Abstract:
Driving a quadrapole mass spectrometer includes obtaining an air core transformer with a primary and a secondary, matching the secondary to the mass spectrometer, and driving the primary based on first and second voltage levels. Driving of the primary is via an isolating stage that minimizes low level drive signal coupling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of U.S. application Ser. No. 09/813,654, filed Mar. 20, 2001 (allowed), which is a divisional of U.S. application Ser. No. 09/392,351, filed Sep. 8, 1999 (now U.S. Pat. No. 6,205,043), which claims the benefit of U.S. Provisional Application No. 60/099,630, filed on Sep. 8, 1998. 
     
    
     STATEMENT AS FEDERALLY SPONSORED RESEARCH  
       [0002] The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-515 (35 U.S.C. 202) in which the Contractor has elected to retain title. 
     
    
     
       BACKGROUND  
         [0003]    Certain applications, including a quadrapole mass spectrometer, can require a specialized power supply.  
           [0004]    A power supply for this purpose has specialized requirements. It should be a high frequency power supply that has a variable peak RF amplitude, but is frequency and voltage stable once set. It should also be fully floating. These power supplies should also be capable of driving a primarily capacitive load.  
           [0005]    If the device will be operating unattended or in space, the power supply should also be lightweight and efficient.  
         SUMMARY  
         [0006]    The present disclosure teaches a stable, high amplitude, high frequency radio frequency and direct current power supply system. According to one aspect, the system uses a clocked operation to turn on power from a power supply.  
           [0007]    A high dynamic range power supply is described that has an oscillator assembly operating from a first power supply and produce first and second out-of-phase, gradually increasing, signals, first and second transistors, coupled to receive said first and second signals respectively, and turned on by the signals to produce an oscillating output. The first transistor produces a first part of the oscillating output and the second transistor produces a second part of the oscillating output. A feedback loop has a detector sensing a level of the oscillating output and producing a signal indicative thereof. A second element compares that signal to a reference and produces an error output indicative of the difference, said error output causing a change in said first and second drive signals. The first transistor is referenced to a second power supply, having a different level than the first power supply. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows a schematic of the system. 
     
    
     DETAILED DESCRIPTION  
       [0009]    The system is shown in detail in FIG. 1. A clock  102  produces a basic high frequency output  104 , here shown as a 20 megahertz clock. It should be understood that any other frequency could be used. A flip flop amplifier  106  divides the oscillating output  104  into two, out-of-phase 10 megahertz signals  108  and  110 . The in-phase 10 megahertz signal  108  is taken as a baseline (zero) phase shift, while the out-of-phase 10 MHZ signal  110  is shifted by 180 degrees relative to signal  108 .  
         [0010]    The output signals  108  and  110  are provided into two analogous, but out-of-phase circuits.  
         [0011]    The integrator/summing amplifier  120  is shown as an operational amplifier with a capacitor C 1  and resistor R 4  in its feedback loop. This effectively changes the square wave output  108  into a gradually increasing signal such as a sawtooth shape having a similar frequency to the driving signal. The sawtooth frequency is applied to the gate of MOSFET  130 , and periodically turns on the MOSFET  130 . When MOSFET  130  is turned on, it drives current from the power supply  140  to one end of the primary  152  of an air core transformer  150 . The return for the power supply  140  is coupled to the center tap  154  of the air core transformer  150 . Use of an air core transformer can reduce the weight of the system.  
         [0012]    MOSFET  130  begins conducting when the sawtooth level reaches the threshold voltage (Vth) of the MOSFET  130 . As the level of the sawtooth increases at the gate of MOSFET  130 , the conduction angle increases. As MOSFET  130  turns on more completely, it conducts more current. The phase-shifted signal  110  is analogously coupled through an amplifier  122  to an analogous MOSFET  132 . The two circuits operate similarly, but 180 degrees out-of-phase. When MOSFET  130  is in its active phase, MOSFET  132  is off. Conversely, when MOSFET  132  is in its active phase, the MOSFET  130  is off. In this way, the primary  152  of transformer  150  is being alternatively pushed and pulled from opposite directions by two out-of-phase 10 MHZ signals. The output is therefore proportional to the amount of pushing and pulling that occurs.  
         [0013]    The secondary  154  of transformer  150  is connected to a load  156  which can be a quadrapole mass spectrometer for example. If a quadrapole mass spectrometer is used, then the inductance of the air core transformer  150  can be adjusted to resonate with a capacitance of the analyzer. The inductance of T1 can be adjusted either mechanically or by changing the windings ratio of the transformer. Use of an air core transformer reduces the weight, and makes it feasible to use such a device. A transformer-coupled output ensures floating output.  
         [0014]    The secondary  154  output is also connected to an RF detector  160 , which produces a detection signal  162  with a DC level that is proportional to the amplitude of the RF signal  158  produced at the secondary  154  of the transformer  150 . The RF detector can include, for example, a rectifying diode. The RF detection signal  162  is coupled to one input of an error amplifier  170 . The other input of the error amplifier  170  receives a command  176  indicative of the desired RF level. A serial input command  172  is connected to digital to analog converter  174 , which is converted to an analog level  176  indicating the desired level. This analog level  176  is coupled to the second input of error amplifier  170 .  
         [0015]    The error amplifier  170  produces an error output  178  indicating the difference between the commanded level  172  and the actual level. This difference is coupled through resistors R 8  and R 5  to the input node of the respective sawtooth amplifiers  120  and  122  where it sums with the flip-flop outputs  108 ,  110 . When the error amplifier output  178  is high, it increases the oscillation signal to a higher level, thereby increasing the drive to the input of the amplifier  120 . This effectively produces more conduction from the transistor  130 , thereby Increasing the amplitude of the RF signal. The increased-amplitude RF signal is reflected by an increase in the output  162  of the RF detector  160 , which hence lowers the error signal  178 .  
         [0016]    This control loop provides extremely stable RF and DC voltages. Hence, this system can be used for long term unattended operation in a changing external environment, such as in space or under highly variable temperatures.  
         [0017]    An important feature of this circuit is its ability to obtain a large dynamic range output signal. At low levels, the drive signal can couple through the gate of the MOSFET, and generate an output signal, which is much greater than the desired minimum signal. In fact, the desired minimum signal for a quadrapole mass analyzer is about that necessary to separate one atomic mass unit. In order to avoid the coupling-through operation, a cascade stage MOSFET  134  is placed in series with a diode  136 . The MOSFET is biased to bias level VB. This provides the isolation to avoid the punch through phenomena noted above.  
         [0018]    Another problem is based on the characteristics of operational amplifiers that are commonly used for this system. Most operational amplifiers have peak voltages of about 3 to 4 volts for the sawtooth wave produced by the amplifiers. This level might not be high enough to bias the available MOSFETs to drive enough power at the output levels. The peak voltage of the sawtooth is hence increased, by referencing the return of the main power supply to a negative voltage at node  131 . By so doing, the peak value seen by the MOSFET is increased by the level of the negative voltage present at the return of the driving power source.  
         [0019]    Other embodiments are within the disclosed system.