US2459208A - Modulation control system - Google Patents

Description

Jan. 1s, 1949. R G WULFSBERG '2,459,208 MODULATI ON CONTROL \S YS TEM l Filed Aug. 25, 1944 Syvum/MM Patented Jan. 18, 1949 MODULATION CONTROL SYSTEM Paul Gerhard Wulfsberg, Cedar Rapids, Iowa, as-

signor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application August 23, 1944, Serial No. 550,793

3 Claims.

My invention relates broadly to transmission systems and more particularly to a circuit arrangement for maintaining a high percentage of modulation in order to provide a maximum of intelligibility in telephonie transmission.

This application is a continuation-impart of my application Serial No. 457,370, iiled September 4, 1942, for Volume limiter circuit, now Patent 2,372,243, dated March 27, 1945.

One of the objects of my invention is to provide a circuit arrangement for a modulation system in which a substantially constant modulation level is maintained despite changes in input level, amplifier gain or modulation transformer voltage drop due to loading effects of the associated power amplifier of the transmitter.

Another object of my invention is to provide a circuit arrangement for an amplier modulator unit in a modulated signal transmission system in which a control Voltage developed in the unit is utilized to compensate, by an automatic change in gain through the system, all effects tending to change the modulation level.

A further object of my invention is to provide a circuit arrangement for a modulation control system in which a control voltage is derived from the point at which the modulating voltage is generated and utilized for compensating for any effects tending to change the modulation level.

Other and further objects of my invention reside in an electronic gain control system actuated by the modulating voltage for holding the modulation level constant as set forth in the specification hereinafter following by reference to the accompanying drawing which diagrammatically shows the circuit arrangement of my invention.

In radio transmitting equipment, it is important to maintain a high percentage of modulation in order to provide a maximum of intelligibility. Overmodulation, on the other hand, is undesirable because of spurious radiations, distortion and overloading of components. The requirement, then, is to provide some automatic means of maintaining a high average percentage of modulation without causing overmodulation.

A number of methods have been proposed .to solve this problem. Among them are various peak limiters and volume compressors designed tosecure a desired input-output level characteristic. These are eiective when the equipment is operating under iixed conditions where they may be adjusted so as to not exceed the audio level to the modulator necessary for 100% modulation. Whenever the power amplier is subject to variable loading, these systems of audio control are not effective.

In my aforesaid Letters Patent 2,372,243 of March 27, 1945, I have shown a volume limiting circuit. In the present invention this limiter is applied to modulation control by takingthe control voltage Afrom the point at which the modulating voltage is generated. A diode rectifier is connected to the output of the modulation transformer through an insulated winding. The D. C. output from this diode is proportional to the audio voltage across the modulation winding. This is true because of the tight coupling that exists between two windings on a common iron core.

It is necessary to supply a peak audio Voltage equal to the plate voltage of the power amplier for 100% modulation. This voltage is subject to variation as the power amplifier load changes, assuming constant audio amplier gain. As the power amplifier draws increased current, increased gain is needed while for decreased loads a smaller gain is required.

This system using an electronic gain control actuated by the modulating voltage is very eiective in holding the modulation level constant. The gain of the amplifier increases or decreases to provide more or less audio power as required by the power amplier. It is possible with this automatic control to maintain a high level of modulation without danger of overmodulation.

Referring to the drawing in detail, the audio frequency input circuit leading from the audio frequency control channel such as a microphone circuit, program distribution system, television control circuit, facsimile system or telegraphic tone channel is indicated by reference character I connected through audio frequency transformer 2 to the input circuits of the single duo-triode 3.

The duo-triode 3 includes cathodes 3a and 3a', l

control grids 3b and 3b and anodes l3c and 3c'. Tube 12SL7 serves particularly well for the position 3. A xed grid bias is obtainedfrom the drop across the cathode resistors 4-5-6 in the lbiasing circuit 25 of the ampliiier tube l as will be more fully explained hereinafter. In the embodiment described, tube 12A6 serves particularly well for the position 1.

'I'he secondary Winding 8 of input transformer 2 is paralleled by a resistance network constituted by resistors 9-I0-II and I'2 symmetrically arranged and connected to ground at I4. Resistors 9 and I2, in the embodiment of the circuit of my invention employing the 12SL7 tube in the position 3, each have a value of 33,000 ohms while resistors l0 and II each have a value of 18,000 ohms. The opposite terminals of secondary winding 8 are connected to anodes 3c and 3c through resistors I5 and I 6, each of which in the vembodiment described have a value of 68,000

ohms.

Balanced output circuits are connected to the anode circuits of tube 3 as shown, that is, primary Winding I'I connects between anode 3c and tap I8 intermediate resistors 9 and I0 an-d primary winding I9 connects between-anode 3c" and tap 20 intermediate resistors II andA I2.

, v25, of.V tube I yandthe :electron .discharge Pali/ll Primary windings l1 and I9 are closely coupled through iron core 2i with secondary winding 22 of the couplngtransformer 23 which couples the output ofthe balancing tube 3 with the amp-liner tube 1.

Secondary winding 22 23 connects at one end with controlgrid 'ib of, amplier tube l and at the other'end to tap 24 intermediate the cathode resistors and 5. of the biasing circuit 25 connectedfbetW-een'cathode 'la and ground 26 for applying the required operating bias to grid 'lb of tube Y'L In the embodiment of my invention as described, resistors 4 and 5 each have the value of 470 ohms while resistor `E has the value of`600 ohms. Resistors 4, 5 and 5 are shunted by condenser 2'! which in the embodiment described has -a value of 25 mi. The fixed bias voltage for the grids 3b and 3b of tube 3 is secured through tap 28 intermediate resistors 4 and 5 of biasing circuit v25 4and through the resistor 29 of 4.7 megohms leading to the control grids 3b and 3b of tube 31. The output circuit .of

"of coupling transformer f through tube 3. The path is completed from the cathode circuit 54 of full lwave rectifier tube 5i through lead 56 extendingztogcontrol grids 3b and 3b .of tube 3 and through resistor 29 and tap 28 and resistors 5 and 6 in grid biasing circuit 25 of The biasing circuit to tube 3 is completed by the circuit that extends through biasing resistor 29 and resistor 4 of biasing circuit 25 to thecathodesa and 3a' of tube 3. Thus in additube 1 extends from anode 'lc and shield gridlol to the plate potential source indicated at `3l) through primary winding 3|- of audio-frequency output transformer 32.

Transformer 32 hasan iron core 33 and a secondary winding 34 wound thereon. A mid-tap 35 is provided in secondary winding 34 leadingto the cathodercircuit Sliaand 31a of the push-pull modulator system which comprises tubes 35 an-d 3l. In the embodiment ofthe circuit described I employ two 1625 tubes as the push-pull modula tors. The control grids 36h and Bib of the pushpull modulators connect to opposite ends of the secondary winding 3,4. The path to the cathodes 36a. and 31a.fromtap35 includes the 150 ohm resistor 58 shunted by condenser 3S of 25 mf. capacity. The cathode circuit is grounded at the tap 35 as shown at 40. y

The output of the push-pull modulator system includes a modulation transformer 4l having a secondary winding 42 divided into two symmetricalportions 43 and 44 with a mid-tap'45 therebetween. The portion of the -secondary winding I' shownf at .43 connects between mid-tap and anode 36o while the portion of the secondary winding show-n at 44 connects between mid-tap 45 and anode 31e. The intermediate grids 35d andB'ld connect to thesource of potential 3i! and the mid-tap 450i transformer 4l connects to the source of potential 51 from which the circuit is completed through ground connection `416 to the ground connection 40 returning to the cathode circuit c35u- 3M through biasing resistor 38 and condenser 39.

.The modulation transformer 4l has an iron core 4l and an outputsecondary winding 48 that leads tothe power amplier system Vand in addition there is a special insulated winding 49 wound ony `the transformer but substantially insulated therefrom by an insulation layer schematically designated at 5i). The winding 49 serves to take control voltage from the push-pull modulator tubes 35--31 and impress this Voltage upon the full wayerectier. This full wave rectifier inthe embodiment of my invention described herein is a 12H6 tube having an 9.5 mf. -condenser52 ccnnected between theV tap 53 on secondary winding 49 and the cathode circuit 54 of the full wave rectifier 5I. The rectier circuit is 'grounded at 55 thu-s establishing a circuit` from the anodes 55 of the full wave yrectifier 5l through ground 55 and ground 25 and i4 to the grid biasing circuit taining a vsubstantially constant tion to the fixed bias maintained on control grids bsand 3b by normal operation of bias resistor 4 inthe grid cathodepath of tube 3; and upon control grid lb of tube 'I by the potential drop through resistors 4 and 5, a compensatory bias is impressed upon grids 3U and 3b' by the rectied control voltage supplied from ther modulation transformer. A constant level will thus be maintained across .the output winding r48 of the trans-4 former. As a result of the co-action of 'the vseveral portions of; the circuit a .constant -modulation level is maintained despite changes in, input' level,` amplifier gain, or modulation transformer voltage drop due toloadingeiects ofthe power amplier of the transmitter. In other words, the control voltage isV takenfrom -a Vpoint inthe circuit such that alinostany eifectf tending, to change the modulation level is .compensatedby any automatic change in gain throughfthe system. While I have described my invention inr one of its preferred embodiments, Ilrealize thatv changes in circuit arrangement may be made and Idesire that it be understood that no limitations Vuponmy invention are intended other than may be imposed'by the scope of the appended claims.

What I claim as new and desire to secureby Letters Patent .of the United States isas'follows; 1. -An amplification control system. comprising a pair of balancedelectron tubefamplier units each connected in push-pull circuit arrangement, a single electron tube unit Yincluding-a.cathode, a control grid and ananode with one circuit-,path interconnectingA said l,cathode and control `grid and another H.circuit path interconnecting said anode and cathode, a resistive .circuitconnected in both offsaid circuit paths,.said unit couplingl the output circuit of one push-pull amplier unit with the input circuit of the otherofusaidpushpull amplifier units, a bias control circuit-connected between the rst of said push-pull amplifier units and the resistive circuit'in the lcircuit. paths leading to the cathode of saidsingle-elec,` tron tube unit, -a signal input circuit` connected with the input circuit of the first of said pushpull amplifier units, al signaloutput circuitconnected with the output circuit of vthe second of said push-pull ampler units and' a full Wave rectier system connected With'the output circuit of the second of Vsaid push-pull ampliiier unitsfor superimposing on said biascontrolcirl cuit compensatory biasing potentials for main# signal leveli at the signal outputcircuit. V

2. An amplic'ation control system comprising a pair 'of balanced electron tube amplier'unit's, each connected in push-pull circuit arrangement andeachL-inc'luding cathodes with associated-cone trol grid electrodes and coacting anodesran elec tron tube system coupling the output of 'Y one of said balanced electron tube amplierunitswith the input of the other of said balanced electron tube amplifier units said electron tube system including a cathode, a control grid, and an anode, a resistive circuit connected yin series with said last-.mentioned cathode, a -tap fon said f resistive circuitxconnected `with'the grid electrodes iof's'aid rst-mentioned electron tube amplier unit, means coupled with the output circuit of said last-mentioned pair of balanced electron tube amplifier units for deriving a biasing current therefrom, and a connection from said means to the grid electrodes of said first mentioned electron tube amplier unit for impressing compensatory bias potential on said iirst mentioned grid electrodes in excess of the bias potential impressed upon each of said grid electrodes from the tap on said resistive circuit, a signal input circuit connected with the iirst of said electron tube ampliiier units, and a signal output circuit connected with the last of said electron tube amplier units.

3. An amplification control system comprising a pair of balanced electron tube amplifier units, each connected in push-pull circuit arrangement and each including cathodes with associated control grid electrodes and coacting anodes, an electron tube system coupling the output of one of said balanced electron tube amplifier units With the input of the other of said balanced electron tube amplier units, said electron tube system including a cathode, a control grid, and an anode, a biasing connection leading to the grid electrodes of said rst mentioned electron tube amplier unit, a multiple tap resistor connected in series with said last mentioned cathode, an independent resistor, one of said taps being connected with the biasing connection leading to said grid electrodes of said rst mentioned electron tube amplifier unit through said independent resistor, means connected with the output circuits of said last mentioned pair of balanced electron tube amplifier units for deriving a biasing current therefrom and a connection from said means to the biasing connection leading to REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,959,062 Place May 15, 1934 2,087,316 Doba July 20, 1937 2,221,541 Hathaway Nov. 12, 1940 2,298,126 Hodnette Oct. 6, 1942 2,346,020 Gillespie Apr. 4, 1944

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