US2422449A

US2422449A - Frequency modulated transmitter

Info

Publication number: US2422449A
Application number: US456859A
Authority: US
Inventors: George L Usselman
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-10-30
Filing date: 1942-09-01
Publication date: 1947-06-17
Anticipated expiration: 1964-06-17
Also published as: GB552039A

Description

June 17, 1947.

(.7! L. USSELMAN FREQUENCY MODULATED TRANSMITTER Original Filed Oct. 30, 1940 s Sheets-Sheetl EMILE: moprjafid O24 llAA-AA IIvIvIv OZFdJDOOE INVENTOR GEORGE 1.. USSELMAN BY Allllll vvvvvvv ATTORNEY June 17, 1947.

G. L. USSELMAN FREQUENCY MODULATED TRANSMITTER :s Sheets-Sheet 2 Original Filed 001. 50, 1940 IvIIIvIv vIIvvIv INVENTOR GEORGE L/ussewmu BY 3mm 17,; 1947. USSELMAN 2,422,449

' FREQUENCY MODULATED TRANSMITTER Original Filed Oct 50, 1940 5 Sheets-$heet 3 OUTPUT LIMITER MULTIVIBRATOR OSCILLATOR SiGNAL SOURCE mvzmon GEORGE L. USSELMAN av )7. ATTORNEY Patented June 17, 1941 mronnncr MODULATED rasnsmrrnn George L. Usselman, Port Jeiferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application October 30, 1940, Serial No.

363,447, now Patent No. 2,297,926, dated Octoher 6, 1942. Divided and this application September 1, 1942, Serial No. 456,859

10 Claims. (Cl. 179171.5)

The present invention relates to frequency modulators and, more particularly, to a frequency modulator using the multi-vibrator principle.

connected by condenser C2 to the grid 8 of tube V2, while the anode I of tube V2 is connected by condenser CI to the grid I2 of tube VI. The grid I2 of tube VI is connected to its cathode II by a resistance RI, while the grid 6 of tube V2 This application is a division of my United is connected to its cathode I3 by a resistance R2. States application SerialNo. 363,447, filed Oc- The anolies Bdand IIliare conriifigtgd by resistors tober 30, 1940, now United States Patent No. I and an the pr mary w g of a trans- 2,297,926, dated October 6, 1942. former 2|). A source of potential I! is connected Most prior art frequency modulators use rebetween a point on the primary winding of transactance tubes to ,vary the effective reactance of former 20 and the cathodes H and I3 to supply an oscillator circuit to obtain frequency moduplate potential t anodes 5 ML The lation. This usually requires oneor more extra ndary winding of this transformer 20 supplies tubes. In the present invention, in the preferred t output of the modulated multi-vibrator to a. circuits, I apply the audio modulating potentials pair of amplifier t b V5 and V6 directly to the multi-vibrator tubes; th t $0 The condensers CI and C2 and resistances I4, the tubes-which produce the oscillations and g R| and 2 form t t constants of t wherein modulatlon takes Placemy F circuit which determines the frequency of opzz f g g ieactlve and eration thereof. ,The condensers CI and C2 are at a termmed by their size and the size of resistances at ii ii fi g g g freguency of It, I8 RI and R2 During each impulse the era ion 0 e mu 1- ra or circu In a modification, means is included for bal- I condensers Cl and C2 are discharged and ancing amplitude variations produced by change enlarged m revers.e sense {ugh frequency of potential on one tube electrode by opposite n mpmfvlbra'tor clrcmt the condenser and equal variations of the potential on another chargmg mmmt consists 9 the condenser the tube electrode to thereby produce substantially grid reslstor and the opposlte tube anode. frequency modulation only of the oscillations 9 3 g gg a zfigfi gzgg g g z ggg g generated with little or 'no amplitude modulatiom 1f the value of condenser or any resistor is In describing my invention. reference will be 2 1 fi g g g g oscilnator made to the attached drawings wherein: e y W1 0 9 p0 en 5 are Figs. 1 to 4, inclusive, each shows a frequency changed in such a Way as to change the charging modulator comprising a pair of multi-vibrator f the charging rate and frequency of tubes with means for modulating elements in the eration will be changed. If the tube resistances multivibrator circuits to thereby modulate the baplilymg ig s fidtentitals frequency at which they operate. 0 e u e 9 9111611 at W 10 ange he In Fig.4, an external grid circuit resistance of charging rate of the Wildensers by changing the the tubes is modulated, this being one of the ele- Voltage drop in the amide resist due t0 the ments which determines t t constant of the 40 change in tube current. In other words, if the multhvibretor circuits and, consequently, the anode potential is considered as the source of frequency of operation thereof. g l' a flll 'l l t th u lll the condenser CI or In Fig. 2, a resistance in'the plate circuit of an mug he grid eak resistolRl R2 to each tube in the multi-vibrator circuits is varied. it can be seen that a change in the In Fig. 3, the screen grid potentials are moduanode P ten ial, uch as would Occur during lated .to thereby modulate the tube impedan modulation of the tube current, changes the an means 1 provided for stabilizing t mean charging rate of the condensers and thereby frequency of operation of. the multi-vibrator. changes the Oscillator q y- In Fig. 4, the screen grid potentials, are d Another way to look at it is to consider that lated, and means is provided for balancing out the condenser CI or C2 and grid leak resistor RI any amplitude modulation produced. or R2 are connected in l l to the opposite The modulators of all of the systems may feed. tube o e t 084110116 ,tube s tance. This the frequency modulated wave'energy to the n ecombination is connected in series with the tube essary and well known a l yin amplitude v anode supply resistance. This latter resistance limiting and frequency multiplying circuits, some 65 serves to limit the total current so that when the of which have been illustrated. tube current increases, the condenser current In Fig. 1, two tubes. VI and V: hav th i decreases and vice versa. Whena higher conanodes and grids cross-connected by condensers denser discharge rate exists, then a higher fre- CI and C2. That is. the anode 6 of tube VI is quency of push-pull oscillations exists in the multi-vibrator circuit. The cl ent! 9! iiml-llfl vibrator circuit is well known to the art. It is a' respondingly modulate the frequency of oscillation of the multi-vibrator. I modulate the resistances RI and R2 by means of a modula- II. The inductance LI and the variable condenser.C3 form a tank circuit which is tuned to the carrier frequency. Of course-condensers CI, C2 and resistors RI, R2, R3, R4 must also have th correct values to operate at the carrier frequency. The tank circuit LI, C3,acts both I as a frequency stabilizer and as a filter to give tion amplifier 30 having its cathode grounded and having a pair of anodes 32 and 34 connected to points on RI and R2, respectively. The anodes 32 and 34 of tube 30 may derive their potentials from resistors RI and R2 and their currents from the grid circuits of tubes VI and V2 as, shown, or the cathode of tube 30 may beconnected to the negative terminal of a source shunted by an audio frequency bypass condenser, the negativ terminal of which is grounded. The control grid 36 of tube 33 is connected to a source of modulating potentials 31 by way of transformer 38. The source of modulating potentials may represent voice signals, television signals, video signals, etc.

The frequency modulated oscillations are supplied to the grids All and 42 of amplifiers V5 and V6 for amplification therein. The anodes of tubes V5 and V6 supply the amplified modulated oscillations to a transformer Ml and from the transformer 44 by way of a filter 50 to a frequency multiplier and amplifier and limiter 60. I

In Fig. 2, I also illustrate a frequency modulator circuit using a multi-vibrator oscillator. In this circuit the modulation is applied to the anodes 6 and ID of the oscillator tubes VI and V2 through resistors R3..and R4 and R5 and R6. Modulation of tubes 30 and 30' in effect varies the resistance in the anode circuit of the multivibrator, thereby varying the frequency. The output is passed through amplifiers and filters also before it reaches the load.

In Fig. 3, I modulate the screen grids 3 and 5 in phase by modulatingpotentials from the source 31. Changing the screen grid potential changes the resistance or impedance ;of the tubes VI and V2. These tube resistances are part of the time constant circuits and, consequently, partly control the frequency of operation thereof. Thus, the frequency variations correspond to the amplitudes of the modulating potentials. In Fig. 3 the output circuit comprising inductance of the primary of transformer 20, condenser C3 and resistances R5 and R6 is a broadly tuned resonant circuit, the purpose of which is to introduce some degree of frequency stability to thereby hold the oscillator at an approximately fixed mean frequency, to improve the output wave shape of the modulator and yet to permit wide band frequency modulation to be accomplished.

In Fig. 4 the modulating circuits are arranged in such a way as to balance outamplitude modulation.

Now, as a rule, the current and voltage wave shape of a multi-vibrator is not like a sine wave and the frequency stability is poor because it changes with power voltage variations. In the circuit of Fig. 4 it may be noted that the cathodes II and I3, control gridsI2 and I8 and the screen grid electrodes 3 and I5 of tubes VI and V2 are, used for the multi-vibr'ator oscillator circuit of th modulator, while the anodes of tubes VI and V2 are usedin the output circuit of the modulator. The anodes 6 and III of tubes VI and V2 are connected in. push-pull relation to. tuned circuit LI, C3. vThecenter point ,of inductance Li is connected by R1 to power source a substantially sine wave shape output. In other words, the sine wave current oscillations in tank circuit LI, C3 are maintained by and guided by the multi vibrator current oscillations-which are not sine wave shape. To obtain correct operation. the proper ratio of voltages supplied from source I1 must be maintained between the anodes Ii and I0 and the screen grids 3 and 5. Too much control by the oscillator screen grid circuit will result in poor wave shape output. Too much control by the anode circuit will cause reduction of modulator band width.

The signal modulating potentials'are applied in phase to the screen grids 3 and 5 of tubes VI and V2 of the 'multi-vibrator oscillator circuit of the modulator to produce the frequency changes. The signal modulating potentials from source 31 'are applied to audio transformer 38. One terminal of the secondary winding of transformer 38 is coiziected to the screen grids 3 and 5 of tubes VI and V2 through resistors R3 and R4. The other terminal of transformer 38 is I connected to the anodes 6 and III of tubes VI and.

V2 through resistor R1 and through coil LI. The power source I! is connected between the oathodes II and I3 of tubes VI and V2, and some suitable point on the secondary winding LLof transformer 38.

During the modulating cycle, take an instance when the screen grid potential is made more positive. This causes a change in the modulator frequency but it would also cause more current to reach the anodes of tubes VI and V2. However,

the'fact that power source I1 is tapped somewh'ere between the output terminals of transformer 38 means that when the screen grid potentials are modulated in the positive direction, the anodes of tubes VI and V2 are modulated in a negative direction and vice versa. tapped position of power source I! on the secondary winding of transformer 31 is properly chosen, the amplitude modulation is balanced out so that only frequency modulated carrier reaches the antenna. The variable resistor R1 is placed in series with the anode circuit of tubes VI and V2 in order that the averageratio of power delivered to the screen grid circuit and the anode circuit of the frequency modulator, may be properly adjusted. This is essential for proper operation of th( modulator. The arrangement of Fig. 4 does not require a limiter stage for limiting out the amplitude modulation.

What is claimed is:

1. In a frequency modulator, an electron discharge device having an anode, a cathode, a

control grid and a screen grid, circuits including,

capacity and resistance interconnecting the screen grid, control grid, and cathode of said device to form a multi-vibrator oscillator, a source of direct-current potential connecting the anode and screen grid of said device to the cathode. of said device, connections for modulating the potentials on the screen grid and anode of said device in accordance with signals to thereby modulate a constant of said multi-vibrator oscillator and, correspondingly vary the frequency of operation thereof,- and anoutput circuit connected to the anode of said device; w

Now if the 2. In a wave length modulator, agenerator of wave energy to be modulated including a pair of electron discharge tubes each having electrodes including a cathode, a control grid, a screen grid, and an anode, a condenser couplin the control grid of one tube to the screen grid of the other tube, a condenser coupling the control grid of the other tube to the screen grid of said one tube, a source of direct current Dotential, a resistance coupling the screen grids of each of said tubes to a point of positive potential on said source, a resistance coupling the con- .trol grid of each of" said tubes to the cathodes of said tubes, a connection tying the cathodes of said tubes together and to a point on said source of direct current potential, an output circuit coupling the anodes of said tubesin pushpull relation, and a source of modulating potentials coupled to the anodes and to thescreen grids for modulating the potentials thereon.

3. In a frequency modulation system, a pair of electron discharge devices each having a, heated cathode and cold electrodes including an electrode serving as an anode, and a control grid, capacities interconnecting the electrode serving as an anode of each of said devices with the consaid devices to the cathodes of said devices, an

' generated.

trol grid of the other of said devices, resistances connected between electrodes of each of said devices, means for applying direct-current potentials to the electrodes of said devices, an output circuit coupled in push-pull relation with corresponding cold electrodes of said devices, a modulator tube having a control grid, a cathode ,and two anodes, connections for applying modulating potentials to the control grid and cathode of said modulator tube, and couplings between the two anodes of said modulator tube and corresponding electrodes of said devices.

4. In a frequency modulation system, a pair of electron discharge devices each having electrodes including a screen grid, an anode, a cathode, and a control grid, capacities cross-connecting corresponding electrodes of said devices, resistances connected. between the cathodes and control grids of said devices, a source of direct current potential connected to the cathodes of said devices, resistance connecting the anodes of said devices to said sourceof direct current potential for applying direct current potentials to the said anode electrodes of said devices, an output circuit coupled in push-pull relation with the anode electrodes of said devices, a transformer having a primary winding excited by modulat- -ing potentials and having a, secondary winding, a

coupling between a point on said secondary winding and the cathodes of said devices, a coupling between one end of said secondary winding and the screen grids of said devices, and a coupling between the other end of said secondary winding and the anodes of said devices.

5. In a frequency modulation system, a multivibrator including a pair of electron discharge devices each having electrodes including an anode, a cathode and a control grid, a capacitor connecting the control grid of each device toan electrode of the other device serving as an anode in said multi-vibrator, a source of direct current potential having a negative terminal connected to the cathodes of said devices, a resistor connecting each of the electrodes serving as anodes of said multi-vibrator to a positive termiml of said source of direct current potential for applying positive direct current potentials to the said electrodes oi. said devices serving as anodes,

a resistor connecting the control grid of each of 6,. A system as recitedin claim 5 wherein said corresponding cold electrodes modulated in synchronism are the control grids of said devices.

7. In a frequency modulation system, a multivibrator including a pair of electron discharge devices each having electrodes including an anode, a cathode, a screen grid and a control grid, a capacitor connecting the control grid of each device to an electrode of the other device serving as an anode in said multi-vibrator, a source of direct current potential having a negative terminal connected to the cathodes of said devices, a resistor connecting each of the electrodes serving as anodes of said multi-vibrator to a positive terminal of said source of direct current potential for applying positive direct current potentials to the said electrodes serving as anodes, a resistor connecting the control grid of each of said devices to the cathodes of said devices,

an output circuit coupled with the anode electrodes of said devices, the arrangement being such that oscillations are generated in said de-' vices and circuits of a frequency depending in parton the circuit impedance, connections to corresponding cold electrodes of said devices for varying the potentials thereon in accordance with signals to correspondingly vary the circuit impedance and the frequency of the oscillation generated and reactance in said output circuit tunedto the frequency of the oscillations generated to stabilize the same and to provide output oscillations of substantially sine wave form.

8. A system as recited in claim '7 wherein said reactance includes an inductor and capacitor in pull relation.

-9. In a frequency modulation system, a multi-v parallel connecting the output electrodes in pushvibrator including a pair of electron discharge devices each having electrodes including an anode, a cathode, a screen grid and a control grid, a capacitor interconnecting the control grid of each device to the anode of the other device, a source of direct current potential having a negative terminal connected to the cathodes of said devices, a resistor connecting the anode of each device to a positive terminal of said source of direct current potential for applying positive direct current potentials to the anodes, a resistor connecting the control grid 01' each of said devices to the cathodes of said devices, an output circuit coupled in push-pull relation with the anodes of said devices, the arrangement being such that oscillations are generated in said devices and circuits of a frequency depending in part on the circuit impedance, connections to the screen grids of said devices for varying the potential thereon-in phase in accordance with signals to correspondingly vary the circuit impedance, and the frequency of the oscillation generated, and an inductor and capacitor in parallel in said output circuit tuned to the frequency of the oscillations generated to stabilize the same to provide output oscillations of substantially sine wave form.

10. In a frequency modulation system, a multivibrator including a pair of electron discharge devices each having electrodes including an anode, a cathode, a screen grid and a control grid, a. capacitor connecting the control grid of each device to the screen grid of the other device, a. source of direct current potential having a. negative terminal connected to the cathodes of said devices, a resistor connecting the screen grid of each device to a positive terminal of said source of direct current potential for applying positive direct current potentials to the said screen grids of said devices, a resistor connecting the control grid of each of said devices to the cathodes of said devices, an output circuit coupled in pushpull relation with anodes of said devices, the arrangement being such that oscillations are generated in said devices and circuits of a frequency depending in part on the circuit impedance, connections to the screen grids of said devices for varying the potential thereon in phase in accordance with signals to correspondingly, vary the, circuit impedance and the frequency of the oscillation generated, and an inductor and'capacitor in parallel in said output. circuit, tuned to the frequency of the oscillations generated to stabilize the same to provide output oscillations of substantially sine wave form.

GEORGE L. USSELMAN.

REFERENCES CITED The following references are. of record in the file of thispatent:

UNITED STATES PA'I'ENTS Bartelink Jan. 4, 1944