Multichannel time-voltage converter
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a multichannel time-voltage converter. 
     The multichannel time-voltage converter is used, for example, when a sample is excited by pulsed light and photoelectrons generated from the sample are detected so as to thereby measure the lifetime of the emission from the sample. 
     2. Description of the Prior Art 
     Conventionally, this kind of multichannel time-voltage converter, for example, as shown in FIG. 1, has been proposed, in which when a first terminal T 1  is fed a start signal, a start switch 1 is used to start the charging of capcitors C 1 , C 2 , and C 3  by a power source 2, and when a second input terminal T2 is fed stop pulses one after another, stop switches S 1 , S 2  and S 3  are respectively sequentially turned off in correspondance with the stop pulses so as to thereby stop the charging of capacitors C 1 , C 2  and C 3 . 
     Such a construction, however, is arranged such that a plurality of time-voltage converters are connected in parallel to only one power source so as to be multichanneled, whereby the circuit constants based on capacitors C 1 , C 2 , and C 3  and resistances R 1 , R 2 , and R 3  at each channel is slightly different so as to cause a shift in the charging waveform at each capacitor C 1 , C 2 , or C 3 , thereby creating a problem in that it is extremely difficult to make these waveforms closely correspond. In addition, the circuit construction is complex. 
     SUMMARY OF THE INVENTION 
     In order to solve the above problem, this invention has been designed. An object thereof is to provide a multichannel time-voltage converter which is so constructed that one sawtooth wave generator is connected to a plurality of tracking hold circuits so that its stop switches are adapted to be turned off sequentially each time a stop pulse is fed to the circuit, whereby a single charging sawtooth wave generator capacitor is usable in common with a plurality of channels, thereby resulting in a simplified circuit in which the conformation adjustment of the charging waveform for each channel is not required. 
     The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional multichannel time-voltage converter. 
     FIG. 2 is a circuit diagram of an embodiment of a multichannel time-voltage converter of the present invention. 
     FIG. 3 is a circuit diagram exemplary of a sequencer used in the present invention. 
     FIG. 4 is an illustration of the operation of the FIG. 2 embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 is a circuit diagram of an embodiment of a multichannel time-voltage converter of the invention, in which reference numeral 10 designates a sawtooth wave generator circuit, which generates voltage in proportion to the lapse of time and comprises a constant current source 11, changeover switches 12a and 12b, and a charging capacitor C. Elements 201, 202, . . . , and 20N designate tracking hold circuits (to be hereinafter called the TH circuit) connected to the output line of the sawtooth wave generator circuit 10 and forming a plurality of channels, the first TH circuit 201 comprising a stop switch S 21 , a capacitor C 21 , and a buffer amplifier B 1  ; T 21  designates an output terminal of the first TH circuit 201. Thus, the TH circuits are similarly formed up to the Nth circuit 20N. In addition, 30 and 40 designate buffer amplifiers. All of the buffer amplifiers may or may not be necessary, depending on the particular application. 
     It is noted that the constant current source 11 and switch 12a may comprise Part No. SP9768 which is manufactured by the Plessy Company Ltd. and the switch 12b may comprise Part No. HI-201 HS which is manufactured by the Harris Corporation. The track and hold circuits (e.g. - switch S 21  and capacitor C 21 ) may comprise Part No. HTS-0025 which is manufactured by ANALOG DEVICES, Inc. 
     FIG. 3 is a circuit diagram exemplary of a sequencer for generating the necessary sequence for operation of the aforesaid converter, the sequencer comprises flip-flops FF 0  to FF N , monostable multivibrators MM 1  and MM 2 , and AND circuits A 0 , A 2 , . . . , A N . 
     Next, an explanation will be given on the operation of the converter in accordance with the present invention with reference to FIG. 4, in which all of the TH circuits 201 to 20N are assumed to be in the hold condition as their initial condition. 
     Step 1--Upon feeding a set pulse P 0  into an input terminal T 3 , signals at a high level are delivered from the Q terminals of the flip-flops FF 1  through FF N  to the switches S 21  through S 2N  so as to turn on each stop switch. As a result, capacitors C 21  to C 2N  of the circuits 201 to 20N are respectively connected to the output line of sawtooth wave generator 10 through the buffer amplifiers 30 and 40. 
     At the same time, since a signal at a high level is output from the Q terminal of flip-flop FF 0  to an AND circuit A 0 , the circuit A 0  is kept in a condition for allowing a signal from input terminal T 1  to pass therethrough. 
     Step 2--Next, upon feeding a start pulse P S  into input terminal T 1 , an output from AND circuit A 0  is fed into the monostable multivibrator MM 2 , and from the Q terminal thereof, a high level signal is fed to change-over switches 12a and 12b, whereby the switch 12a is turned on and switch 12b is turned off. As a result, the constant current source 11 charges the capacitor C and a voltage V C  across both terminals thereof rises, so that when the current of constant source is represented by I 0 , the capacity of capacitor C is represented by C, and the time is represented by t, the voltage is given as ##EQU1## 
     At the same time, voltage V 01  to V 0N  across the terminals of capacitors C 21  to C 2N  of the TH circuits 201 to 20N all rise in waveforms similar to that of the voltage V C . 
     Step 3--When a first stop pulse P 1  (to be generally called P) is fed into the input terminal T 2 , since the AND circuits A 2  to A N  are not in a condition for passing a signal, the pulse P 1  is only fed to the flip-flop FF 1  and its Q terminal that outputs a low level. As a result, only the stop switch S 21  is turned off so as to disconnect the capacitor C 21  from the sawtooth wave generator circuit 10 so as to thereby stop its charging, but hold the terminal voltage V 01 . 
     At the same time, the AND circuit A 2  is fed the signal from the Q terminal of flip-flop FF 1 , thereby being putting it in a condition for allowing a signal to pass through the AND circuit A 2 . 
     Step 4--Next, upon feeding a second stop pulse P 2  into the input terminal T 2 , since only the AND circuit A 2  is in a condition for passing a signal, the pulse P 2  is only fed to the flip-flop FF 2  and its Q terminal thereby outputs a low level. As a result, only the stop switch S 22  is turned off so as to disconnect the capacitor C 22  from the sawtooth wave generator circuit 10, whereby the charging is stopped, but the terminal voltage V 02  is held. 
     At the same time, an AND circuit A 3  (not shown) is fed the signal from the Q terminal of the flip-flop FF 2 , thereby putting it in a condition for allowing a signal to pass. 
     Step 5--Thereafter, each time a stop pulse P is fed in the input terminal T 2 , the Nth stop pulse P N , in the same operation as in steps 3 and 4 above, turns off the Nth stop switch S 2N . 
     In addition, even when the stop pulse P does not occur, after the lapse of setup time T FS  by the monostable multivibrator MM 1 , the output from the Q terminal of the monostable multivibrator MM 1  resets all of the flip-flops FF 1  to FF N  so as to thereby turn off all of the stop switches S 21  to S 2N . 
     Step 6--Voltages proportional to the time from the arrival of the start pulse P S  to arrival of the respective stop pulses P 1  through P N  are developed as the respective output voltages V 01  to V 0N  for the aforesaid setup time T FS . When the stop pulse P does not occur, as abovementioned, its output voltage appears as a voltage corresponding to the setup time T FS , which is not used as data. 
     Step 7--Since each data is effective for a time period T AC  from a finish of setup time T FS  to the input of the next set pulse P 0  after the stop switch S 2N  is turned off, the processing, such as A-D conversion, may occur as needed within this time period. 
     Step 8--After the lapse of the setup time T FF  (T FF  &gt;T FS  ) set by the monostable multivibrator MM 2 , the output from the Q terminal of the monostable multivibrator MM 2  turns the changeover switch 12a off and turns switch 12b on, thereby resetting the voltage of capacitor C and restoring the converter to its initial state. 
     In a case where a distortion is developed in the waveform of voltage V C  at the capacitor C during the switching of changeover switches 12a and 12b, the stop pulse P is delayed for a predetermined time so that a voltage value corresponding to the delay time may be subsequently deducted from the data. 
     While a preferred embodiment of the inveniton has been described using specific terms, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.