Apparatus for improving the efficiency of the modulation stage of a transmitter
BACKGROUND OF THE INVENTION 
     The present invention is directed towards a method and apparatus for improving the efficiency of the modulation stage of a transmitter. 
     The dependence of the anode current I a  of the modulator tube upon the grid voltage U g  is represented, at least in a certain range, by the expression: 
     I a  =k·U g   m , where m≈1.5 and k is a material constant. 
     The modulation systems generally used today, anode modulation with push-pull Class AB modulation amplifiers, provide very stable operation, without problems, from the point of view of modulation characteristics. In order to keep the coefficient of harmonic distortion within reasonable limits, the grid bias of the modulator tubes must be adjusted so that the resulting control characteristic curve is linear. This leads to relatively high quiescent currents (see FIG. 1) and hence to the corresponding pure power dissipation: 
     
         P.sub.VO =(I.sub.O &#39;+I.sub.O &#34;)·U.sub.a 
    
     With the present-day trend towards energy saving measures, however, other circuits are now being sought, for example pulse modulators, in order to improve the total efficiency of the transmitter. This approach is not without problems and the expense is high. 
     Since the above-mentioned power dissipation P VO  results mainly from the measures to reduce the coefficient of harmonic distortion, this invention proposes a circuit in which this unwanted quiescent power can be considerably reduced by means of an appropriate deemphasis of the input signal and dispensing with any linearization of the characteristics of the output tubes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with this invention, there is provided a modulator circuit for a transmitter, having a modulator tube for which, at least within a predetermined range, the anode current I a  varies as a function of the grid voltage U g  in the following manner: 
     
         I.sub.a =k·U.sub.g.sup.m 
    
     k being a constant, and a deaccentuator is provided such that the anode current I a  is given by: 
     
         I.sub.a =k[h(U.sub.E).sup.n ].sup.m 
    
     where h and n are constants, U E  is the signal at the input to the deaccentuator and n=1/m so that I a  =k·h m  ·U E  is a linear function. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is an I a  -U g  characteristic of a typical prior art modulator; 
     FIG. 2 is a schematic diagram of one form of deaccentuator for a modulator circuit in accordance with this invention; 
     FIG. 3 is a diagram of a circuit in accordance with this invention, using a different deaccentuator; and 
     FIG. 4 is an I a  -U g  characteristic of the FIG. 3 circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the circuits in accordance with the invention, a deaccentuator is provided to which an input signal U E  is applied, the deaccentuator providing an output (U E ) n , where n=1/m. Since in certain circumstances the exponent m is an irrational number, the technical possibilities for realizing the above condition are limited. An indirect means, through logarithms, is recommended, as indicated in FIG. 2; raising the input signal to a higher power n is replaced by a simple multiplication carried out by a potentiometer. 
     In practice, a circuit which functions to simulate the above equation is obtainable in the form of an IC chip (for example Model 433 of the firm Analog Devices). The transfer function is given by: ##EQU1## where, in our case, constant values can be inserted for E ref , U y  and U x . Thus, using this chip, the relation: 
     
         E.sub.O =k(U.sub.z).sup.n 
    
     is obtained with the following condition for n: 
     
         0.2&lt;n&lt;5 
    
     Thus the values which can be utilized in practice wherein E O  is the output of the chip, Uz is its input, and k is a constant in this chip are favorable for the present invention: for example n=2/3 for m=1.5 or some other value of n is set depending on the value of m for the particular tube involved. 
     FIG. 3 diagrammatically shows a suitable deaccentuator. Its input circuits 1 and 2 consist of respective &#34;ideal diodes&#34; each composed of an operational amplifier and a diode; thus the positive and negative half-waves of the signal are further treated separately. By appropriate poling of the inputs of the operational amplifiers, positive signals (half-waves) are obtained at their outputs. This is followed by means raising each of the signals to a higher power, comprising the above-mentioned function generators (IC chips) 3,4, the power values n 1  and n 2  of which are controlled by the position of potentiometers 5,6 and must correspond to the values 1/m 1  and 1/m 2  of the respective tubes V 1  and V 2 . 
     Since each function generator 3,4 uses logarithms to generate its output, its output signal Uo takes the form: 
     
         Uo=k[sign(U.sub.E)][U.sub.E ].sup.n 
    
     wherein sign (U E ) is the sign of the input signal applied to the respective function generator 3,4 (the sign into generator 3 being positive and the sign into generator 4 being negative), U E  is the magnitude of the input signal U E , and k and n are as defined above. 
     By means of this circuit, the residual currents of the two tubes V 1  and V 2  are adjusted to zero, so that the whole I a  /U g  tube characteristic curve has the form illustrated in FIG. 4. In FIG. 3, V is the deaccentuator, M the modulator and HG the high-frequency stage. 
     If the zero points are displaced slightly, in the negative direction by any external influences, this may lead, in certain circumstances, to interference effects in the passage through zero of the combined signal. So for practical reasons, the zero points may be displaced slightly in the positive direction (by reference voltages REF 1 , REF 2 ), but only to such an extent that I o  &#39; and I o  &#34; do not assume importance. Furthermore, the reference voltages REF 1  and REF 2  must be applied to the inverting and non-inverting inputs of the input operational amplifiers 1 and 2, respectively, by means of two adjustable voltage dividers, as shown in FIG. 3. 
     A further advantage of this circuit consists in that the two tube factors m 1  and m 2  can be balanced out separately. 
     The output U O  of each function generator 3,4 is applied to the grid of tubes V 1 , V 2 , respectively, via potentiometer 7, 8, respectively. Particularly, the voltage Ug 1  at the slide arm of potentiaometer 7 defines the grid voltage of tube V 1  and the voltage Ug 2  at the slide arm of potentiometer 8 defines grid voltage of tube V 2 . The anode currents Ia 1 , Ia 2  of tubes V 1 , V 2 , respectively, are applied to the primary windings of transformer 9 with the polarities shown in FIG. 4. The resultant voltages across the secondary winding of transformer 9 is applied to the high frequency stage HF which transmits the desired waveform. 
     In the majority of cases, the I a  /U g  curve does not follow the theoretical exponential function, but &#34;flattens&#34; or straightens out upwards. In other words, this means that the lower portion of the curve follows an exponential function while, from a certain point on, the curve continues linearly as a tangent. 
     Such a &#34;two-part&#34; curve can also be dealt with by means of the above-mentioned IC chip. The electronic expense is greater but technically practicable. With modern electronic components, it is also possible to make the transition between the exponential and the linear parts of the curve continuous, so as to avoid unwanted interference effects. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.