Abstract:
A pulse jitter reduction circuit employs a low jitter system clock coupled to synchronize a pulse generating device and an ultra low jitter flip-flop to generate substantially jitter-free trigger signals employed to generate high voltage pulses for a flight tube of a time-of-flight mass spectrometer. By eliminating time fluctuations due to jitter in the triggering signal, the predictability of the arrival time of ions along a flight tube of a time-of-flight mass spectrometer is greatly improved, thereby improving the resolution of the mass spectrometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119(e) on U.S. Provisional Application No. 60/719,128 entitled L AST  S TAGE  S YNCHRONIZER  S YSTEM , filed on Sep. 21, 2005, by Timothy A. Hall, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a pulse jitter reduction circuit which is employed as a last stage synchronizer for synchronizing a pulser circuit for a time-of-flight (TOF) mass spectrometer with the data acquisition circuits to improve the signal resolution of the spectrometer. 
   A TOF mass spectrometer relies upon precise timing between the high voltage acceleration pulse applied to the flight tube to accelerate ions along the flight tube and the subsequent detection of the time of arrival of the ions by the data acquisition system. The high voltage pulse employed for accelerating the ions, therefore, must be synchronized with the data acquisition timing, such that ions corresponding to particular elements can be accurately identified. The more precise the timing relationship of the respective signals, the more precise and higher the resolution of the mass spectrometer. With conventional pulse-trigger systems employed to provide the high voltage pulses to the flight tube, inherent uncertainty exists in the pulse initiation. This inherent fluctuation in the pulse initiation time is referred to as “jitter” and is a limiting factor of the resolution of a TOF mass spectrometer. Jitter as high as 100 pico seconds (ps) or higher is common and adversely affects the resolution of a mass spectrometer, particularly where samples having closely grouped elemental ions are involved. 
   Thus, there exists a need for an improved triggering circuit which eliminates or greatly reduces jitter existing in conventional triggering circuits. 
   SUMMARY OF THE INVENTION 
   A pulse jitter reduction circuit employs a low jitter system clock coupled to a pulse generator and an ultra low jitter flip-flop to generate substantially jitter-free trigger signals employed to generate high voltage pulses for the flight tube of a TOF mass spectrometer. By eliminating time fluctuations due to jitter in the triggering signal, the predictability of the arrival time of ions at the detector in a flight tube of a TOF mass spectrometer is greatly improved, thereby improving the resolution of the mass spectrometer. 
   These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS. 
       FIG. 1  is an electrical circuit in block form of a TOF mass spectrometer incorporating a low jitter pulse generator of the invention; 
       FIG. 2  is a waveform diagram of electrical signals in the circuit of  FIG. 1 ; and 
       FIG. 3  is an electrical circuit in block form showing additional details of the circuit of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a TOF mass spectrometer  10  incorporates the circuitry of the present invention and includes a flight tube  12  (shown schematically in  FIG. 1 ) in which ions are grouped in an ionization chamber at one end. The ion chamber generates and holds ions for subsequent acceleration by applied high voltage pulses from high voltage pulser circuit  14 . The ions are accelerated down the flight tube to a detector  16  within the flight tube. The details of one TOF mass spectrometer which could benefit from the circuitry of the present invention is disclosed in U.S. Pat. No. 5,981,946 entitled T IME - OF -F LIGHT  M ASS  S PECTROMETER  D ATA  A CQUISITION  S YSTEM , the disclosure of which is incorporated herein by reference. As used herein, the expression “ultra low jitter” means the initiation of a pulse with a certainty of less than about 6 pico seconds (6 ps). When used in connection with a circuit definition, it means a circuit capable of such a performance level. 
   The circuit for generating an ultra low jitter trigger pulse includes an ultra low jitter clock  20  coupled to a pulse generator  22  which can be of conventional design and incorporated into a field programmable gate array (FPGA) to provide raw trigger pulses  52  (shown in  FIG. 2 ). The raw trigger pulses  52  from generator  22  are shown in  FIG. 2  with the shaded area representing the uncertainty in the initialization and termination of the pulses. This represents “jitter” which can be 100 pico seconds (ps) or more in the typical 4 nano second (ns) pulses  52 . The raw trigger pulses  52  are frequency controlled by the clock pulses  50  and are applied to ultra low jitter flip-flop circuits  24 ,  26 , and  28 . The resultant low jitter trigger pulse  54  from circuit  24  is applied to the high voltage pulser  14  of the TOF mass spectrometer  10 . 
   As illustrated by pulses  54  in  FIG. 2 , the jitter present in the raw trigger pulses  52  has been substantially eliminated. The high voltage pulses  56  generated by circuit  14  in response to pulses  54  exhibit a slight but very reduced amount of jitter as represented by the shaded areas on the leading and trailing edge of the pulses. This jitter is estimated to be in the neighborhood of about 5.4 ps representing about a 95% reduction in the jitter existent in the raw trigger signal. 
   The pulser circuit  14  applies high voltage pulses  56  to the ion chamber to accelerate ions down the flight tube  12  to the detector  16 . The output of detector  16  is an analog signal  58  which is applied to a switched preamplifier  18  having an output coupled to the input of an analog-to-digital (A/D) converter  30 . The signals  59  from the A/D converter  30  are synchronized with the high voltage pulses from pulser  14  by the ultra low jitter clock signals  50  from clock  20 . 
   Pulses identical to the raw trigger pulses  52  shown in  FIG. 2  are applied to two additional ultra low jitter flip-flops  26  and  28 , which are employed for providing a test signal to the system for detecting the accuracy of the application of the low jitter pulses  54 , which is outputted separately from circuits  24 ,  26 , and  28 . One of the test trigger pulses  54  is applied to a measuring instrument, such as an oscilloscope  27 , while another test pulse  54  from circuit  28  is applied to the switched preamplifier, which can be switched from looking at the signal from detector  16  and coupling them to the A/D circuit  30  or to transmit signals from circuit  28  to circuit  30  for calibrating the system. 
   The pulse generator, including the FPGA  22 , is coupled to an external PC  40 , which is conventionally programmed to receive data from the A/D converter  30  and FPGA  22  representing the ions detected by detector  16 . In addition, however, the FPGA controls the preamplifier  18  to look at either the signals from detector  16  or from the test pulse output from circuit  28 . By employing a test signal, the data acquisition system can be calibrated to great precision to assure the detected ions are accurately identified with their elements. The signals from the circuit shown in  FIG. 1  are shown in  FIG. 2 , with the clock pulses  50  having a frequency of from about 250 MHz to about 1.5 GHz in a typical TOF embodiment. In a preferred embodiment, the pulse frequency employed was 375 MHz. The trigger pulses  52  have a delay from the clocked pulses of about 500 ps due to the generation delay in the pulse-generating circuit  22 . 
   The subsequent low jitter trigger  54  from the ultra low jitter flip-flops  24 ,  26 , and  28  are substantially jitter-free, as shown in  FIG. 2 . The high voltage pulse  56  from high voltage pulser  14  is delayed approximately 1000 ps due to the inherent delay in a high voltage pulser circuit. 
   The data output signal from preamplifier  18  is shown by analog waveform diagrams  58  in  FIG. 2  in which amplitude of the signal indicates the quantity of ions of a particular element have been detected. Finally, the output from A/D converter  30  is schematically illustrated by waveform  59  in  FIG. 2  and comprises a digital number representing the number of and the timing of arrival of ions at detector  16  for two sampled ions (as an example). These signals are applied to the FPGA  22 , which outputs them as data to the input of the PC  40 , as shown by connection  21 . 
   The PC  40  is programmed as in prior Leco Corporation TOF mass spectrometers, such as Leco Model No. Pegasus® IV, to receive the data and provide an output to a printer and/or monitor for analytical samples under test. The PC  40  also applies control signals via conductor  23  to the FPGA  22  for initiating the test pulses and calibrating the instrument. The details of one embodiment of the ultra low jitter pulse generator is shown in  FIG. 3 . 
   In  FIG. 3 , the external PC  40  is shown coupled to the FPGA pulse generator  22 . In the preferred embodiment of the invention, the FPGA employed was a Virtex IV Series, Model No. XC4VLX100-12FF151 3C, available from Xilinx Inc. and which is driven by the ultra low jitter clock  20 . Clock  20  is a Model No. SAN K-A2907-500 available from Nel Frequency Controls Inc. and provides clock pulses to a clock driver circuit  25  comprising a Motorola MC100LVEP14, which applies the clock signals to the FPGA  22 . The same clock signals are applied to the D input of the ultra low jitter D-type flip-flop  24 . In one preferred embodiment of the invention, flip-flop  24  and flip-flops  26  and  28  were Model No. NB4L52 from Semi-Conductor Components Industries. 
   The ultra low jitter trigger pulses from the Q output of circuit  24 , represented by signals  54  in  FIG. 2 , are applied to a signal level converting circuit  29  for converting the signal to a low voltage TTL signal, with circuit  29  comprising a Model No. MC100EPT21 circuit, whose output signals are coupled to a second level converting circuit  31 , which converts the low voltage TTL signals to a higher TTL level signal and comprises a Model No. 74ACT11244 circuit having output signals comprising the input to the high voltage pulser circuit  14 . Pulser circuit  14  comprises a Model 666-561 circuit available from Leco Corporation of St. Joseph, Mich. 
   The FPGA  22  is programmed via an external computer, such as PC  40 , to generate a repetitive raw trigger signal  52  ( FIG. 2 ) at a typical frequency of from about 500 Hz to about 100 KHz. The FPGA and the ultra low jitter flip-flop  24  are coupled to receive clock pulses  50  ( FIG. 2 ) from the output of the ultra low jitter system clock  20 , as seen in  FIG. 1 . The signal  52  from FPGA is applied to the input of flip-flop  24  that has excellent jitter characteristics. The shaded areas on the leading and trailing edges of the raw trigger signal  52  represents the typical uncertainty in the pulse trigger initiation and termination and can vary up to 100 ps or more in a conventional pulse trigger circuit. This can lead to the problem discussed above,. namely, the loss of resolution for the TOF mass spectrometer. By controlling the jitter on the high voltage pulse  56  employing the circuit of the present invention, the uncertainty of the arrival time of accelerated ions to the detector  16  at the end of the flight tube  12  is reduced, thus increasing the resolution of the mass spectrometer. 
   It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.