US2002209A - Modulating system - Google Patents

Description

y 1935. J. G. NORDAHL 2,002,209

' MODULATING SYSTEM Filed April 4, 1935 //v VENT-0R J. G. NORDAHL ZZX M A TTORNEV Patented Ma 21,1935

MODULATING SYSTEM John G. Nordahl, Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, a corporation of New York Application April 4, 1933, Serial No. 664,292 Claims. (01.179-171) This invention relates to modulating systems particularly to modulating circuits for radio tele phone transmitters. i

An object of the inventionis to simplify the '5 control of the power output of a radio telephone transmitter.

In a specific preferred embodiment of thisinvention in a radio telephone transmitter, the

voice or signal wave. is amplified and supplied 10 to the plate circuitof a radio frequency amplifier or oscillator tube to modulate the radio frequency Wave in the plate circuit of the tube, boththe radio frequency and audio'frequency tubesbeing supplied with space current from the same direct current source. In order to control the power output of the transmitter without varying the degree or percentage of modulation, a

variable impedance element is included'in the common direct current and audio frequency circuit of the modulator tube. This permits'the simultaneous control offthe direct current and audio frequency voltages supplied to the modulatorand consequently the simultaneous control of the radio and audio frequency currents. With a proper design of the circuit elements, it ispossible by adjusting the variable impedance to so varythe audio frequency voltage supplied to the radio frequency tube in proportion to the variation in the direct current plate voltage as to per ,mit the transmitter to be adjusted for different power outputs while maintaining the same percentage modulation.

The invention can be more readily understood by reference to the following detailed description in connection with the drawing which shows diagrammatically one embodiment of the inventtion in a radio telephone transmitter. a

l A source of radio frequency wave I, such as a crystal controlled oscillator and buffer amplifier of theusual type, supplies radio frequency waves tolthe grid circuit of a tube 2 of. the screen grid type which is operated as a modulating .amplifier. The modulated radio frequency output of the tube 2 is applied to the grid. circuit of 45fa power amplifier tube 3 through a tuned circuit I14. and the resultant amplified energy issupplied to the transmission line ficon'nected to an antenna f.(not shown) through a tuned, output circuit 6. Q. Modu'lation"'is accomplished in t-thehmodulating "amplifier tubev 2 by superimposing the amplified audio frequency energy onthe direct current volt- Ya g essupplied to the plate and screen grid electrodes thereof. The modulating audio frequency 55. voltage is obtained ,from the audio frequency ;,jampl ifier This amplifiercomprises twotubes 8 and 9 connected in push-pull relationand having a grid circuit supplied with the audio frequency wave from a microphone ll] through the input transformer H. The plates of the tubes 8 and 9 are connected to the primary wind- 5 ing of the output transformer I2.

The cathodes of the tubesZ, 3, 8 and 9 are heated by means of alternating current supplied from the source 13 through the transformer {4.

Two condensers I5 and I6 connectedin series 10 across the cathode circuit and having their midpoints connected to ground and to the midpoint of the secondary winding of transformer l4 provide direct and alternating current connections to the effective midpoint of the cathodes of the 15 tubes. Space current for the audio frequency amplifier tubes 8 and 9 and the modulator tube 2 is supplied from the battery I! through a circuit which will be described in detail later. Grid biasing potential for the modulator tube 2 is ob 20 tained from the battery l8 through the choke coil l9. Grid biasing potential for the audio frequency amplifier tubes 8 and 9 is obtained from the battery 20. Space current for the power amplifier tube 3 is obtained from the battery 2| 5 through the connection to the midpoint of the inductance coil of the tuned circuit 6. A bypass condenser 22 provides an effective alternating current ground connection to themidpoint of the inductance of the tuned circuit 6 and bypasses the alternating current around thebattery 2|. Grid biasing potential for the power amplifier tube 3 is obtained from the battery 23 through the right-hand inductance coil of the tuned circuit 4; the condenser 24 providing alternating current bypass around the battery 23, and an effective alternating current ground connection to an intermediate point of the inductancesof the tuned circuit 4. Blocking condensers 25 and 26 are provided in the tuned circuit 4 for isolating the plate of the tube 2 from the grid of the tube 3 for direct current voltages.

the tuned coupling circuit 4. A condenser 28 is provided to balance the plate to cathode capacity of the tube 2 to keep the coupling circuit 4 balanced with respect to ground. In the output cir- 0 cuit of the power tube 3 a condenser 29 is provided to similarly balance the anode to cathode capacity of the tube 3 to keep the outgoing line 5 balanced with respect to ground.

The direct. currentspace circuit of the audio frequency amplifier tubes 8 and 9 is completed I from the grounded cathode terminal through battery I! and the two halves of the primary windings of the transformer l2 in parallel to the anode of the tubes 8 and 9, the alternating current plate path being completed from the mid terminal of the primary winding of transformer l2 through resistance 30 and the bypass condenser 3|.

The direct current is furnished to the plate and screen grid circuits of the modulator tube 2 fromv the grounded cathode terminal through battery I l, resistance 30, variable resistance 3 2; left-hand induction of the tuned coupling circuit 4 to the plate of the tube and resistance 33 to the screen of the tube 2. Radio frequency in the 'pl'ate cir cult of the modulating tube 2 is bypassed through the condenser 34 and in the screen circuit through the condenser 35. The audio frequency voltage for modulating the carrier wave in the modulating tube 2 is supplied from the secondary winding 'of the transformer l2, the audio frequency circult being completed from the lower terminal of the secondary winding of the transformer l2,

variable resistance 32 and bypass condenser 3 l to the grounded cathode terminal. Thus modulation is attained by impressing the audio frequency voltage on both the plate and screen circuits of the modulating amplifier as disclosed and claimed in thecopending application of W. L. Lawrence, Serial No. 584,099 filed December 31, 1931, Patent 1,923,53 of August 22, 1933.

It will be observed that the variable resistance 32 is included in both the direct current and audio frequency circuits of the modulator tube 2 so that "an adjustment of the resistance 32 will vary the direct current and audio frequency components of the plate and screen voltages for the modulating amplifier tube simultaneously, thus permitting the power output of the transmitter to be varied while maintaining constant the percentage modulation.

Since the efficiency of the modulating amplivfier 2 varies with the direct current plate voltage,

the radio frequency voltage available in the plate circuit of the modulating amplifier tubev will not be a linear function of the direct current voltage. As aresult, it is necessary, in order to maintain the percentage modulation constant for varying power outputs, to vary the direct current and audiofrequency components of theplate voltage .atdifferent rates. This is accomplished by the use of the resistance element 30 which is included in the direct current path of the plate circuit of the modulating amplifier tube 2 but not in the audio frequency path since it is shunted by the audio frequency bypassing condenser 3|.

L thepercentage modulation is further facilitated by the fact that the audio frequency amplifier "I can be designed to have an output which is inversely proportional to its load impedance. When so operated, an increase in the resistance of the variable resistance element 32 will decrease the audio frequency voltage applied to the modulating amplifier 2 not only by reason of its voltage dividing action but also because of the decrease in the output of the audio frequency amplifier resulting from the increase in the load impedance. By taking advantage of either or both of these factors; namely, the effect of resistance element 30 and design of the audio frequency amplifier 1, great flexibility in design may be attained.

What is claimed is: 1. A modulating system comprising an electric discharge device, means for impressing a carrier wave upon said device, a transformer, means for impressing a signal wave upon the primary winding of said transformer, a source of direct current, a series circuit including said source of direct current and the secondary winding of said transformer connected between the anode and cathode of said device, and means included in said series circuit for simultaneously varying at different rates the direct current voltage from said source and the signal frequency voltage from said transformer impressed upon said device.

2. A modulating system comprising an electric discharge device, means for impressing a carrier wave upon said device, a series circuit connected between the anode and cathode of said device, said series circuit including a source of signal waves for modulating said carrier wave and a source of direct current for supplying space current to said device, and means including a variable resistance element for simultaneously varying at different rates the signal current voltage and the direct current voltage impressed upon ,said device.

3. A modulating system comprising an electric discharge device, means for impressing a carrier wave upon said device, a series circuit connected between the anode and cathode of said device, said series circuit including a source of direct current for furnishing space current to said device and a source of signal waves for modulating the source of direct current for supplying space current and shielding electrode biasing potential to said device, means for supplying carrier Waves to the input electrode of said device, a source of signal frequency voltage for modulating said carrier waves, a circuit including said source of direct current and said source of signal frequency voltage for supplying direct current and signal frequency voltages between the cathode and anode and between the cathode and shielding electrode of said device, and a variable impedance elementconnected in said circuit to simultaneously vary the direct current and signal frequency components of the voltages supplied between said cathode and anode and between said cathode and shielding electrode,-

j 5. A modulating system comprising an electric discharge device having a cathode, an anode and an input circuit, means for impressing carrier waves on said input circuit, a circuit including a source of'direct current and a source of signal frequency waves for impressing a voltage having a direct current component and a signal frequency component between said cathode and anode to modulate the carrier waves, and means for simultaneously varying the direct and signal current components of said voltage in such proportions as to vary the power output of said device while maintaining substantially constant the percentage modulation of the resultant wave.

6. A modulating system comprising an electric discharge device having a cathode, an anode, a control electrode and a shieldingelectrode located between the control electrode and anode, means for supplying carrier waves between said cathode and control electrode,-a source of direct current, a source of signal frequency waves for modulating said carrier waves, a circuit including said source of direct current and said source of signal frequency waves and having terminals connected to said anode, shielding electrode and cathode, respectively, for supplying between the cathode and anode and between the cathode and shielding electrode voltages each having a direct current component and a signal frequency component, and means including a variable impedance element connected in said circuit for simultaneously varying said direct current and signal frequency components in such proportions as to maintain substantially constant the percentage modulation while varying the power output of said electric discharge device.

7. A radio telephone transmitter comprising a source of audio waves, an electric discharge device for amplifying audio waves from said source, a second electric discharge device having an anode, a cathode and control electrode, means' for impressing carrier waves between the cathode and control electrode of said second device, a source of direct current, connections from said source of direct current to the first electric discharge device for supplying space current thereto, other connections from said source of direct current to the cathode and anode of said second electric discharge device for supplying space current thereto and including means for impressing between the anode and cathode the amplified audio frequency output of said first electric discharge device, and means included in said other connections for simultaneously varying the audio frequency and direct current voltages impressed upon said second electric discharge device in such proportion as to maintain substantially the perfrom said source of direct current to the first electric discharge device for supplying space current thereto, other connections from said source of direct current to said second discharge device for supplying direct current voltages between the cathode and anode and between the cathode and shielding electrode thereof, said other connections including means for impressing between the cathode and anode and between the cathode and shielding electrode of said second electric discharge device, the amplified audio frequency output of the first discharge device, and having a circuit portion common to both the direct current and audio frequency components of the voltages supplied between the anode and cathode and between the shielding electrode and cathode of said second electric discharge device, and a variable resistance element connected in said circuit portion for simultaneously varyingthe direct current and audio frequency components of said voltages in such proportions as to maintain substantially constant the percentage modulation while varying the power output of said second electric discharge device.

9. A,modulating system comprising an electric discharge device having a cathode, an anode and a control electrode, a cathode-control electrode circuit therefor including a source of carrier waves, and a cathode-anode circuit therefor including'a source of direct current, a source of signal frequency current, a path individual to the direct current, a second path individual to the signal frequency current, a variable resistance element common to the direct current and signal frequency paths, and an impedance ele-- ment in the path individual to the direct cur-- rent and having such a value of impedance that the ratio of the resistance value of said variable resistance element to the total signal frequency impedance of said anode-cathode circuit is greater than the ratio of said resistance value to the total direct current impedance of said circuit so that the power output of said electric discharge device may be varied while maintaining substantially constant the percentage modulation.

10. A modulating system comprising an electric discharge device, means for impressing a carrier wave upon saiddevice, a series circuit connected between the anode and cathode of said device, said series circuit including asource of signal waves for modulating said carrier wave and a source of direct current for supplying space current to said device, and means for simultaneously varying at different rates the signal current voltage and the direct current voltage impressed upon said device, said signal current voltage being varied at a greater rate than said direct cur-: rent voltage.

JOHN G. NORDAHL.

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