance of a transformer is often selected according to a function of the impedance provided by the transformer's load circuit so that the transformer may operate at "maximum power transfer efficiency", or more simply, at "maximum efficiency" . As is well known, when a drive circuit applies a drive signal to the

primary of a transformer, and the secondary of the transformer is electrically connected to a capacitive load circuit, the transformer will operate at maximum efficiency for a signal at a particular optimum frequency (called the "resonant" frequency) when the transformer's inductance is selected to match the load circuit's capacitance so that the transformer secondary and the load circuit form a resonant circuit at that frequency. When this maximum efficiency condition is achieved, the transformer primary presents a purely resistive (i.e., neither capacitive nor inductive) input impedance to the drive circuit, or drive signal at the resonant frequency. Equation (1) shows the well known relationship between the inductance -γ of a transformer, the capacitance C _L of a load circuit, and the natural resonance frequency f _R of the circuit formed by electrically connecting the secondary of the transformer in parallel with the load circuit.

1

Λ (i)

From Equation (1) it can be seen that the resonant frequency is a function of both the transformer's inductance and the capacitance of the load. When the circuit is operated at a constant frequency of f _R , and when the capacitance of the load circuit is known, Equation (2), which is generated by rearranging Equation (1) to solve for Lj, provides a formula for calculating the inductance L,- of the transformer that will operate at maximum efficiency for a given resonant frequency and capacitive load.

L _τ = - (2)

Ferrite core transformers are advantageous because they provide a relatively large inductance for a given physical size, they are operative at high frequencies and high power levels, and they produce relatively small amounts of radiated fields. However, one disadvantage of ferrite core transformers is that their inductance varies according a function of temperature as well as the strength of the magnetic field surrounding the transformer (or the electrical current flowing through the coils of the transformer). So while the inductance of a ferrite core transformer may be selected according to Equation (2) so that the transformer operates at maximum efficiency for

a particular operating temperature and power level delivered at the determined resonant frequency, when the temperature or power level deviate from these particular values the inductance also deviates and the transformer then functions less efficiently since the value of the resonant frequency f _R in Equation (1) changes, but the frequency of the drive signal does not. When the inductance of a transformer deviates from the value selected for maximum efficiency, the transformer may be said to "de-tune" since as will be evident from Equation (1) the new resonant frequency as a function of the new inductance value has shifted from the resonant frequency as a function of the old inductance value. The tendency for ferrite core transformers to de-tune prevents ferrite core transformers from being used in many applications. Since the inductance of air core transformers is relatively independent of temperature and power level, air core transformers are often preferred over ferrite core transformers for many applications. However, despite the stability of their inductance, air core transformers suffer from several disadvantages. For example, to provide a given inductance, an air core transformer must have a relatively large physical size as compared with a corresponding ferrite core transformer. Further, air core transformers are operative only at relatively low power levels, are characterized by relatively large leakages, and generate relatively strong radiated RF interference fields. Systems for automatically preventing a transformer from de-tuning, or "self tuning" systems, would therefore advantageously facilitate the use of ferrite core transformers and permit the substitution of ferrite core transformers for air core transformers. Figure 1 illustrates a prior art system that, as will be discussed in greater detail below, includes a ferrite core transformer and suffers from de-tuning. Specifically, Figure 1 shows a block diagram of a prior art partial pressure transducer 1 of the type that is currently manufactured by the assignee of the present invention, and sold under the trademark PPT. Transducer 1 includes a residual gas analyzer, in the form of a quadrupole mass spectrometer 2, a quadrupole control system 3, and a microprocessor 4. In operation, transducer 1 measures the partial pressures of each of the constituent gasses in a partial vacuum, which in turn is a function of the mass of each constituent gas.

Quadrupole mass spectrometer 2 is a well known device and is often implemented using a spectrometer of the type described in U.S. Patent No. 5,302,827, which is assigned to the assignee of the present invention and which is hereby incorporated by reference. Briefly, spectrometer 2 includes an ion source 10, a focus plate 12, and a quadrupole mass filter 14 that includes a set of rods 16. One of the rods 16 is electrically connected to a terminal A and the opposite rod is electrically connected to a terminal B. In operation of spectrometer 2, the constituent gasses are enclosed within ion source 10 at a partial vacuum. The ion source generates an ion current 18 directed so that the ions pass through an aperture 20 in focus plate 12 and through filter 14 towards a partial pressure detector 22, which may be implemented for example using a Faraday cup. A current measurement device 24 measures the ion current received by detector 22 and provides an indication thereof. The ion current 18 that enters the quadrupole filter 14 includes ions from each of the constituent gasses present within ion source 10. However, quadrupole filter 14 generates time and amplitude varying electrical fields in response to electrical signals received via terminals A and B so as to insure that at any given time only ions having a particular charge-to-mass ratio arrive at partial pressure detector 22. Since ions from each constituent gas are typically characterized by a corresponding unique charge-to-mass ratio, spectrometer 2 generates measurements of the partial pressures of each of the constituent gasses, and thus the mass of each constituent gas, one constituent at a time. Microprocessor 4 and control system 3 control the operation of spectrometer 2 by generating and applying an electrical signal to terminals A and B that includes a constant frequency, sinusoidally modulated, voltage component (hereinafter, the "RF component") superimposed on a constant voltage component (hereinafter, the "DC component"). Microprocessor 4, in combination with a digital-to-analog converter (which for convenience of illustration is not shown in Figure 1) preferably generates a + DC signal, a -DC signal, and a Desired RF Amplitude signal, and applies these three analog DC signals to control system 3. Microprocessor 4 preferably generates these signals so that the +DC and the -DC signals are of equal magnitude and of opposite polarity. In response to these three analog signals, control system 3

generates and applies an electrical signal to terminals A and B including the constant frequency RF component, the amplitude of the latter being equal to the amplitude of the Desired RF Amplitude signal, superimposed on the DC component, the amplitude of which is equal to the amplitude of the +DC signal. In operation, when the amplitudes of the RF and DC components are set to particular values, the mass spectrometer 2 scans for ions having a corresponding charge-to-mass ratio. Microprocessor 4 varies the amplitudes of the Desired RF Amplitude, +DC, and -DC signals over time so that mass spectrometer 2 scans for ions having a selected range of charge-to-mass ratios and thereby generates a spectrogram representative of the partial pressures of the constituent gasses embraced within ion source 10. Microprocessor 4 typically varies these amplitudes quickly and continuously so that spectrometer 2 sweeps the selected range of charge-to-mass ratios (i.e. , obtains a partial pressure measurement for each charge-to-mass ratio in the selected range) in a range from every 10 milliseconds to 10 seconds, and so that spectrometer 2 continuously updates the spectrogram. Figure 2 shows the quadrupole control system 3 in greater detail. System 3 includes a subtractor 40, which receives the analog Desired RF Amplitude signal at its positive input terminal, and which receives a feedback signal (the generation of which will be discussed below) at its negative input terminal. Subtractor 40 generates therefrom an error signal as a function of the difference between the signal applied to the positive input terminal and the feedback signal applied to its negative input terminal. The error signal is applied through a resistor 42 to an input terminal of an integrating amplifier 44. An integrating capacitor 46 is electrically coupled between the input and output terminals of amplifier 44, and the latter generates an output signal that is applied to an amplifier 48. Amplifier 48 generates a periodic output signal that is applied to a primary of a transformer 50. A secondary of transformer 50 generates two periodic output signals that are applied to the input terminals of a class AB amplifier circuit 52. Amplifier circuit 52 includes two transistors 54, 56, each having a gate, a drain, and a source. The two periodic output signals generated by the secondary of transformer 50 are applied to respective ones of the gates of transistors 54 and 56, and the sources of transistors 54, 56 are electrically connected to ground.

The drains of transistors 54 and 56 generate RF drive signals that are applied to respective ones of two input terminals of a primary of a ferrite core transformer 58. The +DC signal generated by microprocessor 3 is applied to one input terminal of the secondary of transformer 58 and the -DC signal is applied to another input terminal of the secondary of transformer 58. The secondary of transformer 58 generates two output signals at levels which are applied to respective ones of terminals A and B. A mechanically actuated variable capacitor 60 is electrically connected between terminals A and B. A capacitor 62 is electrically connected between terminal A and an internal node C. A Schottky diode 64 is electrically connected between ground and node C, and diode 64 has its cathode and anode oriented so as to permit current to flow into node C. Another Schottky diode 66 is electrically connected between node C and an input terminal of an operational amplifier 68, and diode 66 has its cathode and anode oriented so as to permit current to flow from node C towards amplifier 68. A capacitor 70 is electrically connected between the input terminal of amplifier 68 and ground, and a resistor 72 is electrically connected between the input and output terminals of amplifier 68. The output terminal of amplifier 68 is electrically connected to the negative input terminal of subtractor 40. The capacitor 62, diode 66, and operational amplifier 68 form a feedback loop from the secondary output provided across the capacitor 60 between terminals A and B, as will be evident hereinafter. A twenty-four volt power supply (not shown) is used to supply power to control system 3, and system 3 uses ferrite core transformer 58 to generate the high voltage electrical signal that is applied to mass spectrometer 2 via terminals A and B. The primary of ferrite core transformer 58 includes two coils each having a single turn and the secondary of ferrite core transformer 58 includes two coils each having between eighteen and twenty-two turns. The voltage of the electrical signal generated by ferrite core transformer 58 and applied to terminals A and B ranges between 0.5 and 500 volts, and transformer 58 generates this signal from the relatively low voltage (i.e. , less than twenty-four volts) periodic drive signals generated by class AB amplifier circuit 52. Capacitor 62 is a precision measuring capacitor having a capacitance of 5 pF (Pico-Farads), and capacitor 70 has a capacitance of 4,700 pF. Since the grounded

terminal of capacitor 70 forms a RF ground, and since Schottky diodes 64, 66 rectify the current i ₆₂ flowing through capacitor 62, the current i ₆₂ is representative of the RF component of the electrical signal applied to terminals A and B. Amplifier 68 and resistor 72 filter the current i ₆₂ so that the voltage V _6g of the signal generated at the output terminal of amplifier 68 and applied to the negative input terminal of subtractor 40 is described by the formula shown in Equation (3) V _a = T; ₂ R ₇₂ (3) wherein i ₆₂ equals the average value of the rectified current flowing through capacitor 62, and R ₇₂ equals the electrical resistance provided by resistor 72. So, the voltage Vgg of the signal generated at the output terminal of amplifier 68 and applied to the negative input terminal of subtractor 40 is representative of the average amplitude of the RF component of the electrical signal applied to terminals A and B. Subtractor 40 subtracts the voltage V ₆₈ (which is representative of the actual amplitude of the RF component) from the analog Desired RF Amplitude signal (which is representative of the desired amplitude of the RF component) and thereby generates an error signal that is representative of the difference between the desired and actual amplitudes of the RF component of the electrical signal applied to terminals A and B, i.e., in the case the voltage applied across the terminals. The error signal is applied through resistor 42 to integrating amplifier 44 which generates an output signal representative of the time integral of the error signal. This output signal is applied to amplifier 48 which generates a sinusoidally oscillating output signal, the amplitude of which is controlled by the integrated error signal generated by amplifier 44. The sinusoidally oscillating output signal generated by amplifier 48 is applied to the primary of transformer 50 the secondary of which generates two sinusoidally oscillating signals that are applied to the gates of transistors 54, 56 of the amplifier circuit 52. In response to these signals, class AB amplifier circuit 52 generates two sinusoidally oscillating drive signals (or RF drive signals) which are applied to the primary of ferrite core transformer 58, and the secondary of the latter generates the electrical signal levels that are applied to spectrometer 2 via terminals A and B.

In operation, if the actual amplitude of the RF component drops below the desired amplitude of the RF component, the error signal becomes positive and thereby increases the output signal generated by integrating amplifier 44 which in turn increases the amplitude of the RF drive signals generated by class AB amplifier circuit 52 and increases the amplitude of the RF component generated by the secondary of ferrite core transformer 58. Similarly, if the actual amplitude of the RF component grows larger than the desired amplitude of the RF component, the error signal becomes negative and thereby decreases the output signal generated by integrating amplifier 44 which in turn decreases the amplitude of the RF drive signals generated by class AB circuit 52 and decreases the RF component generated by the secondary of ferrite core transformer 58. So, control system 3 essentially provides a feed back loop that stabilizes the amplitude of the RF component, and as stated above, control system 3 generates an electrical signal at terminals A and B having an RF component, the amplitude of which is equal to the amplitude of the Desired RF Amplitude signal, and a DC component, the amplitude of which is equal to the amplitude of the +DC signal. The inductance of ferrite core transformer 58 is selected according to Equation (2), where C is equal to the capacitance of spectrometer 2 an ^f is equal to the frequency of the RF component, so that the secondary of transformer 58 and spectrometer 2 form a resonant circuit and transformer 58 thereby operates at maximum efficiency at the frequency f _R . When this condition is achieved, the input impedance of the primary of transformer 58 (as seen by class AB amplifier circuit 52) is purely resistive. Variable capacitor 60, the capacitance of which may be manually varied by using a screwdriver to turn a screw (not shown) mounted in capacitor 60, provides some limited ability to manually tune system 3 and thereby compensate for any slight mismatches between the inductance of ferrite core transformer 58 and the capacitance of spectrometer 2. In control system 3, ferrite core transformer 58 is selected so that it provides an inductance of p for a particular operating temperature and power level. However, as the operating temperature and power level deviate from the particular values, transformer 58 de-tunes and the inductance provided by transformer 58 deviates from

the desired value of L _f . This causes the primary of transformer 58 to present an impedance that appears either inductive or capacitive to class AB amplifier circuit 52 and thereby decreases the efficiency of transformer 58. Figure 3 shows a graph illustrating the power applied to ferrite core transformer 58 as a function of the mass of the ions being scanned for by spectrometer 2. In Figure 3 the vertical axis represents the power applied to transformer 58 measured in Watts, and this power may be calculated by computing the product of the voltage at, and the current flowing in, the drain of transistor 56. In Figure 3 the horizontal axis represents the mass of the ions being scanned for by spectrometer 2 (assuming that each of the ions has an identical charge) measured in terms of the atomic number of the ions. As is well known, the mass of the ions scanned for by spectrometer 2 is an increasing function of the amplitude of the RF component (i.e., as the amplitude of the RF component increases, the mass of the ions detected by spectrometer 2 increases). If the inductance provided by the ferrite core transformer 58 remained constant at the desired value of Lp, the curve illustrated in Figure 3 would be parabolic, i.e. , the power applied to transformer 58 would monotonically increase as the mass of the ions increased from unity (i.e. , for atomic hydrogen) to a midpoint at approximately atomic number equal to 150, and the power would monotonically decrease as the mass of the ions increased beyond the midpoint. However, the curve illustrated in Figure 3 is not parabolic and this illustrates the inefficiency, or the de-tuning, of ferrite core transformer 58. At higher masses, e.g. , greater than an atomic number of 150, the inductance provided by ferrite core transformer 58 deviates from the desired value of Lj and the power transfer efficiency of transformer 58 decreases. Control system 3 insures that the amplitude of the RF component provided by transformer 58 to spectrometer 2 remains at the desired level. However, to provide this component, control system 3 applies drive signals to the primary of transformer 58 that are higher in amplitude than they would otherwise have to be if the inductance of transformer 58 remained at the desired value of r. In operation, transformer 58 is selected and variable capacitor 60 is statically tuned so that the transformer 58 operates at maximum efficiency for a mass of atomic

number equal to 25 to 150 (i.e., the point at which the power used to drive transformer 58 is maximum). In other words, transformer 58 is statically tuned for operation at the maximum power level. However, when the amplitude of the RF component is set so that spectrometer 2 scans for ions having other masses, the inductance provided by transformer 58 deviates from the desired level, i.e., transformer 58 de-tunes, and transformer 58 consumes additional power. When transformer 58 operates at less than maximum efficiency, the additional power applied to transformer 58 is converted to heat. As a result of the de-mning of transformer 58, control system 3 consumes excess power and transformer 58 tends to operate at an undesirably high temperature. The high temperature of transformer 58 shortens the life span of transformer 58 and also has the undesired effect of heating other temperature sensitive components in transducer 1 and thereby adversely affects the accuracy of transducer 1.

Objects of the Invention It is an object of the present invention to substantially reduce or overcome the above-identified problems of the prior art. Another object of the present invention is to provide an improved self tuning electrical system. Yet another object of the present invention is to provide an improved electrical system including a transformer having a primary coupled to receive a drive signal and a secondary electrically connected to a load circuit, and including components for tuning the system as a function of the input impedance of the primary . Still another object of the present invention is to provide an improved electrical system including a transformer having a primary coupled to receive a drive signal and a secondary electrically connected to a load circuit, and including components for adjusting the inductance provided by the transformer as a function of the input impedance of the primary. And yet another object of the present invention is to provide an improved electrical system including a transformer having a primary coupled to receive a drive signal and a secondary electrically connected to a load circuit, and including

components for adjusting the capacitance provided by the load circuit as a function of the input impedance of the primary. And still another object of the present invention is to provide an improved electrical system including a ferrite core transformer and components for automatically tuning the transformer during operation. And yet another object of the present invention is to provide an improved electrical system including a transformer that defines an aperture and devices for selectively moving a metallic slug into and out the aperture to control the inductance provided by the transformer. And still another object of the present invention is to provide an improved self tuning electrical system for controlling a quadrupole mass spectrometer.

Summary of the Invention These and other objects are provided by an improved self tuning electrical system that includes a transformer having a primary and a secondary. The transformer primary is coupled to receive an electrical drive signal at a predetermined frequency and the transformer primary is characterized by an input impedance that presents an electrical resistance to the drive signal. The transformer secondary is electrically connected to a load circuit that is characterized by an impedance. The system includes tuning components for adjusting the relationship between the transformer inductance and the load circuit impedance according to a function of the input impedance. In one aspect the tuning components may tune the system at the desired predetermined frequency by adjusting the inductance provided by the transformer, and according to another aspect the tuning components may tune the system at the desired predetermined frequency by adjusting the capacitance provided by the load circuit. In one preferred form, the tuning components tune the system so that the transformer primary input impedance is purely resistive (i.e. , neither capacitive nor inductive) at the desired frequency. According to yet another aspect, the invention provides a self tuning electrical system useful for controlling a quadrupole mass spectrometer.

Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.

Brief Description of the Drawings For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which the same reference numerals are used to indicate the same or similar parts wherein: Figure 1 shows a block diagram of a prior art partial pressure transducer which includes a ferrite core transformer and suffers from de-tuning; Figure 2 shows a schematic of the quadrupole control system shown in Figure 1 ; Figure 3 shows a graph of power versus mass illustrating the power consumption of the ferrite core transformer in the transducer shown in Figure 1 ; Figure 4 shows a block diagram of a self tuning partial pressure transducer constructed according to the invention; Figure 5 shows a partial schematic, and partial block diagram of a preferred embodiment of the quadrupole control system of the transducer shown in Figure 4; Figure 6 shows a partial schematic and partial block diagram of another embodiment of the quadrupole control system of the transducer shown in Figure 4; Figure 7 shows a partial schematic and partial block diagram of a self tuning system constructed according to the invention; and Figure 8 shows a block diagram of another self tuning system constructed according to the invention.

Detailed Description of the Drawings Figure 4 shows a block diagram of a preferred partial pressure transducer 200 constructed according to the invention. Transducer 200 is similar to prior art transducer 1 (shown in Figure 1), however, rather than control system 3, transducer 200 is constructed using an improved quadrupole control system 203. As will be discussed in greater detail below, control system 203 includes a ferrite core transformer, and in addition to generating the electrical signals applied via terminals A and B to spectrometer 2, control system 203 automatically and dynamically varies the relationship between the inductance provided by the transformer and the impedance of spectrometer 2 to maintain transducer 200 tuned to the desired operating frequency for all operating conditions. Figure 5 shows a partial schematic, partial block diagram of a preferred embodiment of improved quadrupole control system 203 constructed according to the invention. Control system 203 is similar to prior art control system 3 (shown in Figure 2), however, system 203 includes a ferrite core transformer 258, a phase shift detector 280, a resistor 282, an integrating amplifier 284, an integrating capacitor 286, a motor 288, and a ferrite slug 290. The gate of transistor 56 is electrically connected to one input terminal of phase shift detector 280, and the drain of transistor 56 is electrically connected to another input terminal of phase shift detector 280. The latter generates an output signal that is applied through resistor 282 to an input terminal of integrating amplifier 284. Capacitor 286 is electrically connected between the input and output terminals of amplifier 284, and the latter generates an output signal that is applied to motor 288. Motor 288 mechanically controls the position of slug 290 via rod 289, or some other positioning device. The ferrite core of transformer 258 defines a gap 292, and ferrite slug 290 is disposed so that motor 288 may translatably move slug 290 into and out of gap 292 in a direction indicated by arrow 294. System 203 is shown as including variable capacitor 60, however, those skilled in the art will appreciate that capacitor 60 may be eliminated from system 203. As those skilled in the art will appreciate, the gap 292 prevents core saturation within transformer 258, and the position of ferrite slug 290 relative to gap 292 affects the inductance provided by transformer 258. The inductance of transformer 258

increases as ferrite slug 290 is translated towards the center of gap 292. In operation, motor 288 positions slug 290 under the control of phase shift detector 280 so as to dynamically maintain the inductance provided by transformer 258 equal, or nearly equal, to the desired value of L _f , even under changing operating conditions. The phase difference between the sinusoidally oscillating signals present at the gate and drain of transistor 56 is indicative of whether transformer 258 is optimally tuned at the frequency of the input drive signal, the Desired RF Amplitude signal. When the input impedance of the primary of transformer 258 is purely resistive (i.e. , when the input impedance of transformer 258 as seen by class AB amplifier circuit 52 is purely resistive at the frequency of the input drive signal) transformer 258 is optimally tuned and the sinusoidal signal applied to the gate of transistor 56 is exactly 180° out of phase with respect to the sinusoidal signal generated at the drain of transistor 56 (e.g., as the amplitude of the signal applied to the gate of transistor 56 increases, the amplitude of the signal generated at the drain of transistor 56 decreases). When the inductance of transformer 258 deviates from the desired value of L _τ , the input impedance of the primary of transformer 258 becomes at least partially inductive or capacitive, and the phase difference between these signals deviates from the desired value of 180°. Control system 203 uses this phase difference to adjust the inductance provided by transformer 258 and thereby maintains the transformer 258 tuned to the frequency of the drive signal. Phase shift detector 280 measures the phase difference between the signals present at the gate and drain of transistor 56 and generates a phase error signal representative thereof. The phase error signal is applied via resistor 282 to integrating amplifier 284 which generates an output signal representative of the time integral of the phase error signal. This output signal is then applied to motor 288 which controls the inductance provided by transformer 258 by adjusting the position of ferrite slug 290 so as to maintain a 180° phase difference, or a phase difference that is as close as possible to 180°, between the signals present at the gate and drain of transistor 56. So phase shift detector 280, resistor 282, amplifier 284, capacitor 286, motor 288, and slug 290 essentially form a control loop 300 for dynamically controlling the inductance of transformer 258 so that the transducer remains tuned to the frequency

of the drive signal. Control loop 300 maintains the transducer tuned to the frequency of the drive signal by maintaining the input inductance provided by transformer 258 substantially constant with changes in environmental conditions such as a change in temperature and/or magnetic fields . In one preferred embodiment, phase shift detector 280 may be implemented using commercially available integrated circuits such as the CD4046BM/CD4046BC sold by National Semiconductor. As with prior art transducer 1 , in improved transducer 200 microprocessor 4 preferably varies the amplitudes of the Desired RF Amplitude, the +DC, and the -DC signals quickly and continuously so that spectrometer 2 sweeps a selected range of charge to mass ratios (i.e., obtains a partial pressure measurement for each charge to mass ratio in the selected range) every 10 milliseconds to 10 seconds, and so that spectrometer 2 continuously updates the spectrogram. Control loop 300 preferably responds on a time scale that is relatively fast compared to the rate at which microprocessor 4 varies the amplitudes of the Desired RF Amplitude, the +DC, and the -DC signals so that control loop 300 maintains the transformer 258 at an inductance necessary to maintain the transducer tuned at the desired frequency of the drive signal, regardless of the signal values provided by microprocessor 4. Figure 6 shows a partial schematic, partial block diagram of another embodiment of an improved quadrupole control system 203' constructed according to the invention that may be used in transducer 200. Whereas the system shown in Figure 5 maintains the circuit tuned to a predetermined resonant frequency by adjusting the inductance provided by the transformer 258 so as to maintain the inductance substantially constant, in system 203' the inductance of the transformer 258 is allowed to vary, and thus de-tune; however, compensation for any such de-tuning is provided by adjusting the capacitance of the load coupled to the transformer so as to maintain the system tuned to the predetermined frequency of the drive signal. System 203' is similar to system 203 (shown in Figure 5), however, system 203' does not include ferrite slug 290 and also does not include variable capacitor 60. Instead, system 203' includes a variable capacitor 298 electrically connected between terminals A and B, and motor 288 uses rod 289 (or some other positioning device) to selectively displace one of the plates of capacitor 298 in a direction indicated by arrow 296.

Motor 288 therefore varies the distance between the plates of capacitor 298, and thereby controls the capacitance provided by capacitor 298 so that the product of L - and C of Equation (1) remains constant, which in turn maintains the circuit in tune at the frequency f _R the frequency of the drive signal. Since capacitor 298 is electrically connected in parallel with spectrometer 2, adjusting the capacitance provided by capacitor 298 essentially adjusts the capacitance provided by the load coupled to transformer 258. Motor 288 varies the capacitance provided by capacitor 298 according to a function of the error signal generated by phase shift detector 280 so as to compensate for changes in the output impedance, i.e., the inductance of the transformer. Prior art control system 3 (shown in Figure 2) includes variable capacitor 60, which is used to statically tune system 3 based upon an initial value of L so as to statically tune the system to a predetermined value of f _R . Since the capacitance provided by capacitor 60 is adjusted by using a screwdriver to turn a screw in capacitor 60, it would have been impractical to use capacitor 60 to dynamically maintain system 3 in tune at the desired frequency. Rather, capacitor 60 is only suited for statically tuning system 3 for a particular set of operating conditions. In contrast to the prior art, systems 203 and 203' (shown in Figures 5 and 6, respectively) automatically and dynamically maintain the system tuned at the frequency of the drive signal for maximum efficiency for any operating condition. Thus far the invention has been discussed in connection with a self tuning system useful for controlling a quadrupole spectrometer. However, those skilled in the art will appreciate that the principles of the invention may be applied to many other types of systems. For example, Figure 7 shows a partial block, partial schematic diagram of a preferred embodiment of a self tuning system 400 constructed according to the invention, which as those skilled in the art will appreciate, is a generalized version of system 203 shown in Figure 5. System 400 includes a ferrite core transformer 410 having a ferrite core 412, and a primary coil 414 and a secondary coil 416 are wound around respective portions of core 412. System 400 further includes a drive circuit 415 electrically connected to the transformer primary 414, and a load circuit 417 electrically connected to the transformer secondary 416.

A sensing circuit 418 is coupled to the transformer primary 414 and to a motor 420. The transformer core 412 defines an aperture 422 and system 400 further includes a metallic (e.g., ferrite) slug 424 disposed proximal to aperture 422. Motor 420 is mechanically connected to slug 424 via a rod, or other positioning device 426, and motor 420 selectively adjusts the position of slug 424 with respect to apermre 422 under the control of sensing circuit 418. As with transformer 258 (shown in Figure 5), the inductance provided by transformer 410 varies according to a function of the position of slug 424 with respect to aperture 422, and the inductance generally increases as the slug is moved towards the center of aperture 422. So, motor 420 selectively adjusts the inductance of transformer 410 under the control of sensing circuit 418 so that the inductance of the transformer can be carefully controlled. In operation, drive circuit 415 applies a periodic drive signal at the desired resonant frequency and characterized by a first amplitude to the primary 414, and in response to this drive signal the secondary 416 generates a periodic signal at the resonant frequency and characterized by a second amplitude that is applied to load circuit 417. The voltage gain characteristic of transformer 410 is selected to provide a desired relationship between the first and second amplitudes and transformer 410 may amplify or attenuate the drive signal applied to primary 414 so that the second amplitude may be greater than, less than, or equal to the first amplitude. Sensing circuit 418 senses the input impedance provided by the primary 414 of transformer 410 and controls the inductance of transformer 410 (by adjusting the position of slug 424 via motor 420) so as to control the product of L - and C _L of Equation (1). Sensing circuit 418 may be implemented using a phase shift detector as in the case of system 203 (shown in Figure 5), or by using any other device for sensing the input impedance of primary 414. Transformer 410 is preferably constructed so that it provides an inductance of Lp, as described by Equation (2), for a particular set of operating conditions, where C _L is the capacitance provided by load circuit 417 and; f is the frequency of the periodic signal applied by drive circuit 415 to primary 414. When the inductance of transformer 410 deviates from the desired value of Lj- as a result of changing operating conditions (i.e. , when transformer 410 de-tunes), sensing circuit 418

automatically compensates for this de-tuning and tunes the transformer by adjusting the position of slug 426 so as to return the inductance of transformer 410 to the desired value of Lq-, or at least to adjust the inductance of transformer 410 so that it is as close as possible to the desired value of Lq-. Sensing circuit 418 maintains the system 400' in tune (and thereby maximizes the efficiency of transformer 410) by adjusting the transformer's inductance so that the input impedance provided by the transformer primary 414 is substantially purely resistive, or at least so that any inductive or capacitive components of the transformer primary's input impedance are reduced or minimized. Figure 8 shows a partial schematic partial block diagram of another embodiment of self tuning system 400' constructed according to the invention. This embodiment is similar to the embodiment illustrated by Figure 7, however, in this embodiment motor 420 adjusts the capacitance of a variable capacitor 428 rather than adjusting the inductance provided by transformer 410. Those skilled in the art will appreciate that system 400' may be tuned by adjusting either the inductance provided by transformer 410, the capacitance provided by the load circuit, or both. Motor 420 may adjust the capacitance provided by capacitor 428 by, for example, selectively varying the distance between the conductive plates of the capacitor. Capacitor 428 is electrically connected in parallel with load circuit 417, and may be regarded as part of the load circuit. Motor 420 therefore selectively adjusts the capacitance of the load circuit by adjusting the capacitance provided by capacitor 428. Transformer 410 is preferably constructed so that it provides an inductance of Lq-, as described by Equation (2), for a particular set of operating conditions, where C _L is the capacitance provided by load circuit 417 plus the nominal value of the capacitance provided by capacitor 428, and f _R is the frequency of the periodic signal applied by drive circuit 415 to primary 414. When the inductance of transformer 410 deviates from the desired value of Lq- as a result of changing operating conditions (i.e., when transformer 410 de-tunes), sensing circuit 418 automatically compensates for this de-tuning and tunes the system 400' by adjusting the capacitance provided by the load circuit so that the product Lq- and C _L remains constant. Sensing circuit 418 maintains the tuning of system 400' (and thereby maximizes the efficiency of

transformer 410) by adjusting the capacitance C _L so that the input impedance provided by the transformer primary 414 is and remains substantially purely resistive at the predetermined frequency of the drive signal, or at least so that any inductive or capacitive components of the transformer primary's input impedance are reduced or minimized. The invention has been discussed in connection with ferrite core transformers, however, those skilled in the art will appreciate that the techniques provided by the invention and discussed herein may also be used to maintain the tuning of other types of transformers as well. Further, Figures 5 and 7 show a motor positioning a ferrite slug in an air gap defined by a transformer, however, those skilled in the art will appreciate that there are many methods of affecting the inductance provided by a transformer rather than by moving metallic slugs in the proximity of the transformer, and all of these methods are embraced by the invention. Similarly, whereas Figures 6 and 8 show a motor adjusting the capacitance provided by a capacitor by selectively varying the distance between the plates of the capacitor and thereby tuning the system, the invention embraces all methods of mning a system by automatically controlling the capacitance provided by a capacitor coupled to the transformer. Further, whereas the invention has been discussed in terms of preventing de- tuning in constant frequency systems, those skilled in the art will appreciate that the invention embraces self-tuning variable frequency systems as well. Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not a limiting sense.