US2248045A

US2248045A - Frequency modulation

Info

Publication number: US2248045A
Application number: US282406A
Authority: US
Inventors: Orville E Dow
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-07-01
Filing date: 1939-07-01
Publication date: 1941-07-08
Anticipated expiration: 1958-07-08
Also published as: FR866850A; DE876716C

Description

July 8, 1941. o. E. now

7 FREQUENCY MODULATION Filed July 1, 1939 2 Sheets-Sheet 1 I MODULATOR V bi. J 1 R R L ww a A" v F 90 A j 6 Z l M 41 M iv J, A I I A J C M Z DIRECT CURRENT v SOURCE s I BL INVENTOR. 0 VI E E. DOW

ATTORNEY.

FREQUENCY 42 MM TIPUERS c AMPLIFIERS, L-rc. Y 5

July 8, 1941. Q ow 2,248,045

' FREQUENCY MODULATION Filed July 1, 1939 2 Sheets- Sheet 2 Fig.5!

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MODULATION ,4, SOURC? (2L I F 55 1 r ,2 f o zzfsws R Fr, +4 D/REC'TCURRENT ZZ saunas 4 Z3 T-\\/90 e =-6Z2 g l 1/ P INVENTOR. ORV/LL'E E. 00w BY ATTORNEY.

Patented July 8, 1941 FREQUENCY MODULATION Orville E. Dow, Port Jefferson, N; Y., assign'or to Radio Corporation of America; a corporationof Delaware Application July 1, 1939, Serial NQ."282,406"

(Cl. Mil -171.5)

13 Claims;

This application concerns a new and improved circuit and means for modulating the frequency of wave energy of any frequency. The system of this invention isparticularly applicable to ultra-high frequency wave signalling. By this system, an extremely wide band of signal potentials may be used and the frequency of wave energy may be deviated linearly in accordance with the said wide band of signal potentials. The frequency range of the modulating potentials may be as used in telephony, television, facsimile; etc.

In its broadest aspect my invention involves an oscillation generator of any desiredtype by means of which wave energy of the desired frequency is produced and modulated in frequency. The frequency of operation of the generator is determined in part at least by reactance in the generator circuit. Where a tube generator is used the react-ance may comprise the electrode leads within and without the tube envelope. The extent of external connections forming the reactance depend in part at least upon the frequency generated. In my invention one of the reactances comprises what has been termed in the art a tube reactance. This tube reactance comprises an electron discharge device having a pair of electrodes connected with the frequency determining circuit to form a part thereof with means for impressing generated oscillations on a pair I of electrodes in the tube in phase displaced relation to produce a reactive effect between the said electrodes connected with the frequency determining circuit. By modulating theimpedance of the reactance tube at signal frequency, the reactance in the frequency determining circuit is varied correspondingly to thereby vary the frequency of the oscillations generated.

A system somewhat similar to the improved modulator of the present application has been disclosed in Crosby United States application #209,919, filed May 25, 1938.

In describing my invention reference will be made to the attached drawings wherein Fig. 1 is a wiring diagram including the principal elements of a frequency modulated oscillation gen erator arranged in accordance with the present invention. Fig. 2 is a vector diagram used in illustrating the relation of the several voltages, currents, etc, involved in the operation of the circuit of Fig. 1.- Fig. 3 is a fundamental diagram equivalent to the circuit of Fig. 1. Fig. 3a is an additional current and voltage vector diagram further illustrating-the relation of the var ious currents and Voltagesinthe oscillator cir-' cuit I of Fig. 1 forthe-prcduction of oscillations and also illustrates themanner'in' which the reactive effect produced in circuit I I-an-d reflected in-cir-cuitl changes the frequency ofopera-. tion of the generator circuits. 1 Fig; e 'illustrates a modified formof" the frequency modulator "of Fig. 1. In Fig. 4-the reactive eifectisinductive whereas it is capacitive inFig. 1;;whi-leFig; 4c is a: vector diagram illustrating the operation of themodiiication of- Fig; 1. I

InFig; 1 numeral I is-an oscillator an'd II is amoduating means for the: osciliator. The 0s: cill-ator may. comprise an electron discharge -,dei vice 4 havingits control grid 6 and anode 8 connected inreactive circuits for the production of oscillations; The circuits may include an inductance. H) which is coupled to an inductance [2; The latter inductance maycbe ofthe output circuit having;; c-onnected thereto-*a pair of output leads. One P int on-the inductance I*2-ris connected as shown to the anode Wot-a discharge device l6 having a pair of electron systems in avsi-ngleenvelope. Separate electron dischargesyst-ems in" individual envelopes may be used "here;-- A point on the-inductance I2 is also connected: by a ph-aseshifting condenser C to the control grid 2010f one of the electron discharge systems. The. condenser C cooperates with a phase shifting resistance R shunted" by an inductance L2, thepurpose of which will be given later. Another point on" the inductance I2 is simiiarly 'connected' to the anode 22 and grid 24 of the other electron 'd'isch-argesystem. A phase shitting? condenser C and resistance R and in du'ctanceLZ is'used here also.

Aswill be described more in detail hereinafter, these two tubesystems form a symmetrical reactance infishunt to. the inductance [2 which is coupled toatheinductance 10 so that the symmetricalfreactanwhas an effect on the-oscillation .generatin'gand frequency modulation circuits includinginductance Ill; As will be pointed out morein detail hereinafter, when the im pedanc-esof the electron discharge systems of thetube are modulated in phase by in-phase modulating potentials; that is, varied in phase at signalnfrequency, the reactive efiectbetween the anodes and cathodes of the electron discharge systems is varied and this reactance eifectbeing apart ofthe reactance of the-circuit including reactance Ill'variesthe frequency of the oscillations generated at signal frequency. Modulat ing potentials are supplied from any desired source, such as for example, as derived by scanning-asubject, by lead 30 through resistances R and inductances L2 to the grids or

control electrodes

20 and 24 in phase.

'As stated above, the frequency of operation of the oscillator is varied by changing the amplitude of the electron current flowing to the two anodes of the tube I'B in accordance with the modulating voltage.

The radio-frequency voltages on the anodes M and 22 and

control grids

20 and 24 of tube l6 are in push-pull, i. e., the voltage on one anode is 180 out of phase with respect to the voltage on the other anode, and the voltages on the two grids are 180 out of phase also, but the voltages 1 on the grid and anode of each system are displaced in phase by substantially 90. An advantage of the balanced circuit is that the mid-point of the grid coils L2 does not need to be by-passed 1 for radio frequency. This keeps the capacity I shunting the video modulator at a minimum.

A voltage is produced across the anodes of l6 j due to the circulating currentin circuit I. The 1 voltage-applied to the control grids of 16 is ad- 1 vancednearly 90 by the condensers 0 audio- 1 sistor R. Therefore, the pulses of current which irea-ch each anode will lead the voltage on the said anode byabout 90 and-the reactance due I to the electron current will be a capacitivereactance; pulses is controlled by the modulating voltage which is applied at 30' tothe control grids in The amplitude of the said current parallel. Thus, if the control grid-sbecome-more positive due to the modulation voltage; the current pulses increase in amplitude and the reactance due to the electron current decreases since the voltages on the anodes of 16 remain about constant in amplitude. This reactance is equal to the other radio-frequency voltage from anode to ground divided by the fundamental component of the current pulses to each anode. This change in reactance results in a shift of the oscillator frequency.

The radio-frequency voltage on the control grids can be increased and shifted more nearly 90 by tuning out the capacity reactance from .grid to cathode which shunts the resistor R. In fact, if the impedance from grid to cathode is made slightly inductive, the phase shift can be made to exactly equal 90. This. is accomplished by grid coils L2 shunting R. The voltage on the control grids may be made to lag the voltage on their associated anodes by reversing the positions of R and C or by crossing the phase shifting circuits so they are connectedbetween anodes and grids of opposite sides. This results in an inductive reactance in the oscillator circuit. A modification for producing this inductive reactive effect has been shown in Fig; 4.

The tube I6 may operate either class A or class C. If it operates class A, the control grids must have a remote cut-off characteristic so that the. alternating current component of electron current 'will vary in amplitude as the operating point is varied by the modulating voltage. The

radio-frequency exciting voltage on the control grids is constant in amplitude. If plate dissipation is the limiting factor, a greater degree of a frequency modulation will be obtained if I6 -is f operated as a class "C amplifier. The operation will be similar to an ordinary grid modulated class Cfamplifier. Of course, frequency modulation can also be obtained by plate modulating 16,

f with the resulting advantage of less distortion and the disadvantage of greater modulating power required.

.The relation of 1 the voltagesand currents in Fig. l is indicated in the vector diagram of Fig. 2. The notations used on the vector diagram correspond to those used on the schematic diagram. The peak radio-frequency voltage from anode to ground of one unit is represented by the vector 1 ep. All voltages and currents for the other unit will be 180 out of phase with the corresponding vectors shown. il is the radio-frequency current which flows from the anode through the phase shifting condenser C. After flowing through C the current il flows to ground over three branches, i. e., control grid to ground capacity, resistance R, and inductance L2. These currents are represented by icy, z'R, and 21.2 respectively. eg is the grid to ground voltage and is in phase with 21?. and leads ep by 90 as is required. The emission current pulses through the tube will be in phase with eg and hence lead ep by 90". Thus the tube impedance due to its convection currents will be a capacitive reactance. 2L2 is larger than tag so that il lagszR slightly. The voltage drop across C is the difference between the vectors eg; and ep and is represented by the dotted line connecting their arrows. It must lag il by 90 since C isa pure capacitive .reactance. Thus z'l must lag z'R slightly. I

Fig. 3 is the equivalent circuit of Fig. 1. The condition for oscillation of circuit I is that e'g is .in phase with z'l as illustrated in Fig 3a. Thus if ZT, ,(the reactance due to the electron current of I6) is varied, 12 will change and the factor 7'wMz'2 will throw e'g out of phase with il. A change in the reactance of LI and CI and hence, in the frequency of operation is necessary to restore the resonant condition. .More in detail in Fig. 3 circuit I corresponds, to the oscillator circuit I of Fig. 1. LI is the self inductance of this circuit, CI the circuit capacity and no the equivalent tube plate resistance and circuit resistance. e'g is the equivalent series voltage generated by the oscillator. As a necessary condition for oscillations to take place e'g must be in phase with the circulating current il or I, i. e., the circuit I including the impedances reflected from circuitII must be in resonance. L2 is the plate: to-plate inductance of circuit II, Fig. 1. C2 is the plate-to-ground capacity of the two elements of tube l6 in series. Z'T is the equivalent series impedance of the modulator tube l6 resulting from the convection currents. The generated voltage eg' is equal to the vector sum of the voltage drop il [r'p-l-yKXLL-XCD] due to the self impedance of I and the counter E. M. F. -jwMZ 2 in I due to the circulating current 22- in II. The voltage of e2 is induced in II by the circulating current 12 of I. 22 is the circulating current in II due to the induced voltage e2. 'XL2 is the reactance of L2 at the operating frequency, X02 the reactance of C2, etc.

As stated before an inductive reactance effect may be producedand modulated at signal frequency to frequency modulate the oscillations produced in I. Such an" arrangement has been shown in Fig. 4. In this modification the positions of R and C' are reversed and the inductance L2 is placed in series with R and its reactance at I the operating frequencymade'equal to the reactance of C.- Ablocking condenser B is used in series withR to keep the plate voltage off of the controlgrid. C may then represent the grid-to-ground capacity of the tube. 'B is the directcurrent blocking condenser which has low impedance to radio-frequency currents; R, L2

and C compose'aseries circuit connected between the plate-and ground. The control grids are connected to this series circuit so as to include only C. The reactance of B2 is made equal to the reactance of C at the operating frequency. The current il which flows in the series circuit will be in phase with the plate-to-ground voltage ep as shown in vector diagram of Fig. 4a. The voltage drop eL2 across L2 is equal to the voltage drop eg across C but 180 out of phase. The voltage eg lags 1'1 and ep by 90 and, therefore, the electron current pulses which reach the plate will lag the plate voltage by 90 and the reactance due to the electron current will be inductive.

The inductances L3 have high impedance to radio-frequency currents, i. e., they act as chokes to the currents of the carrier frequency, and have low impedance to the signal or modulation currents. L3 may be adjusted to act as series peaking coils for the modulating video frequencies. C2 is a blocking condenser which has low impedance for the lowest frequency of the modulating voltage. Hg is a grid leak and E is the bias for the

control grids

20 and 24.

The output of the modulator may be supplied to any circuit. In practice the frequency modulated output of the modulator is supplied to a frequency multiplier followed by a buffer stage and an additional frequency tripler. In a system arranged as disclosed here, the following frequencies were used and operation was found to be very satisfactory throughout the range involved. The oscillator per so was operated at 55.5 megacycles per second. This oscillator was frequency modulated in accordance with television signals involving a band width of about 4 megacycles. The modulation index was .111. The output of the oscillator as modulated, was then tripled to a mean frequency of 166.6 megacycles per second, the carrier now having a modulation index of .333. A buffer amplifier stage was excited by this energy and was followed by a tripler so that a carrier frequency of mean frequency of about 500 megacycles per second was obtained with an index of modulation less than 1 covering a total frequency spectrum of the order of 8 megacycles.

What is claimed is:

1. In a frequency modulation system an oscillation generator including a frequency determining circuit, and means for modulating the oscillations generated including a pair of electron discharge tube systems each having an anode, a cathode and a control grid, means coupling the anodes of said tube systems to points of substantially opposed radio-frequency voltages on said circuit, means for applying radio-frequency voltages to the control electrodes of each of said electron discharge systems, the voltages on the control electrodes and anodes of the respective systems being displaced by substantially 90, and means for modulating the impedances of the electron discharge systems in phase at signal frequency.

2. In a frequency modulation system an oscillation generator including a wave length determining circuit, and means for modulating the wave length of the oscillations generated including a pair of electron discharge tube system each having an anode, a cathode and a control grid, means coupling the anodes of said tube systems to points of substantially opposed radio-frequency potential on said oscillation generator circuit, a condenser and resistance in series coupled to said circuit, a coupling between a point on said series connection and the control grid of one of said electron systems, a second condenser and resisttime in series coupled to said circuit, a. coupling between a point on said last series connection and the control grid of the other tube whereby the voltages on the anode and control grids of the respective electron systems are displaced by substantially and means for modulating the impedances of the electron discharge systems in phase at signal frequency.

3. A system as recited in claim 2 wherein the capacitive reactance between the grid and cathode of each system is tuned out by an inductance to thereby obtain a more exact 90 relation between the voltages on the anode and grid of each electron discharge system.

4. In a frequency modulation system an oscillation generator including a frequency determining circuit, and means for modulating the oscillations generated including, a pair of electron discharge tube systems each having an anode, a cathode and a controd grid, means coupling the anodes of said tube systems to points of substantially opposed radio-frequency potential on said oscillation generator circuit, resistive means for applying voltages from the anodes of said tube systems to the control electrodes of said electron discharge tube systems, capacitive; means coupling the control electrodes of said. tube systems to the cathodes of said tube systems,. and means for modulating the impedances of the.- electron discharge tube systems in phase at signal;

frequency.

5. In a frequency modulation system an oscil-- lation generator including an electron dischargedevice having electrodes connected in regenerative circuits for the production of oscillations, an inductance in said circuits, and means for modulating the frequency of the oscillations, including a pair of electron discharge tube systems each having an anode, a cathode and a control grid,- means coupling the anodes of said discharge tube systems to points of substantially opposed highfrequency potential on said inductance, a condenser connecting the anode of each of said tube systems to the control grid of each of said tube systems, a resistance connected between the control grid and cathode of each of said tube systems, the values of said condensers and resistances being such as to produce a substantially phase quadrature relation between the voltages on the anodes and control grids of the respective electron discharge tube systems, and a source of modulating potentials connected in phase between the control grids and cathodes of the tube systems.

6. In a frequency modulation system an oscillation generator including an electron discharge device having electrodes connected in regenerative circuits for the production of oscillations, an

inductance in said circuits, and means for modu-.

lating the frequency of the oscillations including a pair of electron discharge tube systems each having an anode, a cathode and a control grid, a second inductance coupled to said first inductance, means connecting the anodes of said elec tron discharge tube systems to points of substantially opposed high frequency potential on said second named inductance, a condenser connecting the anode of each of said tube systems to the control grid of each of said tube systems, a resistance connected between the control grids and cathodes of each of said tube systems, the values of said condensers and resistances being such as to produce a substantially phase quadrature relation between the voltages on the anodes and control grids of the respective electron discharge tube systems, an. inductance in shunt to each of said resistances the purpose of the said inductances being, to tune out the capacity between thegrid and cathode of each of said tube systems to thereby insure a substantially phase quadrature relation between the voltages. on the anodes and control grids. of each tube system, and a source of modulating potentials connected inphase between the control grids and cathodes of the tube systems.

7. In a frequency modulation system an oscillation generator including an electron discharge device having electrodes coupled in regenerative circuits for the production of oscillations, a reactance in saidcircuits, and means for modulatingthe frequency of the oscillations including a pair of electron discharge tube systems each having an anode, a cathode and a control grid, means'coupling the anodes of said electron discharge tube systems to points of substantially opposed high-frequency potentialon said reactance, a resistance coupling the anode of each of said tube systems to the control grid of each of saidtube systems, a condenser connected between the control grid and cathode of each of said tube systems the values of said condensers and resistances being such as to produce a substantially phase quadrature relation between the voltages on the anodes and control grids of the respective electron discharge tube systems, and a source of modulating potentials connected in phase between the control grids and cathodes of the tube systems. V

Y 8..In a frequency 'modulation system an oscillation generator including an electron discharge device having electrodes connected in regenerative circuits for the production of oscillations, a reactance in said circuits, and means for modulating the frequency of the oscillations including a pair of electron discharge tube systems eachhaving an anode, a cathode and a control grid,

a second reactance coupled to said first reactance, means connecting the anodes of said discharge tube systems to points of substantially opposed high frequency potential on said second named reactance, a resistance and inductance in series connecting the anode of each of said electron discharge tube systems to. the control grid of eachof said electron discharge tube systems,

a condenser coupled between the control grid and cathode of each of said electron discharge tube systems, the values of said condensers, inductances, and resistances being such as to produce a substantially phase quadrature relation between- ,the voltages on the anodes and control grids of i anode of eachrof said tubes to the control grid of each of said tubes, a. condenser coupled between the controlgrid and cathode of each of said tubes, thevalues of said resistances; inductances and condensers being such as to produce. a substantially phase quadraturerelation between the voltages on the anode and control. grid of each of the tubes, a circuit: including; amindnctance. connected in shunt to each of said condensers, and means for impressing controlling potentials on said last named inductances for controlling the gain of said tubes to control the valueof the inductive reactance'between the anode and cathode of each of said tubes.

10. A variable reactance including, a pair of electron discharge tubes each having an anode, a cathode and a control grid, means for applying alternating voltages substantially in phase opposition to the anodes of said tubes, a condenser connecting the anode of each of said tubes to the control grid of each of said tubes, a resistance connected between the control grid and cathode of each of said tubes, the values of said condensers and resistances being such as to produce a substantially phase quadrature relation between the voltages on the anode and control grid of each of said tubes, a circuit including an inductance connected between the control grid and cathode of each of said tubes, the purpose of said inductances being to tune out the capacities between the grid and cathode of each of said tubes to thereby insure the phase quadrature relation between the voltages on the anode and control grid of each of said tubes, and means for controlling the gain of said tubes in phase to controlthe value of the reactances between the anode and cathode of each of said tubes.

11. In a frequency modulation. system an oscillation generating circuit wherein oscillatory energy to be modulated flows, said circuit having reactance, and means for modulating the reactance of said circuit and thereby modulating the frequency of the oscillations generated in'said system including, a pair of similar electron discharge tube systems each having a plurality of electrodes including a control electrode and cathode, means for impressing voltages from said oscillation generating circuit on a first two corresponding electrodes one in each of said electron discharge tube systems, other means for impressing voltages from said generatingcircuit on a second two corresponding electrodes one in each of said electron discharge tube systems, the voltages impressed on the said first two corresponding electrodes and the voltages impressed on the second two corresponding electrodes beingsubstantially in phase quadrature, and'means for applying modulating potentials of substantially like phase between the control electrode and cathode of each of said electron discharge tube systems.

12. In a frequency modulation system an oscillation generating circuit wherein oscillatory energy to be modulated flows, said circuit having reactance, and means supplementing the reactance of said circuit with a modulated reactance to thereby modulate the frequency of the oscillations generated in said system including, a pair of similar electron discharge tube systems each having a plurality of electrodes including a control electrode and cathode, means for impressing substantially phase opposed voltages from said oscillation generating circuiton a first two corresponding electrodes one in each of said electron discharge tube systems, other means for impressing voltages from said generating circuit on a second two corresponding electrodes one in each of said electron discharge tube systems, the voltages impressed on the said first two corresponding electrodesfandthe voltage impressed on the second two. corresponding electrodes being substantially in phase quadrature, and means for modulatingthev potential on the control electrodes of said tube systems relative to the cathodes of said tube systems in phase.

13. In a frequency modulation system an oscillation generating circuit wherein oscillatory energy to be modulated flows, said circuit having reactance, and means for adding a modulated reactance to the reactance of said circuit to modulate the frequency of the oscillations generated in said system including, a pair of electron discharge tube systems each having an anode, a control electrode and cathode, means for impressing voltages of substantially opposed phase from said oscillation generating circuit on the anodes of said electron discharge tube systems, other means for impressing voltages from said generating circuit on the control electrodes of said electron discharge tube systems, the voltages impressed on the anodes and control electrodes of each electron discharge tube systems being substantially in phase quadrature, and means for modulating the potential between the control electrode and cathode of each of said electron discharge tube systems in phase at signal frequency.

ORVILLE' E. DOW.