US2207498A - Modulation system - Google Patents

Description

July 9, 1940. J 2,207,498

MODULATION SYSTEM Filed June 16, 1938 SUPPLEMENTARY MODULATED POWER 40 5 sup smas 3 ILL 1 fico 2 4E RE MODULATING MODULATl/VG .1 Q 2 ma POTENMLS POTENTIAL MODULATOR FREQUENCY AMPLIFIER .souRcs "C I i Q Q g m w fi Q 3 w PERCE/VMGE} MODULATION J Egg/$ BY CARR/ER CONDITION I A TTORNEY.

Patented July 9, 1940 UNITE STATES PATENT OFFICE MODULATION SYSTEM of Delaware Application June 16,1938, Serial No. 213,984 In Great Britain June 16, 1937 12 Claims.

This invention relates to radio and other modulated carrier wave transmitters and has for its object to provide an improved transmitter which shall have a high operating efficiency and shall nevertheless not involve the use of the large and expensive iron-cored components, notably large and expensive iron-cored transformers, such as are required with present day known transmitters of comparable efficiency.

According to this invention a modulated carrier wave transmitter comprises a main tube stage which feeds a load, for example, an aerial, and which receives a high frequency drive, the said stage being operated so as a so-called Class C amplifier; an audio frequency choke in the high tension feed circuit of said main tube stage; a modulator tube stage which is associated with the main tube stage so as to apply anode modulation thereto by the choke method, the said modulator tube stage being so biased as to effect modulation substantially only over the negative half cycles of the envelope; and a supplementary modulated power supply stage which receives a modulated high frequency drive in phase with the drive on the main tube stage and is connected to supply modulated power to the load, said supplementary tube stage being so biased as to take increased feed and thus deliver power to the load during positive half cycles of. modulation, and the Whole arrangement being such that said supplementary tube stage by delivering power to the load, causes the main tube stage to draw on energy stored in said audio frequency choke so that said main tube stage delivers increased power to the load in accordance with the power delivered by the supplementary tube stage. The drive to the main tube stage may be a pure carrier drive or it may be a modulated carrier drive. In the latter case the drive to the said main tube stage and that to the supplementary stage may be taken from the same source. The power supplied by the supplementary stage to the load may be supplied thereto via the anode circuit of the main tube stage.

The invention is illustrated in the accompanying drawing in which Figure 1 shows diagrammatically and by way of example, one circuit arrangement in accordance with the invention and Figure 2 is an explanatory graphical figure.

Referring to Figure 1 the anode i of the tube 2 in the main tube stage is connected to the positive terminal 3 of a high tension supply source (not shown) through a radio frequency choke 3 and an audio frequency choke 5 in series.

The grid 6 of the main tube stage is connected;

to the cathode 1 thereof through a grid leak resistance 8 the cathode i being connected to the negative terminal 9 of the high tension supply. The anode circuit of the main tube stage includes a coupling condenser l which is connected on one side to the anode l and on the other to one end of a parallel tuned circuit l I, I2, whose other end is connected to the negative terminal 9 of the high tension supply. The load circuit is fed from this parallel tuned circuit and may be regarded as a resistance in shunt thereacross. In Fig. l, the load is represented by a resistance I3 thus connected. High frequency unmodulated carrier drive from any suitable source i is supplied to the control grid 6 of the tube 2 through a condenser IB and a carrier drive from the. same source is also supplied to a modulated amplifier ll of any convenientknown type whose modulated output is fed byway of a condenser it to the control grid 19 of the tube 20 of a supplementary modulated power supply stage the anode 2| of which is connected to the high tension positive terminal 3 through a radio frequency choke 22. The cathode 23 of, the tube 20 is connected to the high tension negative terminal 9 and the control grid [9 of this tube is connected to said negative terminal 9 through a radio fre-v quency choke 24 in series with a negative bias source 25 of such value as to bias the tube 29 to twice the cut-off value. The anode 2! of the supplementary tube 20 is connected through a condenser 26 to a point 21 on the inductance II in the tuned circuit l I, I2, this point 21 being about halfway down on the inductance. Speech or other modulating signals are supplied overlead 28 to a modulation potential amplifier 29 of any suitable known type, output from which is taken to the modulated amplifier l1 and also through a condenser 30 to the control grid 3! of the tube 32 of a modulator stage whose anode 33 is connected through the audio frequency choke 5 to the high tension positive terminal 3. The control grid 3| of the modulator-tube 32 is connected through a resistance 34 and a negative bias source 35 to the cathode 36 and the negative terminal 9 of the high tension supply. The bias of the modulator tube 32 is set approximately to cut-oil.

With this arrangement the main tube 2 receives constant drive and operates as a high efficiency Class C amplifier the ratio of peak radio frequency voltage to high tension voltage on its anode I being about 0.9. The drive and bias upon the supplementary tube 20 are such that a small anode feed flows to this stage in the zero modulation or carrier condition and since the tap at 21, from the anode of this stage upon the tuned circuit ll, l2, in the anode circuit of the main tube stage is about half way down on the inductance H the ratio of radio frequency peak voltage to high tension voltage on the anode 2! of the supplementary tubeZD will be about 0.45 in the carrier condition. During positive half cycles of modulation the supplementary stage is driven harder" and accordingly takes increased feed and delivers increased power to the load and accordingly the ratio of radio frequency voltage to high tension voltage on the anode l of the main tube stage is increased and the said main tube stage tends to take decreased feed. This, however, is resisted by the action of the audio frequency choke 5 which automatically raises the potential at its end which is nearer the anode l of the main tube stage so as to maintain the feed constant until, with positive peaks of modulation, the potential at the said end is approximately twice the high tension voltage. In this condition the main tube stage is delivering very nearly twice carrier power into the load and a similar amount apprommately to twice carrier power is supplied by the supplementary stage. The choice of the ratio of 0.45 already referred to, as occurring in the carrer condition, allows a limiting ratio value of 0.9 to be reached in the 100% modulation condition. The results obtained are graphically represented in Figure 2 in which curve AC2 is a curve of anode current in the tube 2; curve A020 is a curve of anode current in the tube 2!]; curve AC32 is a curve of anode current in the tube 32; curve P2 is a curve of power delivered by tube 2 to the load; curve P20 is a curve of power delivered by tube 20 to the load; andTP is a curve of total power in the load; all the curves being drawn against percentage modulation as abscissae. It will be observed that AC=2AB=4ED. As will'be seen the anode current of the tube 20 rises from practically zero at carrier level (point E) to a maximum at the positive peaks of 100% modulation; while for the negative half cycles the anode current of the modulator tube 32 rises approximately rectilinearly from almost zero at carrier level (point E) to a maximum at the negative peaks of 100%. During modulation over the range from 100% negative to 100% positive in the modulation cycle the anode current of the main tube 2 rises approximately rectilinearly from zero (at the negative end of this range) to a maximum at the carrier condition, remaining substantially at that maximum over the positive half of the said range. For any depth of modulation the potential at that end of the audio frequency choke 5 nearer the anode I of the main tube 2, varies symmetrically about the high tension supply voltage and the feed to the main tube 2 and to the modulator tube 32 remains very nearly constant over each modulation cycle. The average resistance provided by these two stages is, therefore, practically unaffected by modulation and the carrier level will not materially vary during changing conditions of modulation. The power drawn from the audio frequency choke 5 during positive half cycles of modulation is applied thereto during negative half cycles.

As already stated the main tube 2 may receive, if desired, a modulated drive instead of a pure carrier drive. To supply this modulated drive the switch S is moved from the contact CD by which carrier drive is supplied to the grid 6 of tube 2 to the contact MOD so that modulated carrier drive is supplied to the control grid 6.

Further, the anode 33 of the modulator tube 32 may be taken to a tap 40 on the audio choke 5 a short distance from that end thereof which is nearer the anode I of the main tube 2 and this tap may be so chosen as to permit 100% modulation without requiring that the anode 33 should fall to zero potential on the negative peaks. To connect the anode 33 of tube 32 to the tap 48 the switch SI is moved from the contact M to the contact 42.

Obviously the various stages need not be single tube stages as shown for any or all of them may include a plurality of tubes in parallel or other suitable connection as may be desired.

It is worthy of note that transmitters in accordance with this invention can be designed to give an efificiency under all conditions of modulation which is as good as and indeed rather better than that obtained with the well known socalled Class B modulator system but nevertheless does not involve the use of the expensive and large push-pull speech transformer required by the said known Class B modulator system. Further, although the main tube stage supplies almost all the carrier power it need be rated at only half the rating of the main amplifier stage in a corresponding known Class B modulator system because in the present case there is no increase in the mean or peak anode current during positive peaks of modulation so that the main tube stage can be designed to approach saturation even in the carrier condition. Broadly speaking it may be said that the main tube stage and the supplementary stage together correspond approximately to the main amplifier stage of a known Class B modulator system while the modulator stage corresponds to one half of the push-pull modulator in the said known system so that the economy in apparatus as compared to the said known system is equivalent to about one half of the modulator of the said known system.

I claim:

1. A modulated carrier wave transmitter comprising a main tube stage having in its anode circuit a tuned circuit from which a load is fed, the anode of said stage being connected to a source of anode potential through a radio frequency choke and an audio frequency choke and said stage being operated as a Class C amplifier, means for supplying a carrier frequency drive to the grid of said main tube stage, a supplementary tube stage which is biased to approximately twice cut-off value and has its anode connected through a radio frequency choke to the source of anode potential, and coupled to a point about half way down on said tuned circuit, means for supplying a modulated carrier drive to the grid of said supplementary stage, and a modulator tube stage biased approximately to cut-off and connected to receive a modulating signal input on its grid, the anode of the modulator stage being connected to the source of anode potential through said audio frequency choke.

2. A modification of a trasmitter as claimed in claim 1 wherein the anode of the modulator stage is connected to the source of anode potential through only part of said choke.

3. A modification of a transmitter as claimed in claim 1 wherein the main tube stage receives a modulated drive instead of a pure carrier drive.

4. In a modulation system, a main amplifier tube having input and output electrodes, an auxiliary amplifier tube having input and output electrodes, a source of modulating potentials and a source of wave energy to be modulated, a

load circuit coupled to the output electrodes of both of said tubes, means supplying direct current voltages to the output electrodes of both of said tubes, means interconnecting said sources and said tubes, means for biasing the electrodes of said auxiliary amplifier tube to such a potential difference that said auxiliary amplifier tube supplies energy to said load circuit only in the presence of wave amplitudes of carrier amplitude or greater whereby the effective impedance presented by the load circuit to said main tube increases on upward swings of the modulating potential, means for holding the direct current supply to said main tube substantially constant on said upward swings, and means for decreasing the plate voltage of said main tube on downward swings of said modulating potentials.

5. In a modulation system, a load impedance, a source of carrier wave energy to be modulated, a source of modulating potentials, a main electron discharge tube having input electrodes and having output electrodes coupled to said load impedance, means for impressing high frequency wave energy on said input electrodes, an auxiliary amplifier tube having input electrodes and having output electrodes coupled to said load impedance, means for supplying a substantially constant amount of direct current to the output electrodes of said main tube, means connected with said sources and the input electrodes of said auxiliary tube for supplying modulated wave energy to said input electrodes of said auxiliary tube, means for biasing said auxiliary tube to pass substantially zero current in the presence of wave energy or carrier wave amplitude or less, a modulator tube having output electrodes connected with said direct current supply means and having input electrodes coupled to said source of modulating potentials, and means for biasing electrodes of said modulator tube to pass current only on the negative portion of the modulation potential cycle.

6. A modulated carrier wave transmitter comprising a main electron discharge tube amplifier stage, means for exciting said tube by a voltage of carrier wave frequency, a load circuit coupled to said tube to be excited thereby, means for operating said tube as a Class C amplifier, and audio frequency choke in a direct current feed circuit for said main tube, a modulator tube connected to said direct current feed circuit to apply anode modulation to said main tube by the choke method, means for biasing the said modulator tube and controlling the same to effect modulation substantially only over the negative half cycles of the modulation envelope, a supplementary modulated power amplifier tube excited by a modulated voltage of carrier wave frequency in phase with the exciting voltage on the main tube, means connecting said last amplifier tube to said load circuit to supply modulated power to the load, and means for biasing said supplementary tube to take increased feed and thus deliver power to the load substantially during positive half cycles of modulation only whereby said supplementary tube stage by delivering power to the load causes the main tube stage to draw on energy stored. in said audio frequency choke so that said main tube stage delivers increased power to the load in accordance with the power delivered by the supplementary tube stage. 7. A transmitter as recited in claim 6 wherein the drive to the main tube stage is a pure carrier drive.

8, A transmitter as recited in claim 6 wherein the drive to the main tube stage is a moduin the main tube has an anode circuit coupling it to the load circuit and wherein the power supplied by the supplementary stage to the load is supplied thereto via the said anode circuit of the main tube stage.

11. In a modulation system, a main amplifier tube having input electrodes and having output electrodes connected with a substantially constant direct current source, a source of modulating potentials and a source of wave energy to be modulated, means for impressing wave energy to be modulated on said input electrodes, a load circuit coupled to the output electrode of said amplifier tube, an auxiliary tube having input electrodes excited by wave energy from said source of wave energy and having output electrodes coupled to said load circuit, means for increasing the output of said auxiliary tube on the upward swings of said modulating potentials whereby said auxiliary tube feeds energy to said load circuit on the upward swings of said modulation potential an additional tube having electrodes connected with said direct-current source and having its impedance reduced by said modulating potentials on the downward swings of said modulating potentials to increase the current drawn by said additional tube from said direct current source and thereby reduce the output of said main tube on the downward swings of said modulating potentials. j

12. In a modulating system, a main amplifier tube having electrodes, a source of high frequency voltage coupled to electrodes of said tube to be amplified by aid tube, means for applying operating potentials to said electrodes, a load circuit for said main tube, a source of modulating voltage, an auxiliary amplifier'tube coupled to both of said sources, means for biasing said auxiliary amplifier tube highly negative sothat said auxiliary amplifier tube amplifies high frequency voltage derived from said source only during substantially one half of each cycle of said modulating voltage, means coupling the output of said auxiliary amplifier tube to said load circuit, a modulator tube having electrodes connected with the electrodes of said main tube and to the means for applying operating potentials to the electrodes of said main tube, and means for reducing the impedance between the electrodes of said modulator tube during substantially the other half of each cycle of modulating voltage to thereby divert current supplied by said means for applying operating potentials to the electrodes of said main tube from said main tube and thus lower said main tubes output during said other substantially half cycle.

JAMES TWA'I'T.

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