Abstract:
A variable pulse width generator generates a periodically recurring and variable mark-to-space ratio waveform, so that the ratio of the difference between the space and mark durations to the sum of the space and mark durations is proportional to a selectable voltage, and substantially independent of any resistor values used in the circuit.

Description:
BACKGROUND OF THE INVENTION 
     A pulse width generator is known from U.S. Pat. No. 3,441,877, issued to Thompson, in which a measurement current I m  passing through a resistor R 1  obtained from a voltage source U m  passes into a Miller-generator, which consists of an amplifier V, and a capacitor C connected across the input and the output of the amplifier V. A Schmitt-type trigger ST is postcoupled to the output of the amplifier V, which in turn controls a switch S of a reference voltage source Q. In one position of the switch S the input of the amplifier V is connected via a resistor R to a positive reference voltage +U r , and in another position of the switch S the input of the amplifier V is connected via the resistor R to a negative reference voltage -U r . Consequently, a reference current I r , which is superimposed upon the current I m , and is either positive or negative in dependence of the position of the switch S, passes to the input E of the Miller-type integrator M. 
     Across the output of the Miller-type integrator M there appears a triangular voltage U i , which according to FIG. 2, ascends or is positive-going, until it reaches an upper threshold value U s1  of the Schmitt-type trigger ST, the Schmitt-type trigger ST subsequently flipping over so as to reverse the switch S, the reference current I r  changing its polarity, and the voltage U i  descending, or becoming negative-going, until it reaches a lower threshold value U s2  of the Schmitt-type trigger circuit ST. At the output of the Schmitt-type trigger circuit ST, there appears a waveform of rectangular pulses, having a mark duraction T a , and a space duration T b . 
     The rectangular voltage waveform is governed by the equation: ##EQU1## 
     The ratio (R)/(R 1 ) represents a constant of proportionality, so that R and R 1  must be precision resistances. 
     The above U.S. Pat. No. 3,441,877 is incorporated into the present application by reference. 
     A pulse-time encoding apparatus is also known from U.S. Pat. No. 3,074,057 issued to Gilbert. 
     There is also known an integrating quasi-sweeping voltage-to-time-interval converter from volume 19, number 2, part 1 of the Russian Journal &#34;Instruments and Experimental Techniques&#34;, March-April, 1976. The integrating quasi-sweeping voltage-to-time-interval converter described in that publication is a variable pulse width generator, including a differential amplifier with two inputs, the input voltage of the integrating quasi-sweeping voltage-to-time-interval converter being connected to one input of the differential amplifier, the other input of the differential amplifier being connected via a capacitor and via a resistor to a reference voltage source. Although the mark-to-space ratio of the integrating quasi-sweeping voltage-to-time interval converter is independent of the value of that resistor, the operation of the integrating quasi-sweeping voltage-to-time-interval converter is subject to disturbances if the input voltage is rapidly changed. The integrating quasi-sweeping voltage-to-time-interval converter is therefore only suitable for applications, where the input voltage changes only relatively slowly. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to devise a variable pulse width generator, which is dependent only on the ratio of a selectable voltage compared to a reference voltage, and independent of any resistor values, which is simple to operate, and which is not subject to any disturbances, when the selectable voltage is changed relatively quickly. 
     This object is attained, according to the present invention, by a variable pulse width generator having features claimed in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be better understood with the aid of implementation examples in which: 
     FIG. 1 shows a circuit of the prior art of a variable pulse width generator; 
     FIG. 2 shows a voltage-time diagram of the variable pulse-width generator according to FIG. 1; 
     FIG. 3 is a first version of a circuit of a variable pulse width generator, according to the present invention; 
     FIG. 4 is a second version of a variable pulse width generator, according to the present invention, 
     FIG. 5 is a voltage-time diagram of the circuit according to a second version of the present invention, and 
     FIG. 6 shows one version of the output stage of the variable pulse width generator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 3, the variable pulse width generator shown in FIG. 3 uses the same reference numerals as the pulse width generator of the prior art shown in FIG. 1. The variable pulse width generator, according to FIG. 3, also includes the Miller-integrator M, a Schmitt-type trigger ST, and a reference voltage source Q supplying the reference voltages +U r  or -U r . The circuit elements are connected to one another in a manner similar to the circuits of the prior art shown in FIG. 1, and also operate in a similar manner. There is also provided a resistance R, which is not, however, connected directly to the input of the Miller-type integrator M, but is connected in series with a load resistor R b  of a current converter StW between the input E of the Miller integrator M, and the reference voltage source Q. A current I passes through the primary winding of current transformer StW, and a measurement voltage U m  is obtained across the load resistor R b . The load resistor R b  acts as a voltage source, and the reference voltage source Q, the resistor R, and the load resistor R b  are connected in series, forming a feedback path which is permanently connected between the input E of the Miller-type integrator M and the reference voltage source Q. Consequently, a current passes through this feedback path to the Miller-type integrator M, which current contains a component proportional to the measuring voltage U m , and a component proportional to the reference voltage +U r  or -U r . 
     The following equations apply: ##EQU2## 
     It should be noted that the equations (1) through (11) of the above-noted Thompson reference (U.S. Pat. No. 3,441,877) apply if 
     
         a=1 
    
     
         V.sub.1 =U.sub.m 
    
     
         V.sub.2 =U.sub.r 
    
     
         R.sub.2 =R 
    
     
         t.sub.1 =T.sub.a 
    
     
         t.sub.2 =T.sub.b 
    
     However, equation (5) of Thompson should read ##EQU3## From equation (11), it will be seen that the ratio of the m=(t 1 )/(t 2 ) mark-to-space duration is dependent on the ratio c=(R 2 )/(R 1 ) [equation (8)]. 
     If the symbols used in the present application shown in FIGS. 1 and 2 are substituted into equation (11), one obtains ##EQU4## In the circuit according to FIGS. 3 and 4, and from equations (a) and (b), it follows that ##EQU5## In Thompson, m depends on the resistance ratio R 2  /R 1  whereas it is substantially independent of any resistance ratios in the present invention. 
     It follows from equation (d) that ##EQU6## wherein U h  =U s1  -U s2 . The ratio of the difference of the mark duration T a  and the space duration T b , to the sum of the mark duration T a  and the space duration T b  is therefore substantially dependent only upon the measurement voltage U m  and the reference voltage U r . The value of the resistance R does not enter into the measurement result. In lieu of the current transformed StW having a load resistor R b , there can also be used a voltage transformer SpW with a load resistor R 1  across its secondary which supplies a voltage U m  proportional to the voltage U, and which is connected in a feedback path via the resistor R to the reference voltage source Q. 
     A comparator K is provided to supervise the voltage U i  of the Miller-type integrator M, as can be seen in FIG. 4, which compares the voltage U i  with a triangular voltage waveform U.sub.Δ, and as shown in FIG. 5, switches over the polarity of the reference voltage U r , if the voltage U i  exceeds, or falls below the instantaneous respective values of the triangular voltage waveform U.sub.Δ. The equation: ##EQU7## applies, where T a  +T b  corresponds to the period of the triangular voltage waveform U.sub.Δ, and is independent of the measurement voltage U m . 
     It should be noted that the period of the alternating voltage applied across the primary of the voltage transformer SpW (or current transformer StW) should be large compared to the period of the triangular waveform. 
     One version of an output stage of the circuit of FIG. 3 is shown, for example, in FIG. 6. The switch S of FIGS. 1, 3 and 4 is formed by two CMOS field effect transistors TR 1  and TR 2 . Transistor TR 1  is a P-channel type, and transistor TR 2  an N-channel type. The output of the Schmitt-trigger ST (FIG. 3) or of the comparator K (FIG. 4) is connected to the gates of the two field effect transistors. The source of TR 1  and its substrate are connected to the reference voltage +U r , and the source of TR 2  and its substrate are connected to the reference voltage -U r . The drain of both transistors is connected to the resistor R. In lieu of the transistors TR 1  and TR 2 , it is alternately possible to use a CMOS gate, such as type MC 14001 B, manufactured by Motorola. 
     I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.