US2226214A - Transmitter - Google Patents

Description

TRANSMITTER Filed Oct. 29,

H. D. ALBRECHT Dec. 24, 1940.

Gttomeg Patented Dec. 24, 1940 UNITED STATES 2,226,214 j TRANSMITTER Harold D. Albrecht, Pennsauken, N. J., assigner to Radio Corporation of Delaware America, a corporation of Application October 29, 19.48, Serial No.'2.37,637

y Claims.

My invention relates to vacuum tube coupling systems, and more particularly to modulation circuits in a transmitting system and inverse feedback circuits. i

In transmitting systems and amplifiers femploying degenerative or inverse feedback circuits 'for reducing distortion and hum, considerable diiculty has been experienced from phase shift in the feedback loop, particularly incases where audio-frequency transformers have been used in the circuit. It has been common practice to employ a conventional push-pullv transformer consisting of a split primary and a closely coupled secondary for coupling from a push-pull or class B high powered modulation amplifier to an R F. amplifier anode circuit in a radio transmitter, for modulating the R. F. carrier in accordance with impressed audiofrequency signal waves. This transformer has prevented realizing the full benefits of feedback in the reduction of distortion and hum, although in some cases special phase correcting networks have been added for the purpose of reducing the phase shift.

It is, accordingly, an object of my invention to provide, in a transmitting system, improved coupling means free from phase shift, for coupling a modulator to an R. F. amplifier or relay employing degenerative feedback in the system. Y

It is a further object of my invention to provide an improved circuit arrangement for coupling a double ended or divided output amplifier or relay into a single ended circuit.

It is a still further object of my invention to provide, in an amplifier system having degenerative feedback, a simplified inductive coupling means with greatly reduced phase shift for coupling a double ended or divided output amplifier to a single ended circuit.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with lfurther objects and advantages thereof, will best be understood by reference tothe following description of a yspecific embodiment, taken with the accompanying drawing, diagrammatic view of a radio transmitting system embodying my invention.

Referring to the drawing, I have shown schematically an oscillator I, an R. F. amplifier 3, both in block diagram, coupled to the input of a push-pull R. F. amplifier stage 5, which in turn is coupled in push-pull to a final high powered push-pull R. F. amplifier 1. 'Ihe amplifier tubes in which the single figure is a include the usual filament 2, grid 4 and anode 6. The output of amplifier 1 is coupled through transformer 9 to the antenna I I. The amplifiers 5 and 1 are coupled by means of a variably tuned circuit I3 including a split tuning capacitor I5 and center-tap R. F. inductance coil I1.

The R. F. amplifier 5 is modulated by means of a class B amplifier stage I9, in accordance with the teachings of Barton Patent 2,063,290 and includes tubes 2I2I and 23-23 arranged in parallel push-push relation respectively. The outputs of tubes 2I and 23 are connected to a center tapped iron core chokev coil 25 in balanced relation. In effect, coil 25 is an auto transformer, the two halves being closely coupled so that the voltage across one half is practically the same as that across the other half, although 180 out of phase. One end terminal of coil 25 is connected, through a stopping capacitor 21, e. g., of the order of 8 mfd., to a modulating reactor 29 at its high potential end, preferably no magnetic coupling existing between the reactor and the coil 25.

The reactor is connected in the usual manner for plate modulation, between the high voltageA anode supp-ly source 3I and the anodes or plates 6 of push-pull tubes 5, through an R. F. cho-ke 33 and resistor 35, to a center tapped terminal on the coil I1. It will be noted that this terminal on coil I1 is at ground potential for the R. F., and that this point is varied at audio potential with respect to ground, by means of the modulator. The anodes of tubes 5 are effectively in parallel so far as the impressed audiofrequency is concerned, and for this reason are considered to be 'a single ended circuit into which the divided output of the modulator I9 feeds. It is within the scope of my invention to employ my auto transformer to couple the output from a push pull audioamplifier into the grid circuit of a conventional single ended audioamplifier. I am well aware of the fact that an auto transformer connection of somewhat similar nature has been employed heretofore in feeding from a single ended output stage into a push-pull stage, as shown 'and claimed by Davis Patent 2,032,921, but I am not aware of any prior arrangement for coupling in -a reverse manner with resulting improvements, and more particularly in an inverse feedback combination, in accordance with my invention. It is also withinthe scope of my invention to employ the arrangement of the Davis patent in a feedback loop circuit for reduction of phase shift.

In the system shown, which has been built and used in accordance with my invention, the amplifier I3 is of the class B type, operated With grid bias, as disclosed in patent of Loughren 1,699,110, although other types of amplifiers, e. g., class A and class AB, could be employed in accordance with my invention. The tubes 2l and 23 were of the low impedance type, but were provided with sufficient negative grid bias so as not to be driven to draw grid current during normal operation, although an amplifier in which grid current is drawn could be employed; for example, as shown in Barton Patents 2,084,180 and 2,084,181. The amplifier 5 as actually used, was a class C type, although here again other types could be employed.

Ahead of power amplifier stage I9 is an audioamplifier, shown in block diagram at 3l. For the purpose of providing inverse or negative feedback, a portion of the final output energy existing in transformer 9 is taken, by means of a coupling coil 39, and fed back, through a rectifier circuit 4I into the A. F. amplifier 3l, in accordance with the teachings of DeWitt Patent 2,093,751.

While I have illustrated my invention inconnection with this type of feedback circuit, it also has advantages when used in a feedback involving the audio amplifier only, for example, as illustrated in Oman application, Serial No. 153,574., filed July 14, 1937 (same assignee). It will be observed that in the Oman application the output audio frequency transformer is not included in the feedback loop. My invention Will permit the inclusion of the output coupling means in the feedback loop by virtue of the reduced phase shift.

Besides eliminating the transformer, and the corresponding difficulties caused by phase shift, I have found that the use of a choke coil is substantially less expensive and simplifies design.

'40 My improvement was applied in a given case to an existing transmitter, in use in a broadcasting station, and it was found that the phase shift was reduced to such an extent that the distortion in the transmitter, at 7500 cycles, was reduced to 45 approximately half of its former value. In making this change, I disconnected the secondary of the existing audio transformer of the system, using the primary as a choke coil, and added a modulating reactor in the manner shown at 29 50 in the present drawing.

Although I have illustrated but one embodiment of my invention, other modifications will be apparent from the disclosure to those skilled in the art. My invention, therefore, is not to 55 be limited except insofar as is necessitated by frequency amplifier characterized by a double ended output circuit balanced for radio frequency currents, an audio modulation amplifier having output electrodes connected in a double ended output circuit, a reactor connected to said electrodes and constituting an auto-transformer for coupling said audio output circuit to said radio frequency amplifier output circuit in single ended relation for audio frequency modulation,

and a degenerative feedback loop circuit in said system including said auto transformer.

2. In a transmitting system, a radio frequency amplifier, a double ended output audio amplifier, said amplifiers including thermionic tubes having cathodes, control electrodes and output electrodes, a power supply circuit for said output electrodes, means for coupling said audio amplifier to the radio frequency amplier in single ended relation comprising a choke coil connected in balanced relation to the output electrodes of said audio frequency amplifier, a modulating reactor and stopping capacitor, and a coupling connection through said stopping capacitor to a point on said modulating reactor, said reactor being connected in the output electrode power supply circuit to said radio frequency amplifier, and said choke coil and reactor being characterized by absence of magnetic coupling, whereby phase shift is reduced.

3.` The invention as set forth in the next preceding claim, wherein a degenerative feedback circuit is connected from the output of said radio frequency amplifier to the input of said audio amplifier.

4. In a transmitting system, a radio frequency amplifier connected toy a source of radio frequency current, a double ended audio frequency amplifier connected to a source of audio frequency current and comprising a pair of thermionic devices connected in balanced relation, said devices having output electrodes, a power supply circuit connected to said radio-frequency amplifier, an audio frequency reactor having end terminals connected respectively to said output electrodes, means for connecting a portion of said reactor to said power supply circuit in such manner that said reactor constitutes an auto transformer for modulating said radio frequency amplifier with audio frequency current in said reactor, and an inverse feedback circuit connected from the output of said radio frequency amplifier to the input of said audio frequency amplifier and forming therewith a feedback loop including therein said reactor.

5. The invention as set forth in claim 4 wherein said audio frequency amplifier is of the class B type.

HAROLD D. ALBRECHT.

Images