US2076685A

US2076685A - Modulated carrier wave transmitter

Info

Publication number: US2076685A
Application number: US753291A
Authority: US
Inventors: Tringham William Stuart Leader
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1933-11-17
Filing date: 1934-11-16
Publication date: 1937-04-13
Anticipated expiration: 1954-04-13

Description

April 13, 1937. w. s. L. TRINGI IAM 2,076,685

MODULATED CARRIER WAVE TRANSMITTER Filed Nov. 16, 1934 POWER MT +F 5 AZ 6 www EDT E ATTORNEY Patented Apr. 13, 1937 UNITED STATES l? I I MODULATED CARRIER WAVE TRANSMITTER William Stuart Leader Tringham, Surrey, England, assignor to Radio Corporation of America, a corporation of Delaware Application November 16, 1934, Serial No. 753,291 In Great Britain November 17, 1933 7 Claims.

This invention relates to modulated carrier Wave transmitters and apparatus for use therewith. More specifically the invention relates to What may be termed modulation monitoring arrangements for use in connection with radio and like transmitters and whereby the correctness of the modulation at diiierent stages of the transmitter may be checked from time to time and during operation of the said transmitter.

According to this invention a modulation monitoring installation, for use for the purpose specifled, comprises a cathode ray tube, means for deriving from a plurality of stages of the transmitter to be monitored, potentials which should be like functions of the degree of modulation, and

switching means or its equivalent whereby any of the potentials so derived may be applied to deflect the ray in the cathode ray tube, the deflection of the ray thus obtained serving as an informative indication of the correctness or otherwise of operation of the transmitter.

Fig. 1 is a diagrammatic showing of the apparatus used in carrying the invention, and Fig. 2 is a modification thereof.

One method of carrying out the invention will now be described with reference to Figure 1 of the accompanying diagrammatic drawing said method relating to the primary intended application of the invention which is to amplitude modulation television transmitters of the kind wherein modulation is effected at the grid circuit of a high frequency amplifier stage.

In a practically convenient and satisfactory form of television transmitter, the said transmitter is modulated on the grid circuit of the penultimate high frequency stage, in such manner that the carrier current in the output circuit of that stage should be at any instant directly proportional to the instantaneous brightness of the picture to be transmitted.

' Thus a certain predetermined low carrier output level corresponds to a black spot upon the picture and a high carrier leve1namely the peak or maximum output which the transmitter can give practically without distortion-corresponds to a spot of maximum whiteness or brightness. In cases where the synchronizing signals are transmitted as modulations of the same carrier wave-the usual casethe said synchronizing signals may be transmitted, as is well known per Se, by modulating the carrier to reduce the carrier level below the minimum value corresponding to black spots upon the picture.

In such a transmitter it is very desirable to be able from time to time to observe instantaneously the levels of modulation at difierent parts of the transmitter installation as a whole, and in particular it is very desirable to be able to compare wave shapes at (1) the input to the radio transmitter (2) the output of the modulated amplifier stage, and (3) the output of a. power amplifier stage which usually succeeds the modulated amplifier stage.

In applying the present invention to provide a monitoring installation for use in connection with a transmitter as just described, there is provided a switch device SW having [2 contacts arranged in a circle, these contactsbeing numbered in the drawing in clockwise direction from I to [2.

Contacts l, 2 and 3 are connected together and earthed at E. Contacts 6, I and 8 are connected together and through a high frequency choke CH1 to the gun or anode point to which .is connected one deflecting plate of a cathode ray tube of ordinary oscillograph type such as shown by Patent No. 1,592,274 to N. Kipping, issued July 13, 1926, and entitled Employment of cathode ray oscillographs. Contacts 9, l0 and II are also connected together and through a choke CH2 to the deflecting plate terminal of the cathode ray tube. The ends of the chokes CH1 CH2 remote from the switch contacts are connected together through a resistance R, the ends of which in consequence constitute the input terminal IT of a cathode ray oscillograph which is the indicating device for the whole installation. It is understood that the other pair of deflecting plates are connected to a sweep circuit as is illustrated in the above cited patent to Kipping. To avoid unwanted pick up of high frequency currents the choke CH1 and CH2 should be connected in circuit as close to the said oscillograph as possible. The switch contacts are uniformly spaced as shown so that contact l is diametrically opposite contact 1 and contact 2 is diametrically opposite co'ntactB, and so on. The switch has two diametrically arranged rotatable bridge pieces A1 and A2 each of which can connect a desired pair of diametrically opposed contacts together. Switch contact I2 is connected to the positive terminal of a bias bat-- tery B and a movable tapping point T1 thereon is connected to a movable tapping point T2 upon a potentiometer resistance M which constitutes the modulation resistance in the grid circuit of the high frequency amplifier valves V1 V2 at which modulation is accomplished. Resistance M is earthed at one end and connected to the grid point of the parallel connected valves V1 V2 at the other. Switch contact 4 is connected through a high frequency choke CH3 to the electrical centre of a coil C1 whose ends are connected to the anodes P1 P2 of a full wave rectifier D1 having two anodes and a cathode K1, as in the usual way, the cathode being earthed. This coil C1 is a coupling coil and is looselycoupled to the anode circuit of the power amplifier 2| following the stage V1 V2 at whichmodulationis effected i. e. the modulated amplifier stage. switch contact 5 is similarly connected through a high frequency choke CH4 to the electrical centre of a second coupling coil C2 whose ends are con-- nected to the anodes P3 P4 of a full wave rectifier D2 having an earthed cathode K2. The coupling coil C2 is loosely coupled to theanode' circuit of the high frequency amplifier V1 V2 at which mod-- ulation is effected. Filament heating currents for the two full wave rectifiers are taken through suitable resistances from the main low tension 0 supply for the transmitter as a whole.

point T are so adjusted that with the minimum amount of grid bias on the grid of'the modulation stage V1 V2 which will give new carrier level from the transmitter, the deflection of the cathode ray tube when its ,inputterminals receive potential through contact I2 is, zero and so that with a given carrier level the deflection of the cathode ray tube is the same whichever of the three supplies is fed thereto by theswitch, i. e. whether the input to the cathodev ray tube is derivedfrom contacts I2, 4 or 5. The two diametrical contact bridges A1 A2 of the switchare arranged to be rotated together by a single handle and to be at right angles to oneanother so that, for example, when contacts 6 and I2 are bridged, contacts 3 and 9 are also bridged. It

will be noted that with the arrangement described the sense of the voltages which are applicable to the cathode ray tube from the three diiferent sources, is such that the deflection will always be in the same direction.

x The value of the resistance R. across the input terminals of the cathode ray oscillograph should be made sufficiently low to ensure that the capacity 'of the various inputs is negligible at all modulation frequencies. If thisbe done theapparatusmay be employed for frequency response measurements.

-In the modification shown in Figure 2 the coupling between the anode circuit of the modulated amplifier stage. V1 V2 and thegridcircuit of the succeeding power amplifier stage (not shown) is effected through a concentric feeder 00, IC, whose outer tubular conductor 0C is earthed. (Generally a similar method of coupling will be employed between the power amplifier and the transmitting aerial). With. this method of coupling it is convenient to dispense with the coupling coils- C1 C2 of Figure 1, and,

6 instead of employing inductive coupling to the rectifier D1 D2, to take the modulated high fre.

quency energy from the appropriate anode circuits via the concentric feeders connected thereto and suitable small condensers. The circuit arrangement is shown in detail in Figure 2 for The remaining the rectifier associated with the anode circuit of the stage V1 V2. As will be seen this rectifier D2 has an earthed cathode K2. and has its anode Pa connectedthrough achoke CH4 to the contact 5 and also through the coupling condensers F1'Fz to the inner conductor 10. The conductors 10, 0C, are coupled through condensers F3 F and coil C3 to the anode circuit of stage V1 V2. The other rectifier D1 has its cathode K1 earthed and its anode P1 connected through chokeCHa to contact 4, said anode being also connected through. coupling condensers F1, F2 to the inner conductor of the tubular concentric feeder (not shown) associated with the anode circuit of the power amplifier stage (not shown). S1 S: are screens. The use of coupling condensers for taking. modulated high frequency energy from the concentric feeders is not necessary and other coupling expedients may be resorted to; for example, resistances may be used although such use islnot. preferred owing totheheating effects in resistance. Again, it is possible 'to use a.combi-. nation of coupling condenser on the live side,

following'by avariable resistance. This arrangea,

ment is preferable to thetuse of coupling resistances simply, but there is little to choose between 1 it and the use of coupling condensers as illustrated.

Preferably the oscillographlused in outthis invention .is mountedon or nearthe' same control board or ,table, as the, main controls. of the transmitter to ,be monitored or checked, in order that thesaid oscillograph may,

bdobserved. while operating, the transmitter ,c0n-. v trols.

. The, oscillograph the carry through or degree of unbalance of high frequency energy when adjusting theneu-.

tralizing. reactances of a neutralized highfrer quency valve amplifier employed in the transmit- That is to say that if the amplifier is un ter. balanced as regards to neutralization, radio. frequency energy will bepassed throughthe grid further it canreadily vbeascertained whatper-v :can. be used for measuring,

The deflection of the ray will bean centage of the normal carrier strength the carry through is.

Havingnow described my inventionQwhat I claim is: I p

.1. A modulation monitoring installation for use with a modulated radio transmitter whereinis provided a;plurality of stages comprising a cathode raytube oscillograph, means for deriving from a plurality of stages of the transmitter to be monitored potentialswhich are -=prede-r termined functions of the degree of modulation and a single switching means whereby only one.

2. A modulation monitoring installation for use with amodulated radio transmitter wherein is provided a plurality of stages comprising a.

cathode ray tube oscillograph, means for deriv be, monitored, means for rectifying said energy,

means for deriving from astage succeeding said modulatedamplifier stage modulated high frequency energy, meansv for rectifying said last.

mentioned energy, means for deriving from an impedance in the transmitter and wherein modulation energy flows, potentials corresponding to said modulation energy, and a single switching means for applying at any instant only one of said mentioned potentials or either of the rectified outputs to the input terminals of a cathode ray oscillograph.

3. A modulation monitoring installation for use with a modulated radio transmitter wherein is provided a plurality of stages comprising a cathode ray tube oscillograph, connections between one input terminal of said oscillograph and three adjacent contacts of a rotary switch having twelve equally spaced contacts arranged in a circle said switch also having two mutually perpendicular bridge arms adapted to be moved together each bridge arm being adapted to connect together diametrically opposite contacts of the switch a connection between three further adjacent contacts of the switch and the other input terminal of the oscillograph a connection between an impedance of the transmitter monitored, through which modulation energy flows, and a seventh contact of the switch, a connection between an eighth contact of the switch and a rectifier connected and arranged to rectify modulated high frequency energy derived from the anode circuit of a modulated amplifier in said transmitter, a connection between a ninth contact of the switch and a rectifier connected and arranged to modify modulated high frequency energy derived from the anode circuit of a stage succeeding said modulated amplifier stage, the remaining three.

switch contacts being connected together and earthed, the whole arrangement being such that by rotating the two bridge arms together to different positions either voltages from the modulation impedance or voltages from either of the rectifiers may be applied to deflect the ray within the cathode ray tube.

4. An installation as claimed in claim 3 and wherein the rectifiers are energized through inductive coupling circuits are of the full wave type.

5. An installation as claimed in claim 3 and wherein the rectifiers are energized through capacitative coupling means connected to concentric feeder coupling devices in the transmitter.

6. An installation as claimed in claim 3 and comprising chokes inserted in the leads to the input terminals of the cathode ray tube oscillograph said chokes being adjacent said terminals.

7. An installation as claimed in claim 3, and

WILLIAM STUART LEADER TRINGHAM.