US2393785A

US2393785A - Carrier modulation

Info

Publication number: US2393785A
Application number: US464341A
Authority: US
Inventors: Laurance M Leeds
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-11-03
Filing date: 1942-11-03
Publication date: 1946-01-29
Anticipated expiration: 1963-01-29
Also published as: GB567517A; BE474679A; FR950308A

Description

Jan. 29, 1946. 1 M. LEEDS 2,393,785

CARRIER MODULATION Filed Nov. 3, 1942 2 Sheets-Sheet 1 7 l VARIED AT MODULATING FREQUENCY CARRIER summon 9 1 LOAD VARIED AT Z MODULATING y FREQUENCY Fig.3.

11/ ll/ T H l l 9') VARIED AT RRIER *gQ MODULATING B i LOAD. *7

REQUENCY Laurence PL Leeds,

His Attorney.

Jan. 29, 1946.

q. M. LEEDS CARRIER MODULATION F iled Nov. 3, 1942 2 Shets-Sheet 2 Inve nt or" w e emaw L m M Wo e m cxMA n 8 mm u t Patented Jan. 29, 1946 UNITED 'STATE s PATENT 1 OFFICE v CARRIER MODULATION Laurance M. Leeds, Rotterdam Junction, N. Y,

assignor to General Electric Company, a corporation of New York Application November 3, 1942, Serial No. 484,341

Claims. (01. ire-171.5)

generator to its load through an impedance matching transformer having a ratio varying at modulating frequency to control the carrier output. With such an arrangement the circuit etliciency is independent of the modulation percentaKe.

The object of my invention is to provide an improved and more efllcient arrangement for modulating a carrier wave, I

The novel features which I believe to be characteristic of my invention are set forth with par-,

ticularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be under- 1 stood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1 to 4 inclusive represent carrier modulation circuits; Figs. 5 to 8 inclusive. represent networks for obtaining a variable inductance through the use of a variable capacity; Figs. 9 to 12 inclusive represent networks for obtaining a variable capacit through the use of a variable inductance; and Fig. 13 represents a variable capacity having its value determined by the instantaneous modulating potential applied to its control electrode.

Referring to the drawings, in Figs. 1 to 4 inclusive there is showna source of radio frequency power I having output terminals 2 coupled through reactance networks consisting of-quarter wavelength artificial transmission lines I, I, I, and 8 to a load, such as an antenna, represented by a resistance 1. The reactance networks are made up of inductances I and condensers s which are varied in magnitude at themodulating frequency to produce a variation in the coupling between the power source and its load, thereby modulating the power output.

In operation the reactances of the inductances and condensers can be varied so that the voltage across the load resistance 1 is given by the equation e,=E'.[1+m cos (pt)] cos (wt) 1 u is the carrier frequency angular velocity. It

will be recognized by those skilled in the art that Equation 1 is the envelope equation of a distortionless modulated signal.

The manner in which this result is obtained is explained in connection with Fig. 1 for the case in which the reactances of the inductances I and condensers 0 are equal. For this condition the input impedance of the network, the impedance at the terminals 2, is given by the equation R. where Z is the input impedance, X is the react-F ance of the network condensers and inductance at the R. F. carrier frequency, and R. is the load resistance 1,

Because the modulation network 3 is made up of reactances, all of the power fed into the network must appear in theload resistance I. From this it follows that the power input is equal to the power appearing in the resistance 1 where e. and 2!:- are, respectively the instantaneous and maximum voltages across the load resistance 1, m is the modulation percentage p is the modulating frequency angular velocity, d

equation Q I z--- 3 where en, the instantaneous voltage across terminals 2 of the R. F. power source is equal to E0 cos .wt. From

Equations

1, 2, and 3 the instantaneous value of the reactances in the network is given by the equation 0. E.(l+m cos pt) cos wt ('4) From Equation 4 is apparent that, in order to obtain the distortionless modulated signal defined by Equation 1, itis necessary that the reactances in the modulation networks vary at modulation frequency in accordance with the equation constant 1+m cos pt (5) equal to zero and the network reactances are infinite.

Because the modulation networks use both variable condensers and variable inductances and because it may be more convenient to use only one type of variable reactance, the circuits shown in Figs. 5 to 12 inclusive will be useful in practical modulation networks.

The circuits of Figs. 5 to 8 inclusive are quarter wavelength artificial transmission lines made up of fixed condensers l and inductanoes it having variable condensers i2 connected across output terminals l3 which, due to the circuit constants, appear at the input terminals H as variable inductances.- Any one of the networks shown in Figs. to 8 may be substituted for the variable inductances in the modulation networks of Figs. 1 to 4 inclusive and will provide at the terminals ll an apparent inductanc given by the equation where L is the apparent inductance, X is the reactance of the fixed inductanoes and condensers in the networks, and C is the instantaneous capacity of variable condenser i2.

The circuits of Figs. 9 to 12 inclusive are made up of fixed condensers l5 and inductances I! having output terminals l1 shunted by variable inductances l8 which, due to the circuit constants, appear as variable condensers at the input terminals IQ of the network. Any of the circuits shown in Figs. 9 to 12 inclusive may be substituted for the variable condensers in the modulation networks of Figs. 1 to 4 inclusive and provide at the terminals i an apparent capacity given by the equation I where C is the apparent capacity, X is the reactance of the fixed condensers and inductances in the network, and L i the instantaneous inductance of the variable inductances l8.

In Fig. 13 is shown a variable condenser of the construction disclosed in Patent 2,243,829, Brett et al., which is capable of modulation frequency variation necessary for operation of the modulation networks although the range of variation is not suillcient for 100 per cent modulation. This condenser comprises an evacuated envelope 2| containing spaced condenser plates 2i traversed by a beam 22 of electrons from an elongated cathode 23. The electrons from the cathode pass through slits in a control electrode 24, in an accelerating electrode 25, and in a screen suppressor electrode 26, and are collected on a plate 21. The screen suppressor electrode 22 shields the condenser plate 2! from the voltages applied to escapes the

electrodes

24 and 2| and also prevents the return of electrons from the plate 21. The beam of electrons passing between the condenser plates varies the dielectric constant and, since the beam is controlled by the potential e. applied to the control electrode 24, the capacity appearing at the condenser terminals 22 is directly related to the potential at the control electrode 24. Due to the inertialess nature of the electron beam, the eflective capacity appearing at the terminals 28 can be varied at a high frequency.

The instantaneous value of the capacity at the terminals 22 is given by the equation o t X E.(l +m cos pt) (9) From this it follows that JBef'EJl-l-mc (11 (10) 4 (1E. e JZ[I+mCOB (111)] If the relation between a; and the capacity of the terminals 22 is linear, the modulation will be distortionless if e; is distorticnless.

, Neglecting the small amount of power required to control the reactances I and 9, the efliciency is the instantaneous efllciency of the power source, i. e., the eillciency of the power source at the particular loading existing at that instant, and is independent of the modulation percentage. The

' power source may be an oscillator, power amplifier, or other device and will have an efficiency dependent upon its design. If the power source is a class C amplifier, its instantaneous efficiency can be very high and in general will be in excess of 70 per cent at any modulation percentage.

The transmission apparatus disclosed above is applicable to any modulated transmitter and should result in a very simple high power broadcast or television transmitter.

While I have shown particular embodiments of my invention, it will be understood that many modifications may be made without departing from the spirit thereof, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Transmission apparatus comprising a source of carrier, a quarter wavelength artificial transmission line, made up of variable series and shunt reactances, for connecting the carrier output with a load, and means for varying simultaneously each of said reactances at modulating frequency to modulate the carrier output while maintaining constant the electrical length of the line.

2. In a modulation system, a source of oscillations of constant amplitude, a load, a reactance network between said source and load, said network having both shunt and series reactanoes and an electrical length equal to an odd multiple of a quarter of a wavelength at the frequency 01 said source, a source of modulating voltage, and means to modulate the voltage across said load in accord with said modulating voltage, said means comprising means to modulate both the shunt and series reactances of said network in accord with said modulating voltage while maintaining constant said electrical length of said network.

3. In a modulation system, a source of oscillations of constant amplitude, a load, a reactance network between said source and load, said network having both shunt and series reactances and an electrical length equal to an odd multiple 01 a quarter of a wavelength at the frequency of said source, a source of modulating voltage. and means to modulate the voltage across said load in accord with said modulating voltage, said means comprising means to vary both the shunt and series reactances of said network in accord with the product 01' a constant and (1+m cos at) where m is the percentage modulation, 1: is the frequency of said modulating voltage, and t is time.

4. In a modulation system in which a source of constant voltage of carrier frequency is supplied through a reactance network to a load, said network having an electrical length equal to a quarter or a wavelength or odd multiple thereof at the frequency of said source, the method of modulating the amplitude of voltage supplied to said load without modulating the phase thereof which comprises varying the characteristic impedance of said network while maintaining constant the electrical length of the network.

5. In a modulation system, a source of carrier wave oscillations, a load, a reactance network connected between said source and load. said network comprising series reactance of one sign and shunt reactance of opposite sign and having an electrical length equal to an odd multiple or a quarter of a wavelength, a plurality of capacitance devices, each corresponding to one of said reactances and each having capacitance of value corresponding to modulating voltage applied thereto, means to apply thereto modulating voltage in accord with which it is desired to modulate said carrier wave oscillations supplied to said load and means to modulate each of said positive and negative reactances in accord with the corresponding one 01' said capacitances thereby to vary the voltage supplied to said load, said positive and negative reactances being varied in such relation as to maintain constant the electrical length of said network between said source and load.

LAURANCE M. LEEDS.