US2383456A - Modulation circuit - Google Patents

Description

Aug- '28, 1945. A. ALFORD Erm. MoDULATIoN CRcUIT Filed Nov. 15. 194:5 2 sheets-sheet 1,

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IN VEN TORS ANDREW Auro/ro Guy fr. P/:rf/Pso/v ATwRNEY 'Aug-28,1945. A. ALFORD ETA. f f 2,383,456

MODULATION CIRCUIT lated Wave.

Patented Aug. 28, 1945 FFICE y MODULATION CIRCUIT Andrew Alford, Cambridge, Mass., and Guy K.

Patterson, New York, N. Y., assignors to Federal Telephone and Radio Corporation,`NeWy ark, N. J., a corporation of Delaware Application November 15, 1943, Serial No.510,268

12 Claims.

(Cl. Z50-11) This invention relates to modulation circuits and Imore particularly to modulation systems particularly suitable for energization of radio beacons.

In certain types of radio beacons, it is often desirable to energize one of the antenna units with carrierc and side band energy and to energize other. cooperating antenna units with side band energy only. This has commonly been done by systems feeding modulated energy to opposed diagonal points of ahighfrequency bridge circuit of such a nature that atoneterminal the carrier Wave 'will Vbalance out andl permit only side band energy to be fed out,'wh'ile`lat the diagonally oppositeterminal the carrier energies willv add in phase'so that both'` carrier and side band output is obtained. i y

. Itis 'a principal object of our invention to provide'a modulating circuit which will produce separate modulated carrier Waves and side band'wave outputs without the use of special bridge circuits.

It is afurther object of our invention to provide a modulating system in Which voice frequency may be applied to a modulator simultaneously With the `application thereto of other modulating signals and carrier frequency waves, and in which there is provided one output carrying the voice frequency signals and the other modulating signalstogether with carrier frequency, and a second output circuit carrying only the side bands of said other modulating signal.

It is a further object of our invention to provide a small and light-Weight modulating unit for energizing a radio beacon.

According to a feature of our invention, We pro'- vide a `vacuum tube arrangement operating su stantially as apusl'i-pull amplifier modulatorto which unit is applied a radio frequency energy in push-pull relation. Accordingly, in the output of the vacuum tube arrangement, there is provided an amplifiedV Vradio frequency excitation. The midpoint of the push-pull output circuit'is coupled with-a further output loop tuned substantially to the'carrier frequency. However, because of the push-pull input and output characteristics, none of the carrier frequency will appear in this circuit. A modulating signal is then applied to-one grid only of the push-pull vacuumtube circuit, thus producing an unbalanced modulated Wave in the o'utput'circuit of the vacuum tube arrangement. This unbalanced voltage will feed to the second output circuit, and in that circuit will consistonly of the side bandenergy 4of the modu- HoWever, in the principal circuit,

there will |be present the carrier frequency as 5,5

Well as the side. bandmodulating frequencies.` In order to prevent intercoupling of the two output circuits, a shielding is generally provided bletfween the two output circuits. Furthermore, in order precisely to balance the push-pull modulation, independent bias control is provided for the push-pull vacuum tube circuit, and in the output circuit, a differential phase balancingcondenser is provided.

A better understanding of our invention and the objects and features thereof may be had from the particular description thereof made With reference Ato the accompanying drawings, in which:

Fig. a block schematic diagram of a radio beacon arrangement incorporating a modulating system in accordance With our invention; and

Fig. 2 is ya Schematic Wiring diagram ofthe modulating circuit in accordance with `our in-y vention.

Turning first to Fig. 1, I0 represents a radio frequency source vcoupled to the input loop of modulator II. In the output of modulator II is provided `a balanced output loop I3 coupled over a loop I4 to a centrallypositioned antenna unit I5. A modulating signal is supplied from source I2 to one side of modulator II, producing inloop I3 a modulated carrier Wave which consists ofy the carrier frequency and the side band energy.V This energy is supplied to unit I5. Coupled to, the midpoint of loop I3.is a second output loop I6. This loop, because of its symmetrical connection With respect to loop I3, Will not receive any of lthe balanced radio frequency energy from source I0. However, the modulating signal is unbalanced in character and therefore there will be developed in loop I6 side band energy. This energy is fed overa coupling line and a keyer I9 to the outer antenna units I1 and I8 arranged on opposite sideso-f central antenna unit I5. The respective lengths of the lines are so chosen that en'ergy in unit I5 is 90- in yphase from energyin units I'I and I8 While the energy inunits I'I and I8 is in phase opposition With respect to one another. Keyer I9 serves alternately to reverse the potential fed tok antenna units I'I and I8, thus prod-ucing a keyed beacon signal.

Should it be desired to also apply voice signal to the beacon, this maybe :done by connecting a voice source 20 across the input loop of modulator- I I so that this voice energy is in parallel. Thus,

the radio frequency energy will 'be modulated so that this voice frequency energy will be applied to central antenna unit I5 and to antennas II and I8.

,A preferred circuit for the modulator I I of Fig.v

l together with the input and output loops is illustrated in Fig. 2. In this arrangement, energy from `the radio frequency source is supplied over a loop 2! to inputloop 22. This input loop applies the carrier wave in push-pull to the control grids 30, 4G of a double tetrode tube 23. If desired, parasitic suppressing resistors 22a may be included in loop 22.1 A condenser S0. is provided in loop 22 so that the bias on the separate grids 39 and 4B may be independently controlled and further so that signals may be independently applied to these grids. The balanced loop 'output circuit I3 tuned to resonance at the carrier frequency by a variable condenser 32 is coupled to anodes 3|, 4I 5of tube 23. A-t the midpoint 33 of loop I3 is provided a connection to loop I6 which is tuned by variable condenser 34 and which may be connected to ground through radio frequency choke 35 and `by-pa'ss condenser 36. A loop I4 is Vcoupled with loop I3 to furnish the output leads for this circuit. Energy from a modulating signal source I2 is applied over lead 46 and radio frequency choke 42 to grid 4I] of tube 23. This serves to apply an unbalanced modulating signal producing unbalanced sideband energy in `output loop I3, thus providing in I3 sideband energy as well as carrierfrequency energy. The unbalanced sideband component in loop I3 is also fed over midpoint 33 to loop circuit IB and from this circuitl is supplied over output leads 38 to a desired load which may be antennas such as I1 and .I8 of Fig. l.

In order that the circuit may be properly balanced, we provide a differential'bias control comprising potentiometers 48 and 49 connected by leads 46, 4l and chokes42, 43, respectively, to grids4 and 30 0f tube 23. A voltage regulating tube 50 is provided to assure that constant voltage is applied across potentiometers 48 and 49. By adjusting this differential bias control, an amplitude balance in the output of tube 23 may be readily achieved. In addition, a differential phase balancing condenser 55 is provided across tuned loop I3. This condenser is adjusted until -thephase balance isachieved in loop I3 so that in the absence of modulating signal from source I2 there is no output in loop I3. By-pass condensers 44V and 45 may be provided to by-pass any radio frequency energy which may pass chokes v42 and 43 tovground.

To prevent any coupling between the input and output lcircuits 22,4 I3 and I5, separate shield arrangements I, 52 and 53 are provided about each of these separate components. This shielding is essential if all the carrier frequency is to be excluded from loop I6 when the parts are assembled, for example, in a compact modulating unit where close spacing is required.v

In an actual full-scale operating model of the modulator in accordance with this invention, it was found ypossible to make the entire unit in such size to be readily portable. The unit, including the modulator, the necessary radio frequency oscillators and the tone generating source, may be housed in a casing about the size of a suitcase and light enough to be readily carried by one man.

While We havedescribed a particular embodiment of our invention related to a particulartype of beacon, it is' clear that this description is made merely by way of example and that many lmodiiications may present themselves to those skilled in the art. For example, if it is desired to have the voice frequency sidebands trans-- vmitted from loop I6 to the exclusion of loop I3, it is merely necessary to make the connections to grids 30, 4I) in parallel instead of pushpull relationship.

Also, instead of keying the output line, the keying device may be arranged to alternately connect source I2 to lines 46 and 41, if desired. What we consider as our invention is embodied in the objects set out above and in the accompanying claims.

What is claimed is:

1. A modulating system for producing two output waves, one of carrier frequency plus side bands, and the other of side band frequencies only, comprising vacuum tube means having two control grids and two anodes, an input circuit coupled to said grids, a center-tapped loop output circuit coupled tov said anodes, a source of carrier frequency energy, means for coupling said source of carrier frequency energy to said control grids in push-pull relation, a source of signal energy, and means for coupling said source of signal energy to one only of said control grids to lmodulate said carrier frequency energy to produce side bands, whereby carrier frequency and side band energy is developed in said loop output circuit, and side band energy `only at said center tap.

2.'A system according to claim l, further comprising separate bias adjusting means coupledv to said, control grids toindependently adjust the bias on said grids.

3. A system according to claiml, further comprising differential condenser means connected across said loop outputicircuit to adjust the balance in said loop.

4. A modulating system according to claim l, further comprising bias control means for independently adjusting the bias of said control grids,

and a differential condenserv coupled across said loop output circuit to adjust the phase balance therein.

5. A modulating system according'to claim 1, further comprising a resonant circuit coupled kto the center tap of said loop output circuit for receiving said side band `energy only, and shielded means for shielding said resonant circuit from said loop output circuit.

A6. -A modulating system accordingto claim 1,

further comprising voice signal means, and means for applying said voice signals to said control grids in push-pull relation, whereby voice modulated energy is developed in said loop output circuit but is balanced out at said center tap.

7. In a radio beacon having a central radiating means, two side radiating means arranged on opposite sides of said central radiating means and a line interconnecting said side radiating means in phase opposition, means for supplying carrier.

and side band energy to said central radiating means and substantially side-band energy only to said side radiating means, comprising vacuum tube means having two control grids and two anodes, an input circuit coupled to said grids, a center tapped loop output circuit coupled to said anodes, a sour-ce lof carrier frequency energy, meansy for coupling said source of carrier frequency energy to said control grids in push-pull relation, a source of signal energy,.means for coupling said source of signal energy to one only of said control grids to modulate said carrier frequency energy to produce side bands, whereby carrier frequency and side band'energy is developed in said loop output circuit, and side band energy only in at said center tap, first coupling means for coupling said loop output circuit to said central radiating means, a second output ycircuit connected to the center tap of said loop in said last named means comprises keying means for alternately reversing said second coupling means. v

9. A modulating system according to claim 7,

-further comprising bias control means for independently adjusting the bias of said control grids, -an'd a differential condenser coupled across said loop output circuit to adjust the phase balance therein.

10. A modulating system according to claim 7, further comprising voice signal means, and means for applying said voice signals to said control grids in push-pull relation, whereby voice modulated energy is developed in said loop output circuit but is balanced'out at said center tap.

11. A modulating system according to claim 7, further comprising bias control means for independently adjusting the bias of said control grids, a differential condenser coupled across said loop output circuit to adjust the phase balance therein, and shielded means for shielding said loop output circuit and said second output'circuits from one another.

12. A modulating system for producing two output waves, one of carrier frequency plus side bands and the other of. side band frequencies only, comprising vacuum tube means having two control grids and two anodes, a loop output circuit coupled to said anodes, a second output circuit coupled to the electrical midpoint of said electric loop circuit, a source of carrier frequency energy, means for coupling said source of carrier frequency energy to said control grids' in push-pull relation, a source of signal energy, and means for coupling said source of signal energy to only one of said control grids to modulate said carrier frequency energy to produce side bands, whereby carrier frequency side band energy is developed in said loop output circuit and side band energy only in said second output circuit.

, ANDREW ALFORD.

GUY K. PATTERSON.

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