US2133410A - Distortion reducing system in modulated amplifiers - Google Patents

Description

Oct. 18, 1938. w. H. WIRKLER ,1

DISTORTION REDUCING SYSTEM IN MODULATED AMPLIFIERS Filed June 15, 1935 3 Sheets-Sheet 1 .2 El RFINPuT AAAAA ggrpur Bms Ami/PUT MLTER H W/R/(LER Oct. 18, 1938.

DISTORTION REDUCING SYSTEM IN MODULATED AMPLIFIERS w. H. WIRKLER 2,133,410

Filed June 15, 1935 3 Sheets-Sheet 2 nfout ut d I 1 yr/d ML TEA H. 'W/RKLER GRMWZM Patented Oct. 18, 1938 UNITED STATES PATENT OFFICE DISTORTION REDUCING SYSTEM IN MODU- LATED AMPLIFIERS Application June 15,

6 Claims.

My invention relates broadly to transmitting systems and more particularly to circuits for substantially decreasing distortion in the output of modulation systems and audio frequency amplifiers in speech frequency systems.

It is often desirable to use either grid-modulated amplifiers or class B linear amplifiers instead of the more usual plate-modulated amplifier in radio-telephone transmitters. One objection to the use of either grid modulation or the class B linear amplifier is that the amplitude distortion is greater than in the plate-modulated amplifier, especially if the design or adjustment of the circuit used is not quite correct. Because the grid-modulated amplifier is much simpler than the linear amplifier; especially when the necessary modulating equipment for the low level stage used to excite the linear amplifier is considered, and because the number of adjustments for the grid-modulated amplifier are fewer, it is desirable to reduce the distortion of the gridmodulated amplifier so as to make its use in high quality broadcast transmitters practical.

One of the objects of my invention is to provide a speech amplifier system having means for reducing the distortion in grid-modulated amplifiers.

The main source of amplitude distortion in a grid-modulated transmitter of proper design is the non-linear relation between the input grid voltage and the radio-frequency output current. This is a result partly of the mechanics of grid modulation. In addition, the non-lnear tube characteristics accentuate this non-linearity. Further distortion is introduced by the fluctuation of the radio-frequency excitation voltage caused by the non-linearly varying grid current load drawn by the modulated stage during modulation. The distortion caused by all of the factors above can be decreased to a minimum by means of the compensating system of my invention.

Another object of my invention is to provide means for compensating variations in factors in a modulating circuit, which changes produce distortion, by withdrawing a portion of the distorted output current to a separate channel, separating the distortion elements from the modulated output, and reintroducing the distortion elements 50 into the modulating circuit in substantial opposition to the variations producing them.

A further object of my invention is to compensate for variations in factors in a modulation circuit, which produce distortion, by diverting a portion of the modulated radio frequency output 1935, Serial No. 26,830

current, rectifying this current in a linear rectifier device, opposing the signal component in the distorted rectified current by the input signal current, and reintroducing the remaining distortion components into the modulating circuit in substantial opposition to the variations which produced them.

A still further object of my invention is to provide substantially linear operating devices for deriving the distortion components from the distorted output of a modulated amplifier without modification thereof so that the distortion components may be reintroduced into the modulating circuit in opposition to the causes producing them.

Still another object of my invention is to provide filter and blocking means disposed throughout the circuit to provide selective paths for audio and radio frequency currents in order to isolate the distortion components of a distorted modulated output so that the distorted components may be employed substantially unaltered to oppose the forces producing them.

A still further object of my invention is to reduce the amplitude and harmonic distortion inherent in modulation systems and audio amplifiers when operated under certain conditions.

Other and further objects reside in the circuits and arrangements hereinafter more fully described with reference to the accompanying drawings in which:

Figure l is a schematic diagram of the circuit connections in the distortion reducing system of my invention as applied to a grid-modulated amplifier; Fig. 2 is a graph showing the normal deviation of the output current from the input voltage in a non-linear amplifying device; Fig. 3 is a trace of an oscillogram showing the distorted operating condition in a grid-modulated amplifier without the compensating system of my invention; Figs. 4, 5 and 6 are traces of oscillograms showing the operating conditions in different portions of the circuit shown in Fig. 1; Fig. 7 is a schematic diagram of the circuit connections in a modified form of my invention employing an additional amplifier stage in the compensating circuits; and Fig. 8 is a similar diagram showing the distortion decreasing system of my invention as incorporated in the intermediate amplifier of a radio frequency transmitter.

Inasmuch as modulation systems and audio amplifiers are often capable of delivering relatively high outputs, but only when operated under conditions which would ordinarily result in high amplitude distortion, the utility of a device for reducing this distortion is at once apparent,

as then such modulation systems and audio amplifiers can be operated at greatly increased efficiency with consequent saving in apparatus and operation costs.

The inherent distortion in the grid-modulated amplifier and the distortion caused by the fluctuating radio frequency grid voltage can be effectively reduced at the source by means of the circuit shown in Fig. 1. Reference character I designates the grid-modulated stage excited by capacity coupling to the tank circuit 2 of a preceding radio frequency amplifier stage. Modulating voltages of audio frequency are introduced through the transformer 8, the secondary winding of which is connected with the grid circuit of the modulating amplifier I through radio frequency choke coil 9. Reference character 3 indicates a linear detector coupled to the output circuit of the modulated amplifier I through the circuit 4. The rectified output of detector 3 produces a voltage across a resistor 5, connected in circuit with the detector 3, which is always proportional to the instantaneous radio frequency output current of the modulated amplifier.

A source of bias potential I 0 is connected in series with resistor 5 in opposite polarity with respect to the voltage produced across the resistor 5. Reference character l2 designates an electron tube amplifier having the grid l2b thereof connected through radio frequency filtering means H! to the cathode of the linear detector 3. The circuit l4, comprising radio frequency choke and by-pass devices, is operative to smooth out the radio frequency component of the rectified wave emanating from the detector 3 and pass the audio frequency components to the tube I 2. The cathode of tube I2 is shown energized by alternating current and having the usual balanced connection therefrom made to the high potential side of the modulating voltage input transformer 8. The anode I2a of tube I2 is connected with coupling coil l5 and radio frequency energy absorbing means I 6 in circuit to the oathode l2c. This circuit is paralleled by a radio frequency choke coil I! connected directly from the anode l2a to the cathode 120.

If the modulation system I were free of dis tortion caused by non-linearity in the operation of the modulating amplifier, that is, if the ampli tude of the modulated wave varied linearly with the modulating voltage introduced into the grid circuit by the transformer 8, there might still remain distortion caused by the varying impedance of the grid circuit of the modulating amplifier. The coupling to the linear detector 3 could be adjusted so that its audio frequency output voltage effective in the tube I2 would equal and balance the effect of that introduced by transformer 8; and the bias voltage introduced by the source I!) could be adjusted to exceed slightly the direct current component of voltage developed by the linear detector 3, thus providing a suitable negative bias for the grid of vacuum tube l2. Since the cathode I20 of tube [2 is connected to the ungrounded side of the modulation transformer 8, it will vary in potential at audio frequency in phase with the grid Nb of tube l2. If the coupling of the linear detector is properly adjusted, the audio frequency potential difference between the grid I21) and the cathode I20 of tube l2 can be made zero, so that tube l2 has only a constant negative bias effective in its grid circuit insofar as the audio frequency voltages are concerned; and its plate circuit, which is inductively coupled to the output circuit of the radio frequency exciting amplifier through coil 15 effects a constant load for the exciting amplifier by inductive absorption of radio frequency energy in varying proportions as the radio frequency current in the coil 2a in the tank circuit 2 varies.

With the adjustments made as above, on the assumption that no non-linearity exists between grid voltage and radio frequency output current of the moduated stage, the action of the circuit when such distortion does exist can be best analyzed by means of an cscillograph. The oscillogram pictured in Fig. 3 has been enlarged in Fig. 2, the ordinates thereof representing radio frequency output voltages and the abscissae, instantaneous grid voltages. The solid line shows the actual modulation characteristic, without distortion compensation, obtained by modulating an amplifier consisting of a single type 211 triode, excited by a radio frequency amplifier consisting of a pair of type 46 triodes, and artificially loaded by means of a resistor in its output circuit. The curvature of this characteristic is plainly evident.

Applying now the compensation circuits of my invention to the system just outlined, the straight dotted line drawn on this same figure (Fig. 2) represents the voltage output from linear detector 3 that is required to neutralize the audio frequency voltage, as supplied from transformer 8, in the grid circuit of tube I2 at every instant during the audio frequency cycle. It is evident that this neutralization occurs at only the three points marked 0 on the modulation characteristic. At a point on the characteristic at which the difference between the two lines is di as indicated by the arrow, the linear detector voltage is less than that required for neutralization, and the grid potential of tube I2 with respect to its cathode is more negative than normal and produces an increase in the impedance of the anodecathode path. The result is that circuit l5 absorbs less energy from the radio frequency tank circuit 2 and more excitation is available for the modulated amplifier I, tending to make the modulation characteristic more nearly coincide with the dotted straight line in Fig. 2. When the difference between the two curves is (ii, the action is reversed so as to decrease the excitation to the modulated amplifier and again make the modulation characteristic fall closer to the straight line. At the points 0 of the modulation characteristic, of course, no such action takes place, since the modulation characteristic already lies on the straight line.

The oscillogram traced in Fig. 4 shows the improved form of the modulation characteristic after the compensating circuit of Fig. 1 has been applied. The transmitter was operated under exactly the same conditions of audio input, carrier output, radio frequency excitation, and input to the radio frequency exciting amplifier as when the cscillograms of Figs. 3 and 4 were taken. The greater linearity obtained by means of the compensating circuit of my invention is clearly evident.

The oscillogram traced in Fig. 5 shows the relation of radio frequency input to the linear rectifier to its rectified output. This relation is here shown to be substantially linear, indicating that the compensation obtained by means of the circuit of Fig. l is not a result of compensating distortion in the linear rectifier, but that the operation of the system is substantially as hereinbefore described.

The oscillogram traced in Fig. 6 shows the voltage effective between grid I21: and cathode I2c of the compensating modulator tube I2; (the horizontal sweep circuit in this case was a 60 cycle wave). This oscillogram suggests another method of describing the operation of the compensating system of my invention as applied in Fig. 1. Consider the voltage wave shown on the oscillogram of Fig. 6 as comprising the distortion components resulting from the modulation frequency impressed on the grid circuit of the modulated amplifier I. These distortion components are then harmonics of the modulation frequency, and the cathode and grid of tube I2, operating under the respective potentials impressed thereon, constitute a harmonic selecting device, which cancels the undistorted component of the output voltage of the linear detector 3, impressed with the distortion components on the grid I2b, with the undistorted input potential supplied through transformer 8 and impressed on the cathode I2c, leaving only the distortion components active in the output circuit of the tube I2. The tube I2, in this case, also acts to amplify the distortion components, and, in effect, to reintroduce them by means of the coupling coil I into the radio frequency excitation circuit of the modulating amplifier I. in such phase as to oppose and reduce the distortion components generated in the modulation amplifier.

This aspect of phase relationship between the reintroduced harmonics and the original modulating voltage gives rise to an important consideration in the design of such a compensating system. It is seen that unless the phase relationship is correct and remains so throughout the audio frequency range of the transmitter, proper action of the compensating system cannot be realized. No difficulty was experienced with the circuit shown in Fig. 1, and it was found to operate satisfactorily over the entire audio frequency range of the transmitter to substantially eliminate distortion in the output and effect linear operation of the modulation amplifier I.

In order to secure almost complete elimination of amplitude distortion, an amplifier stage may be inserted in the compensating modulator system. as shown in Fig. '7. Careful design of the amplifier stage is required in order that the phase relat onship of the opposing audio frequnecy voltages might be maintained constant throughout the audio range of the amplifier.

Fig. '7 provides a three element electron tube 24m to select and amplify the distortion components as described in connection with tube I2 in Fig. 1, and an amplifier tube 25 to further amplify the distortion components; and in addition, the output circuit of tube 25 is so arranged that the amplified distortion components in this circuit act directly through the modulation channel on the grid of the modulated amplifier, in differentiation from the operation in Fig. 1 whereby the distortion components are reintroduced through the radio frequency excitation channel to the grid of the modulated amplifier.

Fig. 8 shows the system of my invention embodied in the intermediate amplifier unit in a radio frequency transmitting system. The unit comprises the modulated amplifier stage I, linear detector 3, and compensation modulator stage I2, associated with the modulating energy input system shown generally at 20, and a stage of the radio frequency excitation system indicated at 2I. The linear rectifier 3 is adapted to be energized from the distorted output of a later stage through connection I9. The compensated output. of the unit is supplied to terminals 23.

The circuit arrangement for separating the distortion components from the undistorted components of the modulations may be, instead of the grid-cathode circuit of an electron tube, as shown in Figs. 1, '7 and 8, a differential transformer, two electron tubes connected in opposition, or any device sensitive to the difference between two voltages, currents, or amounts of power, or to some function of the voltage current or power. The distortion components after separation and amplification, may be reintroduced as a variation in voltage, current, power, resistance, or any other effect which can be used in any part of the circuit to vary the output of the modulating amplifier so as to reduce the distortion components.

Aside from the precaution against phase shift mentioned hereinbefore, it is also necessary that the entire compensating system, including the detector 3 and the compensating modulator I2, and whatever circuit elements intervene and contribute to the final effect of the reintroduced harmonics on the modulated stage, must be capable of passing the high frequencies of the distortion components, which are several times as high as the modulation frequencies, to make the system most effective.

In addition, the whole compensating system should be free of amplitude distortion in itself. If such is not the case, the compensating system will introduce distortion components of even higher frequency; that is, the original harmonics of the modulation frequency causing distortion will be reduced, but harmonics of these harmonics will be generated. Although currents of these higher harmonics will generally be of low amplitude and the total harmonic content of the output wave of the transmitter will be reduced, weak but obj ectionably wide, spurious si-debands may be generated.

I have found by experiment that the simple absorption method of compensation modulation as disclosed in the circuit of Fig. 1 was substantially linear in operation over the range corresponding to the limited percent modulation capability required of the compensating modulator, and no trouble was experienced from this source. In this connection, it might be noted that the overall modulation capability of the compensating system need be only X%, for 100% modulation capability of the main modulator, where X is the sum of all the peak values, in percent of the undistorted output, of the harmonics generated in the modulated amplifier; and that the reduction of distortion is dependent upon the overall gain in the compensating system from the value of the original distortion components, selected as taken from the transmitter output, to the extent of their final effect on the same output, with the relations as given below:

Let the ideal modulation characteristic be the straight line y=a:v, where at: the input modu lating voltage and a, the amplification factor of the main modulation system. For simplicity, let us assume that the actual modulation characteristic with compensation is y=ax+bm where only distortion components of the second degree are considered present, and

is the distortion factor with compensation. Then the linear detector output is caar-i-cba: where c is the linear detector coefficient. Now, 0 is adso that ca=1 and the output of the linear detector, cazr-i-cbx becomes with ca replaced by unity. Thus, the distortion component effective in the input circuit of the compensating modulator, by counteracting the input modulating voltage component, is

YNOW, let K be the overall amplification of the compensating modulator from its grid circuit to the final efiect on the output of the modulated stage, including any intervening amplifiers. The amplified distortion is then Since this has already been subtracted from the normal distorted output of the modulated amplifier to obtain the compensated output azr-l-bx the uncompensated output would have been and the ratio of compensated to uncompensated distortion is rsl This equator,

shows the total reduction of distortion obtained when the gain of the compensating device is times that of the modulation system. It is seen that the equation is assymptotic, and that infinite amplification is required in the compensator to reduce the distortion to zero, but that a reduction in the ratio of approximately 3 to 1 is readily obtained. This is approximately that shown by the difference in the oscillograms traced in Figs. 3 and 4.

Another method for increasing the distortionreducing capability of the system of my invention is to reintroduce the distortion components by absorption modulation of one of the lowerlevel radio frequency amplifier stages, with all radio frequency stages between this absorption stage and the modulated stage adjusted to act as linear amplifiers for a small percentage of modulation. This would require these stages to be operated at cut-off bias as class B amplifiers, but they could be operated as class C linear amplifiers by providing proper excitation and bias so that they are linear over a certain range of vari ation of the radio frequency excitation. This arrangement appears to offer considerable possibilities, and is within the contemplation of my invention as disclosed.

The compensating arrangement of my invention employs the audio-frequency modulating voltage as introduced into the grid circuit of the modulated amplifier, and does not compensate for distortion originating prior thereto in the audio-frequency pickup circuits. This is not a serious problem, however, since an audio amplifier of sufficient power for grid modulation can be coupled into the grid-modulated circuit by means of a transformer designed to minimize amplitude distortion due to the flow of grid current in its secondary circuit. The design problems for such an audio system are similar to those involved in driving Class B audio amplifiers and present no special diificulties. However, an audio driving system involving compensation similar to that used in the triple twin type of positive grid audio circuit might be used if desired.

Thus, while I have described .my invention in certain of its preferred embodiments, I desire that it be understood that modifications may be made and that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. A system for reducing distortion in radio frequency grid-modulated amplifiers due to the varying impedance to ground of the modulated amplifier to the radio frequency excitation, which rovides a compensating load effectively in parallel with the varying impedance, said system comprising means for deriving the distortion components from the distorted modulated output of the modulated amplifier, and a radio fre quency energy absorbing circuit connected with said means and with ground and coupled with the radio frequency excitation circuit, said absorbing circuit operative under the influence of said distortion components to compensate by absorption in varying degrees the variations in impedance of said modulated amplifier to said radio frequency excitation.

2. In a grid-modulated electron tube amplifier connected with a high frequency excitation circuit and a low frequency modulation circuit, means for reducing distortion due to the varying impedance in the amplifier to high frequency excitation, said means comprising a high frequency energy absorbing circuit coupled with the high frequency'excitation circuit in parallel with the varying impedance in the amplifier connected thereto, and means selectively energized by distortion components in the modulated output of said amplifier and connected with said absorbing circuit for controlling the operation of said absorbing circuit for substantially compensating for the changes in impedance in the said amplifier.

3. A system for reducing distortion in a radio frequency grid circuit modulated amplifier due to the varying impedance to. ground of the modulated amplifier to the radio frequency excitation, which system comprises compensating load means connected effectively in parallel with the varying impedance of the grid modulated amplifier, and means for controlling the operation of said lead means in step with the distortion components of the modulation in the output of the modulated amplifier, the last said means including means energized from the output of said modulated amplifier and by the modulation components in the input of said modulated amplifier for deriving the distortion components for controlling the aforesaid load means.

4. A system for reducing distortion in a radio frequency grid circuit modulated amplifier due to the varying impedance to ground of the modulated amplifier to the radio frequency excitation, which system comprises a radio frequency energy absorbing circuit coupled with the radio frequency excitation circuit effectively in parallel with the varying impedance of the grid modulated amplifier, and means for controlling the operation of said energy absorbing circuit to compensate by absorption in varying degrees the variations in impedance of said modulated amplifier to said radio frequency excitation, said means including means energized from the output of said modulated amplifier and by the modulation components in the input of said modulated amplifier for deriving the distortion components for controlling said energy absorbing circuit.

5. A system for reducing distortion in a radio frequency grid circuit modulated amplifier due to the varying impedance to ground of the modulated amplifier to radio frequency excitation, which system comprises a compensating impedance load element connected effectively in parallel with the varying impedance of the grid modulated amplifier, and means for controlling the potential drop across said impedance load element in step with the variations in impedance of said modulated amplifier to said radio frequency excitation, said means including means energized from the output of said modulated amplifier and by the modulation components in the input of said modulated amplifier for deriving the distortion components for controlling the potential drop across said impedance load element.

6. In a high frequency grid circuit modulated amplifier, the method of reducing distortion due to the varying impedance to ground of the modulated amplifier to radio frequency excitation, which consists in providing a parallel impedance load of similar characteristics, and controlling the operation of the parallel impedance load with respect to the radio frequency excitation in accordance with the varying impedance of the modulated amplifier by deriving the distortion components from the output of the amplifier and utilizing them for controlling the parallel impedance load.

WALTER H. WIRKLER.

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