US1808538A - Transmission regulation - Google Patents

Description

June 2, 1931.

J. C. GABRIEL ET AL TRANSMISSION REGULATION Filed March 11, 1929 lNl/ENTUFS HUBBARQ Patented June 2, 1931 um'rso STATES mammen JOHN G. GABRIEL, O! m. YORK, AND IRAN! A. HUBBARD, 0] 13111183008, NEW YORK,

ASSIGNOBB 1'0 am TELEPHONE LABORATORIES, INCORPORATED, OI NEW YORK,

N. Y A. CORPORATION 01 NEW YORK armsinssron BIGI ILA'I'ION- Application filed larch 11, 1828. Serial 110. 845,887.

This invention relates to radio receiving systems and more particularly to means for maintaining at a constant level the output energy supplied to'a load circuit.

Numerous gain control devices have been proposed which utilize different components of. the incoming wave to effect the desired control. One such type is that disclosed in Patent 1,511,015 to Affel, October "7, 1924, in which the unmodulated carrier component is utilized to effect the desired control. Another type is disclosed in an application of E. Bruce, Serial No. 158,169, filed December 31, 1926, Patent 1,778,75Q, October 21 1930, in which the signal or audio frequency component or a component of changed frequency, derived from the received wave, may be used to effect the control function.

The present invention relates to asystem which embodies certain features of the abovementioned patent and application in combination with added features for improving and simplifying the operation of the gain control mechanism.

An object of this invention is to efiect gain control which, while extremely eflici'ent and having a small time lag, requires a minimum ofmanual adjustment to regulate its operation.

Another object is to regulate the operation of the gain control mechanism by a single adjustment, which fixes the relative potentials applied to a plurality of devices included in the receiver.

A feature of the invention relates to the provision, in a circuit includin a space discharge tube thegrid of which 1s biased by a potential derived from a point of fixed potential in the filament heating circuit, of

means whereby the potential difference between the grid and filament may be changed without changing the potential of the fixed point.

The above objects are accomplished in one embodiment of the invention, comprising asuccessive detection receiving system, by util izinga portion of the changed frequency component derived from the received wave.

The selected portion of the changed frequency energy is amplified, preferably in one ormore stages each includin a space dis- 'amplitude of the incoming wave, from which it is derived, a variable voltage dro' will be produced across the resistance. T is variable voltage is applied to the high frequency detector to control its gain, in a manner inverse to fluctuations in the incoming wave, whereby these fluctuations are compensated.

In addition to the variable gain control potential, the various "space discharge devices or tubes, which operate as amplifiers and rectifiers, are supplied with steady biasing potentials derived from resistance elements included' in the circuits through which heating current is supplied to the filaments of the discharge devices.

- In order that the steady biasing potentials and the variable potentials, used to control thegain of the receiver, may always be controlled to have the same relative values in spite of changes in the operating characteristic of the receiver or changes in the circuits thereof, for example, when a space discharge tube is changed, a pair of simultaneously adustable resistances are included in the filament heating circuit of the filament of one of the discharge tubes.

The resistances function to permit the potential difference between the grid and filament of the tube, with which they are associated, to be varied without affecting the potential of the point on the filament heating circuit whence are derived potentials applied tion, will be clear from the following descrip- In the drawing there is shown an antenna 1 connected through a tuned circuit 2'. and

condenser 3 to'ground. r I a The circuit 2 is tuned to the frequency' of the wave to be received. The condenser 3 serves as a low impedance path to ground for the high frequency ener and to prevent the flow of direct or low requency current to ground.

High frequency energy, produced in the tuned circuit 2 by the wave incident upon the antenna 1, is supplied to the coil or autotransformer 4.

The connection from the antenna circuit is made at an intermediate point of the coil 4, which has one terminal connected to the grid of modulator 5 and its other terminal connected through an adjustable condenser 6 to ground.

Oscillations from the source 7, which may be of any type, for example, a vacuum tube oscillator, are supplied throu'h a coupling between the coils 8 and 4 to t e grid of the modulator 5.

The method of connecting the antenna to the tube 5 is similar to that disclosed in application, Serial No. 48,988, filed August 8, 1925 by E. Bruce, Patent 1,710,254, April 23, 1929, and it is used for the purpose therein set forth. Briefly its purpose is to permit the source of local oscillations to be conjugately coupled with respect to the antenna. This serves two purposes, first, it prevents radiation of the locally supplied oscillations, and second, it permits the tuning of the circuit 2 to be adjusted without causing variations in the amount of energy supplied by the local source to the modulator.

The incoming wave and the locally supplied oscillations are combined in the device 5 to produce a changed frequency wave. This wave is selected by the filter 9 and amplified by an amplifier 10 comprising any number of stages. A

One stage of the changed frequency amplifier is shown in detail at 11, as a distortionless amplifier comprising a space discharge device. However, any other type of distortionless amplifier may be used.

The changed frequency wave is applied to the input electrodes of the detector 12 in whichit is-detected to .produce the speechcurrents which are supplied through a transformer or repeating coil 13 to a load circuit source 16.

tended to any subscriber of the system.

Space current issupplied to the discharge dev1ces 5, 11 .and 12 from the direct current For tube 5 the space current path extends from the positive terminal of source 16 through a conductive path within the filter 9, anode of tube 5 to its cathode, the left hand resistance 17 and via ground to the negative terminal of source 16.

The space current circuit for tube 11 extends from the positive terminal of couroe 16, a conductive path in the diagrammatically shown portion of the changed fr uency amplifier, anode of 11 to its catho e, resistance 20, cathode of 5, resistance 17 and via ground to the negative terminal of batterv 16.

The tube 12 is supplied with space current over the circuit extending from thepositive terminal of source 16, primary winding of transformer 13, anode of 12, its cathode and through ground to the negative terminal of source 16.

As shown in the drawing, heating current is supplied to the filaments of tubes 11 and 5, connected in series, from the negative terminal of source 18, resistance19', the filament of tube 11, a portion of resistance 20, filament of tube 5, a portion of resistance 17 and through ground to the positive'terminal of source 18. i

The grid of tube 5 is connected to its filament over the following direct current path. From the grid of .tube 5 through the lower half of coil4, the inductance of the tuned circuit 2, a resistance 21, resistance 22, the filament of tube 11, a portion of resistance 20 to the right hand or negative terminal of the filament of tube 5.

The grid is therefore negatively biased by the voltage drop caused by the flow of filament heating current through the series circuit including the filament of tube 11 and the ortion of resistance 20 associated with the lament of tube 5.

The grid of tube 11 is negatively biased by the voltage drop due to the flow of filament heating current through that portion of the resistance 19 between the contact 23 and its terminal connected to the filament lead.

The output side of'the changed frequenc amplifier 10 isconnecte'd by a lead'24 inclu ing a'blocking condenser 25 to a plural stage auxiliary .Qchanged fre uency amplifier 26. A portion of the change uenc energy is I supplied through the-lead in uding the condenser 25 to the auxiliary unit 26. After being amplified in unit 26, the changed frequency current is supplied by a transformer 27 to a rectifier R. The rectifier is herein shown, by way of example, as including a three electrode space discharge device 28 having its grid and plate electrodes directly connected to constitute, in effect, a two electrode device.

Heating current is supplied to the filament of the rectifying tube 28 from a source '29 over the following circuit. From the negative terminal of source 29 through a resistance 30, ripple suppressing filter 31, the filament, resistance 32 and ground to the positive terminal of source 29.

The circuit through which the auxiliary chan ed frequency current is applied to the reetit ier 28 extends from its grid and plate, through secondary winding of transformer 27 condenser 33 to ground and thence through resistance 32 to its filament and across t e space path within 28 to its grid and plate.

The receivin system is so designed that, when the wave incident upon the antenna has an amplitude of definitely chosen value, speech current of the minimum amplitude necessary for satisfactory commercial operation is supplied to the apparatus or line connected to the terminals 15 and the grids of the various tubes have impressed thereon the steady potentials referred to above.

When the system is in 0 erative condition, switch 35-is thrown to the eft to engage contact 36.

Provided the amplitude of the incident wave is greater than is necessary to produce output speech currents of an amplitude equivalent to the minimum commercially satisfac- 40 tory value, changed frequency current is supplied to the rectifier R. I

The rectified current produced by rectifier R flows over the following circuit. From the grid and plate electrodes of tube 28,

v through the secondary winding of transformer 27, ripple suppression filter 34, switch 35, contact 36, resistance 22, filament of tube 11, resistance 20, filament of tube 5, resistance 17 and through ground'to resistance 32,

v filament of tube 28 and across its space path to its grid and anode.

The drop of voltage across the filaments of tubes 11 and 5 and the included portion of resistance 20 and resistance 22, due to 56 the flow of the unidirectional component of the rectified current therethrough, will be impressed upon the grid of modulator 5, to supplement the biasing poiential applied hereto due to flow of heating current.

60 By virtue of the supplementary biasing potential, the overall gain of the system is controlled to transmit a desired amplitude of speech current to the apparatus or line connected to terminals 15.

Q Should the amplitude of the incident wave tion, Patent 1,778,750, the s increase or decrease, due to fading efiects, the amplitude of the rectified current and the voltage drops across the filaments of tubes 11 and 5 and the resistances 20 and 22 will vary correspondingly. The voltage applied to the grid of modulator 5 will, therefore, increase or decrease to decrease or increase the overall gain of the system and thereby maintain constant the amplitude of the speech current supplied to the apparatus or line connected to terminals 15.

As in the above mentioned Bruce applicaof operation of the control is determined bythe resistance (R) and the capacity (C) in shunt to R. In the present system, B is the resistance 22 and is of 100,000 ohms. The capacity (C) in .shunt to 22, is the combined capacities of condensers 3, 40, 39 and 33 which e uals .4 mf. The time constant (RC) is there ore 100,000 1o- X 10; second,

which is satisfactory for one operating condition. The value of the capacity of combined condensers 3, 40, 33 and 39 may be materially reduced in practice and hence the speed of operation may be increased.

However, if the incident wave fades to such an extent that the amplitude of the speech current is below the value necessary for satisfactory commercial operation, the auxiliary amplifier cuts off to prevent the transmission of changed frequency current to the rectifier tube 28. Hence no gain control potential is applied to the grid of the modulator 5.

The id and plate of tube 28 are connected, y a direct current path, to the left hand terminal of resistance 19 and, therefore, have applied to them a negative potential determined by the source 18 and resistance 19. This path includes the resistance 22.

Any direct current flow through the resistance 22 would change the biasmg potentials applied to the grids of tubes 5 and 11 and thereby interfere with the proper operation of the gain control mechanism,

For example, if the grid and plate of tube 28 are at a positive potential with respect to its filament, direct current will flow through the resistance 22 and thereby increase the biasing potential applied to the grid of tube 5. This would decrease the overall gain of the system and thereby reduce the speech current supplied to the apparatus or line connected to terminals 15, to a value which would be unsatisfactory for commercial operation.

To revent' the flow of direct current 1 throug the resistance 22, under the conditions stated above, the heating circuit of the filament of tube 28 is rovided with resistance units 30 and 32 '0 such value that one terminal of the filament of tube 28 has applied thereto a negative potential substantially the same or slightly less than that applied to is grid and plate. Since the tube 28 5 is unilaterally conductive and current can ..only flow across its space path when the grid and plate are positive with respect to its filament, no current will flow through its output path, including resistance 22, until changed frequency current, adapted to render the grid and plate ositive with respect to the filament, is supplied to the tube.

However, by maintaining equal potentials upon the respective electrodes of tube 28, i. e. grid, plate and filament, rectified current will flow through resistance 22 when the tube is supplied with changed frequency current of the proper polarity to render the id and plate positlve with respect to the lament.

During the operation of the system it may be necessary or desirable to change the steady potential applied to the grid of the modulatlng tube 5. To effect this function, without distributing the otential relations with respect to the recti er tube 28, the filament of tube 5 is provided with two simultaneously adjustable contacts 37 and 38, which respecttively engage the resistances 17 and 20.

As thecontacts 37 and 38 are shifted, the amount of resistance 20, connected between its filament and the negative lead of the filament of tube 11 is varied. Consequently the potential difference between the filament of tube 5 and its grid is changed. At the same time, the amount of resistance 17 included in the filament heating current is changed by an equal amount to maintain the heating current constant.

The method of control, for the biasing pot'ential applied to the grid of tube 5, possesses the advantage that this potential may be varied by a single adjustment to meet varying conditions of operation, without changing the total current flowing through the heating current circuit of tubes 11 and 5 and without varying the potential applied to the grid and plate of rectifying tube 28.

While one embodiment of the invention has been disclosed and certain specific details have been described, it is to be understood that this invention is not limited thereto but only by the scope of the attached claims. i

What is claimed is:

1. In combination a space discharge device including a hot electrode and at least one cold electrode, a circuit through which heating current is supplied to said hot electrode, means for derivin from said heating circuit a normally fixe potential and applying it to said cold electrode, means in the heating circuit for changing the potential difference between said hot and cold electrodes while maintaining constant said normally fixed potential and means actuated by the output current of said space discharge device for varying said normally fixed pofrom said heating circuit a fixed potential and applying it to the cold electrodes of a plurality. of said discharge devices, and means in the heating circuit for changing the potential diiference between the hot and cold electrodes of at least two of said plurality of discharge devices while maintaining constant said fixed otential,-said means operatin to change t e potential of said grid and the potential of the hot electrode of another of said space discharge devices.

3. A system comprising space discharge devices each including a hot and at least one cold electrode, a circuit. through which heating current is supplied to said hot electrodes, means for der1ving from said heating circuit a fixed potential and applying it to cold electrodes of a plurality of said discharge devices, means for maintaining the hot electrode of one of said lurality of devices at substantially the xed potential, and means in the heating circuit for changing the potential difference between the hot and cold electrodes of at least one other of said pluralitiyxof devices while maintaining constant the ed potential.

4. A system comprising space discharge devices each includmg a hot and at least one cold electrode, means for connecting certain of said devices in cascade, a series circuit through which heating current is su plied to the hot electrodes of said casca e connected devices, means for deriving from said heating circuit a fixed potential and applying it to cold electrodes of one of said certain devices and at least one other device, means for controlling said other device for applying a varyin potential to the electrode of said certain evice to which the fixed potential is applied, and means in the heating circuit for changing the otential difference between the hot and co d electrodes of said certain device while maintainig constant said fixed potential.

5. A receiver for signal modulated high frequency carnier waves comprising a space discharge modulating device including a cathode and grid, a circuit through which heating current is applied to-said cathode, means for deriving from said heating circuit a fixed potential and applying it to said id, a source of oscillatlons differing in requency from the wave to be received and connected to said modulator for changing the frequency of said received wave, means for selectively amplifyin the wave of changed frequency, means or detecting said selected wave to produce the signal current means for rectifying a portion of said changed frequency wave, means actuated by said rectified current to control the ain of said receiver, and means in said heatlng circuit for changin the potential difference between said cat ode and said grid while maintaining constant said fixed potential.

In witness whereof, we hereunto subsribe our names this 9th day of March 1929.

JOHN G. GABRiEL. FRANK A. HUBBARD.

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