US2184571A - Modulation system - Google Patents

Description

2 Sheets-Sheet 2 Dec. 26, 1939. A. w. VANCE MODULATION SYSTEM Filed May 27, 1938 F1 3.

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45 II: I w 1 ill" INPUT cHRR/ZR MODl/LHTO)? ill VOL 7776-3 FIXED 519 C If DETECTOR llwcntor Arthur W Vance Cttorncg Patented Dec. 26, 1939 UNITEDT'STATES PAT- ar creme 2,184,571 MODULATION SYSTEM Arthur W. Vance, Haddonfield, N. J., -assignor to Radio Corporation of America, a corporation of Delaware Application May 27. 1938. Serial No. 210,304

13 Claims. (01. 250-17) My invention relates to radio modulation systems, and more particularly to method and means for providing a modulation system which has high operating efficiency and at the same time 5, is economical from the point of View of the numher of tubes and other parts which are required. 'It has been recognized for some time that modulated oscillators or power amplifiers cannot be operated at a high average efliciency. This I is partly due to the widely varying conditions under which they are operated. In order to provide ample reserve power for conditions of maximum modulation, transmitters mustbedesigned to have an output greatly in excess of the average value. This, of course, decreases the average transmitter efiiciency, and increases its cost.'

It is highly desirable therefore to provide.

' means for operating a tube, on the average, more nearly at its optimum rating. The general method of accomplishing this is to divide the output between several tubes, for example, by operating one tube at optimum conditions at 50% modulation and supplying the increased output required for 100% modulation by another tube which is called into operation only at. the high output of a second power tube is also connected modulation levels.

It has been proposed to accomplish this by coupling the antenna to the power output stage through an impedance inverting network. The

across the antenna. The input to-the second power tube is then excited in such a manner that the tube delivers radio frequency current to the antenna in phase with that delivered by the first power tube, for positive peaks of the. audio signal, and out of phase for negative peaks of they frequency voltage, and it will be impossible tomaintain a uniform operating condition.

It is well known that in a pentode tube having a high internal plate impedance the grid voltageplate current characteristic is nearly independent of the plate voltage over theoperating range. However, pento-des are not available .in the high power ratings.

Another difficulty which the present invention overcomes arises from the fact that the phase reversal due to overmodulation downward prevents successful application of overall audio degenerative feedback to transmitters of. thisv type.

In accordance with one modification of the 'invention this difiiculty is overcome.

Most power tubes. are triodes. Consequently, it is desirable'to be able to use a? triode efficiently in the manner mentioned above.

It is an important object of this inventionto provide means for efficiently operating a triode power tube which is subject to widely varying plate voltages.

n is a further object or this invention to provide a variable bias Which-willmaintain constant operating conditions for a triode operating under conditions of variable plate voltage.

Another object is to provide a system for satisfactorily applying overall audio feedbackto a transmitter operating on this principle. v It is a further object of this'inv-ention to provide a simplified and improved modulating means for a radio transmitter.

It is a still further object of this invention to provide a highly efficient modulation means which has a low first cost and low operating and main tenance costs.. 1 1

This invention will be better understood from the following description considered in connection with the accompanying drawings, and its scope is indicated by theappended claims. Sim

ilar reference numerals refer to similar parts throughout the several drawings.

Referring to the drawings, Figure 1 is a schematic diagram of a modulator system according to a preferred embodiment of my invention in which'v'ariable bias is obtained by an unbalanced condition on the grids of the driver tubes; I

Figure 2 is a schematic diagram of another embodiment in which avariablebiasing voltage, obtained from the plate of the first output tube, is applied to the grid of the second tube;.

Figure 3 is another embodiment in which the biasing voltage is obtained from the grid of the.

first output tube; and Figure 4 is a diagrammatic representation of a complete transmitter in which overall audio degenerative feedback is employed to reduce distortion.

Referring to Fig. 1, the input electrodes offa power amplifier triode l are connected to a pair of input terminals 3, 5. A driving voltage of carrier frequency is connected to the input terminals from a source which is not shown. The triode I is a Class C amplifier which drives the load l, which may be an antenna, for example, through an impedance inverting network shown at 9. This network consists of the tank circuit 25 shown across the load I and the tank circiut 21' in the plate of the triode: l. Both are tuned so that they present an inductive reactance XL. at the carrier frequency, which is equal to the capacity reactance X0 of the series capacity ll. It is well-known that a class C amplifier, when suificiently excited, constitutes a nearlyconstant voltage generator for load impedances sufiicient to produce plate voltage saturation. Therefore. for purposes of explanation it, will be assumed that the tube I forces a constant radio frequency voltage Ec across the input to the impedance inverter at all points of the operating cycle.

It can readily be shown that a constant current is forced through the load 7 regardless of its value, or its efiective value when shunted by the second amplifier I3. The magnitude of this current is where Z0 is the characteristic impedance of the network r The load I has a value of so that a two to one voltage transformation is obtained. The current through the load leads the voltage on the input by for a highpass network as shown. a class B, R. F. amplifier triode which is ex cited through a coupling transformer 23 by a balanced'modulator I5 of a conventional type.

The R. F. excitation to the balanced modulator is obtained through the phase shifting network I! shown from the same carrier source as that which drives the tube I g This network I'l introduces 90 phase shift to compensate for the phase characteristic of the impedance inverter 0. The network I'I gives 90 lag but the only difference between lag or lead would be a reversal in the polarity of modulation. For no modulation, the triode I 3 is biased very near to plate current cut-ofi and the modulator is in balance sothat the triodel3 draws negligible plate current. Thus, carrier is supplied to the load i at the class C efficiency of the amplifier I which emciency is limited only by the tubes and voltages used.

The audio input to the balanced modulator I5 is applied to input terminals I9, 2i from a source which is not shown. The audio voltage is applied out of phase to the modulator grids, but the carrier is supplied in phase to the grids. Consequently the balanced modulator delivers no carrier frequency energy to the amplifier I3, but only side-band energy. When the audio input voltage is positive with respect to the grid of one of the balanced modulator tubes it is nega tive with respect to the grid of the other. Each tube is therefore alternately blocked by the audio voltage. The radio frequency voltage which is induced on the gridof the second amplifier 53 will be of one phase when delivered by current Tube I3 in Fig. l is flowing through one of the balanced modulator tubes, and will reverse 180 when delivered to the amplifier by the other modulator tube, due to the push-pull connection of the balanced modulator output transformer primary.

For upward modulation the balanced modulator drives the amplifier I3 so that it delivers current to the load I of like phase to that fiowing in the load due to the amplifier I. As the current in the load increases due to tube It, amplifier I continues to supply the same current as before and so delivers more energy since the same current is driven by it into a higher effective resistance. At modulation upward, amplifier It has approached voltage saturation and is delivering a current to the load I equal to that trom the amplifier I, and each is delivering twice the power that the first amplifier I supplied on no modulation. Both tubes are working at relatively high efiiciencies since both are at, or near, plate voltage saturation.

For downward modulation, the audio voltage drives the balanced modulator so that it delivers radio frequency current to the amplifier I3 of such a phase that the tube I3 delivers current to the load I in phase opposition to that nowing in the load due to first amplifier I. This causes the apparent load resistance to decrease and amplifier I delivers less power since it maintains the same current through a lower resistance. At 100% modulation downward the current from the second amplifier I3 exactly balances that from the first amplifier I, and the apparent load resistance becomes zero. In this case, the load presented to the output of amplifier I becomes extremely small, and the tube-sup-v plies only its circuit losses. The second amplifier I3, however, delivers a current of average carrier value into a short circuit, so it is subject to considerable instantaneous plate dissipation at this point in the modulating cycle. The average dissipation in this second amplifier over a 100% sine wave modulation cycle is not excessive, however. In actual use, with typical program modulation, the dissipation in the second amplifier I3 is always much less than that in the first.

It is to be noted that, as the second amplifier I3 starts upward modulation, it must start to draw plate current at minimum plate voltage (approximately equal to the D. C. applied voltage), and as it starts modulation in a downward direction, it must draw current at maximum plate voltage (about equal to 1%; times the applied D. C. voltage). If the second amplifier I 3 is a triode, and if the D. 0. grid bias is to be adjusted so that the tube will draw negligible plate current at unmodulated carrier condition, as mentioned above, it will have to be biased to cut-ofi for maximum plate voltage, and therefore the tube will be far beyond cut-ofi for minimum plate voltage. Thus the excitation voltage of the phase which produces upward modulation must be very high for any plate current to be drawn by the tube at minimum plate voltage, and upward modulation started. Violent distortion results in such circumstances. This distortion can be overcome by reducing the bias of the amplifier I3 sothat large currents are drawn at maximum plate voltage, and small but definite currents at minimum plate voltage. However, this causes the amplifier to absorb a great deal of power and thereby greatly reduce the efficiency at the unmodulated carrier condition. i

It is therefore desirable that the amplifier I3 be biased ofi so that it'draws the same plate current at both minimum and maximum plate voltage. The solution which I have discovered is to supply a radio frequency bias to the tube such that the grid voltage is more negative when the plate voltage is maximum, and less negative when the plate voltage is minimum. The procedure is first to apply a negative D. C. bias 29 which is sufiicient to bias of]? the amplifier tube I! to the desired extent for the existing applied D. C. voltage with no radio frequency component present on the plate, and then to apply a radio frequency biasing voltage equal to -;LE, where E0 is the normal carrier voltage across the load and is the amplification constant of the amplifier tube I3. The efiect of sucha radio frequency bias is obtained, according to a preferred embodiment of my invention,

.by applying unbalanced D. C. biases and 4| to the grids of the two balanced modulator tubes 3| and 33. This effectively compensates for the changes in operating plate potential of the amplifier I3, for a high excitation voltage from the balanced modulator I5 is applied to the grid of the amplifier I3 during the phase which produces upward modulation. As noted above, this is necessary due to the fact that at the start of the upward swing the plate voltage 1 of the amplifier I3 is a minimum.

mately 66%.

Consider the efiect of a deviation of the carrier bias from the value which provides complete compensation for the varying plate voltage. If the radio frequency bias is less than this amount, the compensation will not be complete, and the plate current will increase during the positive peaks of the radio frequency component of the plate voltage. The amplifier will thus absorb power from the load. On the other hand, if the radio frequency bias is greater than that necessary to compensate for the variations in plate potential, the plate current will tend to increase during negative peaks of the plate voltage. The amplifier therefore delivers some in-phase power to the load.

By taking advantage of this fact, the efficiency of the transmitter can be still further increased. Efliciency of the order of 30%, or better, is possible for the amplifier I3 if a small amount of over-bias is employed. I have found that where the efiiciency of the first class C amplifier I is if the carrier biasis adjusted to exactly compensate for the plate voltage variations, the overall efiiciency of the carrier circuits is approxi- This decrease is due to the fact that the second amplifier I3 is drawing a uniform current through the radio frequency cycle.

liver any current to the antenna.

If the carrier frequency bias of the amplifier I3 is lowered slightly so that more current is drawn during positive peaks of plate voltage, the

overall eificiency drops to about 63%, due to the absorption of power from the load. If, however,

a small amount of over-bias is applied to the amplifier tube, I have found that an increased efficiency may be obtained. By increasing the carrier frequency bias until amplifier I3 is delivering about 5% of the total power to the load, with no modulation, the overall efficiency of the carrier system increases to 70%. Thus, the most efficient condition results when a small excess of radio frequency bias is applied to the second amplifier I3.

Figure 2 shows an alternative arrangement by tion cycle.

This current represents a pure loss as it does not dewhich a carrier frequency bias may be obtained. The circuit of the amplifier I, impedance network this network is derived from the plate of the first Thev amplifier tube I through a capacitor 43. network 3? shifts the phase of the radio frequency voltage in an opposite sense to that caused by the impedance inverting network 9. Consequently, the radio frequency bias which is applied to the grid of the amplifier tube I3 is out of phase with the radio frequency plate voltage component, and tends to maintain a uniform efiective bias;

Fig. 3 illustrates still another embodiment of my invention for obtaining the biasing voltage. The same circuit is shown as that of Fig. 2 except that the input of a phase inverter .45 is connected to the grid of the first amplifier I. Since the grid voltage is 180 out of phase with the voltage on its plate, it becomes necessary to shift the bias 180 also. This is accomplished by utilizing a network 55 which gives a changei in the opposite direction'to that given by the to be expected that the gain of the amplifier I will be higher for downward modulation than for upward, and thereby tend to produce evenharrnonic distortion. This is particularlytrue when the amplifier tube is a triode. is ofi-set by the regulation of the amplifier I. This stage, in practice, is not a constant voltage generator as assumed in the previous discussion, so that the output voltage will rise as the load impedance increases. From zero modulation to modulation upward, the load varies two to one, while from zero modulation to 100% down, the load may vary 20/1, or more. Therefore, in most cases the output voltage will rise more on the downward modulation part of the cycle than it falls on the upward part of the modula- This tendsto compensate somewhat for the increased gain of the amplifier on the downward cycle. The actual compensation so obtained will, of course, depend on the tube, the

load at no modulation, and the exciter charac-- teristics. This distortion can also be reduced by unbalancing the center tap of secondary of the audio input transformer to the balanced modulater so that its output will be greater on the upward part of the modulation cycle. This is illustrated in Figs. 1 to 3 by the variable tap which input to the baltion. In practice, the zero signal current of thesecond amplifier I3 is kept as low as possible to permit the highest possible efficiency, and the resultant odd harmonic distortion removed by This effect the use of feedback in a manner which is well known to those skilled in the art.

The radio frequency bias which I have described is not necessary when two tubes are used to replace the single amplifier tube l3. One tube may then be used for upward modulation and the other for downward modulation. Separate plate voltages and D. C. biases would be required for each tube. The downward modulating tube would then be supplied with only enough D. C. plate voltage to enable it to supply radio frequency current of average carrier value into a short circuit. The upward modulating tube would require the same D. C. plate voltage as in the present system. Each tube would be biased so as to draw equally small zero signal currents. This system has some merit in that the dissipation at modulation down is greatly reduced. However, the extra power tube complement and the necessity for an extra power supply, more than off-set the higher efficiency at 100% modulation down since this condition exists a negligible part of the time.

It is worthy of note that with all systems which operate upon the general principle here described,

overniodulation in the downward direction results in a different type of distortion than that ordinarily produced. If the excitation on the amplifier id, of downward modulating phase, is increased tosuch an extent that this tube delivers more current to the load than the first tube i, then there results a reversal of phase of the load, that is, the load current isover-compensated, and begins to rise again beyond the 100% modulation downward point. Since the amplifier i3 is far from voltage, or emission saturation at this point it is capable of forcing considerable reversed current through the load.

Thus, over-modulation in the downward direc' tion does not result in distortion in the sense usually understood since it represents merely an excessive ratio of sidcbands to carrier and not distortion of the modulating wave form. No new sidebands are produced so no interchannel interference is caused, as is the case when ordinary transmitters are over-modulated downward and the downward peaks are sharply cut off; In the upward direction there is no sharp limitation at 100 modulation. The only limitation is that of emission or voltage saturation of the tubes, so the distortion for over l I'lJ% modulation upward is decidedly less than that for over-modulation downward in the ordinary case. 100% modulation upward is an arbitrary value and, by design, the limitation can be set at any figure, etc. The new system permits similar control for downward modulation. It is possible to design it so that it will over-modulate to almost any desired extent without production of harmonics of modulation frequencies and consequent inter-channel interference.

When an attempt is made to use audio degeneration with a transmitter utilizing a modulation scheme of this type, the effect of over-moclulation must be considered. As pointed out above, overmodulation causes a reversal of the phase of the output voltage in the antenna, and that the antenna current starts to rise again when the wave is modulated over 100% downward. This causes a reversal of the feedback phase and a certain amount of instability. This is, of course, true in all modulation systems which operate on the general principle of which this invention is an improvement.

A solution to this diificulty has been found to lie in applying a fixed, unmodulated carrier to the feedback rectifier along with the transmitter output. Thus the percentage modulation at the feedback rectifier is reduced to the point where 100% modulation of the transmitter output still represents less than 100% modulation of the carrier utilized to drive the feedback rectifier. This is illustrated in Fig. 4.

Referring to Fig. 4, a carrier frequency oscillator is indicated by reference numeral 6|. The output of the oscillator is connected to a modulator which is indicated by reference numeral 63. This modulator may be similiar to one of the systems shown in the preceding figures of the drawing, or any other system which operates on the same principle. The audio modulatingvoltage is derived from a suitable source, indicated by reference numeral 65. This voltage is applied to the audio input of the modulator 63 in series with a rectified audio voltage which is obtained from the carrier by a feedback detector 6?.

The feedback detector consists of a diode rectifier it which is coupled to the transmitter antenna ill by means of a transformer T3 and a coupling coil ii. The detector is also coupled to the carrier oscillator 6i through a transformer 15. Thus, an unmodulated carrier is added to that derived from the antenna, and over-modulation at the detector is prevented in the manner described above.

When this is done, the system operates with feedback in the normal manner. Stability is determined by the phase and gain characteristic around the feedback loop. Thus, with this system, I have effectively reduced the extra sidebands, even when over 100% modulation downward is obtained. This has been impossible with systems known up to the present time.

Thus I have described a modulation system which permits efficient operation with a minimum number of tubes, and which is particularly adapted to the use of power output triodes.

I claim as my invention:

1. In a modulation system which includes a first vacuum tube for supplying main carrier currents to a load and an auxiliary tube supplying auxiliary carrier currents to said load whose phase with respect to said main carrier currents is controlled by a modulating voltage, the plate of said auxiliary tube being subject to varying potentials due to the main carrier currents in said load, the method of maintaining substantially uniform operating conditions for said auxiliary tube in the presence of said varying plate potentials, which includes the step of controlling the plate current of said auxiliary tube by a radio frequency bias, said bias having a phase which tends to oppose a change in the plate current of said auxiliary tube due to said variation.

2. In a modulation system which includes a first vacuum tube for supplying main carrier currents to a load, and an auxiliary tube supplying auxiliary carrier currents to said load whose phase with respect to said main carrier currents is controlled by a modulating voltage, the plate of said auxiliary tube being subject to varying potentials due to the main carrier currents in said load, the method of maintaining substantially uniform operating conditions for said auxiliary tube which includes the steps of deriving a radio frequency bias whose frequency is equal to that of said main carrier, but in phase opposition thereto, adjusting the potential of said bias to be at least equal to the peak potential of the radio frequency voltage on the plate of said tube, due

to the main carrier current in said load, divided by the amplification factor of said auxiliary tube, and controlling the plate current of said auxiliary tube by said derived bias.

3. In combination, a source of main carrier frequency currents, a first amplifier connecting said source to a load, a second vacuum tube amplifier whose ouput is delivered to said load, said vacuum tube having grid and plate electrodes, a source of modulating voltages, means for energizing said grid by a radio frequency voltage Whose phase is determined by said modulating voltages'whereby the resultant current in said load is determined by said modulating voltages, and means for applying a radio frequency bias to said grid to coinpensate for varying plate potentials applied to said second amplifier due to voltages across said load.

4. In combination, a source of main carrier frequency currents, a first amplifier conecting said source to a load, a second vacuum tube amplifier whose output is delivered to said load, said vacuum tube having grid and anode electrodes, a source of modulating voltages, a balanced modulator comprising a pair of tubes whose push-pull outputs are coupled to said grid, means for applying said modulating voltage to said balanced modulator, means for applying carrier frequency currents to said balanced modulator, and means for adjusting said balanced modulator so that the radio frequency voltage coupled into said second amplifier is greater during modulating voltages of one polarity than during modulating voltages of the opposite polarity, whereby the undesired effect of the varying plate potential of said second amplifier, due to currents in said load, is minimized.

5. The combination which includes a source of main carrier frequency currents, a first triode amplifier connecting said source to a load, a second triode amplifier having a plate and grid electrodes whose output is delivered to said load and whose plate is affected by the voltage across said load due to currents from said first amplifier, means including said second amplifier for impressing currents across said load whose phase with respect to currents in said load from said first amplifier is determined by a modulating signal, and means for maintaining constant anode current in said second amplifier in the absence of said modulating signal.

6. The combination which includes a source of main unmodulated carrier frequency currents, an antenna, a first triode amplifier connecting said source to said antenna, a second triode amplifier having plate and grid electrodes whose output is delivered to said antenna and whose plate potential is affected by the voltage across said antenna, a source of modulating voltages, means for energizing the grid of said second triode by a voltage which produces output currents in phase with currents in said antenna due to said first amplifier when said modulating voltage is of one polarity, and output currents out of phase with currents in said antenna due to said first amplifier when said modulating voltage is of the other polarity, and means for maintaining constant plate current in said second amplifier in the absence of said modulating voltages.

7. A device of the character described in claim 6 which is further characterized in that said means for maintaining constant plate current is a radio frequency bias.

8. A device of the character described in claim 6 which is further characterized in that said means for maintaining constant plate current is a radio frequency bias obtained from said source of main carrier frequency current. I

9. A device of the character described in claim 6 which is further characterized in that said means for maintaining constant plate current includes a phase changing network which produces a 90 6 which is further characterized in that said means for maintaining constant plate current includes a phase changing network which produces a 90 phase lead and which derives its voltage from the input of said first triode amplifier.

11. The combination which includes a source of main carrier frequency, an antenna, a first triode amplifier and an impedance inverting network connecting said source to said antenna, a source of modulating voltages, a second triode radio frequency amplifier having grid and anode electrodes, means for impressing the output of said second triode amplifier across said antenna, the phase of the radio frequency output of said second amplifier being determined by said modulating voltages, and means for causing said second triode amplifier 'to deliver the same amount of radio frequency current to said antena independently of the potential appearing on said anode due to currents in said antenna from said first amplifier.

12. A device of the character described in claim 5 which is further characterized by means for obtaining audiodegenerative feedback, and means for stabilizing said feedback in the presence of overmodulation.

13. A device of the character described in claim 3 which is further characterized by means operated by a signal soupled from said load for obtaining overall audio degenerative feedback, and means for reducing the percent modulation of said coupled signal to prevent overmodulation thereof.

ARTHUR. W. VANCE.

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