US1936162A - Transmission system - Google Patents

Description

1933- R. A. HEISING TRANSMISSION SYSTEM Filed Nov. 30, 1925 //7ve/77"a/7 F4 mend 4. l/e/slly I by 4773 Patented Nov. 21, 1933 TRANSMISSION SYSTEM Raymond A. Heising, Millburn, N. 1., assignor to Western Electric Company, Incorporated, New York, N. Y... a corporation of New York Application November 30, 1925 Serial No. 72,229

19 Claims. (Cl. 179-171) her 1, 1919 and December 1, 1924, and which have since matured into Patents 1,567,734, granted December 29, 1925, and 1,620,636, granted March. 15,,

1927, respectively.

The general object of the invention is to insure,

in modulated high frequency generating and transmitting systems, great stability and efliciency in operation as well as economy of plant.

A particular object relates to improved methods of and means for supplying the negative biasing potentials to the impedance control elements of one or more of the electric discharge devices in such a system. By the means and methods of the invention, negative biasing potentials are obtained from the energizing circuits for the discharge devices.

A more specific object is to derive such negative biasing potentials in the form of voltage drops occurring in the normal operation of a transmitting. system including an oscillation generator, and in particular to achieve this result in such manner that the average biasing potentials are independent of the oscillation condition of the oscillator.

These objects are achieved, in a system for gen-- erating and transmitting a modulated carrier frequency wave produced by the constant current method of modulation, by including in the constant current path a resistance the potential drop across which is impressed upon the impedance control electrode of one or more three-element electric discharge devices. The location of the resistance in this path insures a substantially constant current therethrough, hence a constant potential across its terminals, and this resultwill occur without regard to the oscillation condition of the oscillation generator.

The constancy of the difl'erence of potential may be further assured by the use of a condenser connected in shunt to this resistance.

The essential principle of the constant current method of modulation resides in the use of a 4 common space current source for the device upon .55 lator. The variations impressed on the modulating device tend to cause corresponding variations in the current traversing the circuit including the common space current source and therefore to correspondingly vary the potential impressed from said source on the space current circuit of the second device. It is immaterial as to how the output terminals of the second device obtains its high frequency potentials.

In the older and perhaps more conventional arrangement the high frequency variations are produced by the second device. In a modified form they may be produced by a separate oscillator and impressed on the second device, which accordingly functions principally as a power amplifier.

This invention is equally applicable to these two alternate forms of the constant current modulating method.

The invention will be set forth in greater detail, with the elucidation of further objects and 76 features, in the following description taken in connection with the accompanying drawing, the two figures of which illustrate the application of the invention to the two above mentioned modifications, respectively, of. the constant current modulation method.

In the drawing, various elements of the system are labeled with identifying symbols which are suggestive of the functions of the elements. For example, the symbols M, 0, AS, AN and AP are used to indicate, respectively a. modulator, an oscillator, a speech amplifier, an antenna and a power amplifier.

Referring to Fig. 1, the arrangement of this figure, as a whole, is adapted to generate modulating currents in circuit 1, to use these currents in modulator M to modulate the high frequency carrier currents generated by oscillator O and to radiate the resultant modulated carrier waves into space by means of antenna AN.

The oscilator O is of the well known Meissner yp which is characterized in that its frequency is determined by the natural period of a circuit 7 distinct from the circuits immediately associated with the electric discharge tube which comprises 100 a portion of the oscillator.

In the circuit of Fig. 1, this tuned circuit is constituted by the antenna itself, including its capacity to ground, and this circuit is associated with the remaining portions of the oscillator, as in the conventional arrangement, by inductive coupling means.

Specifically, the oscilator comprises an electric discharge device 2 havingin its grid or impedance 110 controlled circuit a coil 3 coupled to coil 4 in the antenna circuit.

Blocking condenser 5 and resistance leak path 6 provide, in a manner well understood in the art, any desired value of negative biasing potential for the grid of the device 2.

The plate circuit of device 2 includes condenser E in series with the parallel arrangement of coil 8 and variable condenser 9, the coil 8 being coupled to antenna coil 4. The function of the condenser 9 is to vary the effective coupling of the coil 8 to the coil 4. Coils 3 and 8 are so arranged that the direct mutual inductance therebetween is negligible. The oscillation frequency is adjusted to any desired value by variation of the inductance of coil 4. The variable condenser 10 may also, although not necessarily, .be inserted in the antenna circuit to increase the capacity reactance of the antenna to such a value that the inductance of coil 4 may be effectively varied to provide the frequency adjustment, and this condenser may itself be made variable to provide an additional adjusting means if desired.

The modulator and oscillator are related to each other, and with respect to their means for producing a normal space current therethrough, to constitute one example of the so-called constant current modulation system. The particular form here illustrated is disclosed in U. S. patent to Heising 1,442,147, January 16, 1923.

The space current paths of the modulator and the oscillator are supplied with space current from a direct current source 11 through a low frequency choke coil 12. The modulator M is, in effect, a variable impedance device, the function of which is to cause a variable flow of current through the space circuit of the oscillator O in response to variations impressed on the input cir- -cuit of the modulator M. Accordingly because of the variations in the potential drop across the choke coil 12, corresponding variations occur in the potential applied to the space circuit of the oscillator. These potential variations in the space circuit of the oscillator are effective to cause corresponding variations or modulations of the carrier oscillations therein produced. The inductance of the choke coil 12 should be such as to insure a substantially constant current supply to the oscillator and modulator. The variations of the carrier oscillations are accurate reproductions of the modulating variations only to the extent that this current is maintained constant.

The invention particularly relates to a means associated with the circuits so far described for deriving negative biasing potentials for the grid of the modulator.

The direct current source 11, which supplies space current to the modulator and oscillator, is used for this purpose. A resistance 13 is inserted between the negative terminal of this source and the cathodes of the modulator and oscillator. The grid of the modulator is connected to this resistance at its junction with the source, or at any other point intermediate its ends, depending upon the desired biasing potential. The space current flowing through this resistance produces a drop of potential thereacross, at least a portion of which, depending upon the particular point of connection, is impressed upon the grid of the modulator. This drop of potential will be substantially constant since, on account of the type of modulation system used, the current traversing the common space circuit is substantially constant.

In order to further insure an absence of fluctuations in the current flowing through the resistance 13, due for example to the operation of the modulator or to variations of the characteristics of the source 11, a condenser 14 is connected in shunt to the resistance. This condenser is of low impedance for current of the frequency of these fluctuations and hence provides a shunt path through which they are diverted from the resistance 13.

The use of the resistance and shunting condenser in this relation accordingly insures that a negative potential of constant value is impressed upon the grid of the modulator and therefore makes it unnecessary to use a separate source for this purpose.

Obviously, this expedient may be employed to supply biasing potentials to any one or all of the electric discharge devices used in the system if found desirable, including the oscillator itself and additional amplifiers that may be found advantageous, on occasion, to amplify the modulating variations before modulation or to amplify the resultant modulated carrier wave after modulation.

Referring now to the arrangement illustrated in Fig. 2, the arrangement of this figure differs from that of Fig. 1 principally in that modulation occurs by variation of carrier oscillations in the space circuit of a power amplifier into which the carrier oscillations are fed rather than in the oscillator itself.

This arrangement does not differ essentially, as to its fundamental principles of operation, from that of Fig. l. The system shown in Fig. 2 is sometimes preferred where it is desirable to maintain a constancy of frequency greater than can be secured by the arrangement of Fig. 1, in which the oscillator oscillates directly into the antenna.

In view of the similarity of the circuits of Figs. 1 and 2 and the explanation already given of the basic principles underlying the operation of each and their important differences, it will not be necessary to describe the circuit arrangement of Fig. 2 in great detail. The elements of this figure, which have similar functions as those of the corresponding elements in the arrangement of Fig. 1, are similarly labeled. The features, counterparts of whose labels are not found in Fig. 1, are specific to the arrangement of Fig. 2 and these elements only will be described.

The speech amplifier AS functions in the ordinary manner characteristic of amplifiers wherever used, and its presence in this circuit does not result in any new operational characteristic of the modulating system. It may equally well be used in the system shown in either figure, and in a practical case would be used when the design of the modulating device, with relation to the device in which the carrier oscillations occur, is such as to require an amplitude of impressed variations greater than could be secured by utilizing the source of modulating currents 1 directly.

On account of the presence of this speech amplifier, opportunity is provided for demonstrating how the grids of more than one low frequency electric discharge device may jointly be supplied with negative biasing potentials in accordance with the means of the invention; that is, the grids of the modulator M and the speech amplifier AS are connected in parallel with respect to each other and the resistance 13. Obviously, the grids of these two devices would not have to be connected to the resistance at the same points. Further, in the circuit of Fig. 2, as in the circuit of Fig. 1, the negative biasing potentials for the grids of the oscillator and power amplifier, either one or both, may be derived in the same manner.

The oscillator 01 illustrated in Fig. 2 is of a different type from that correspondingly illustrated in Fig. 1. It is of the form known as the Colpitts oscillator, which is disclosed for example, in Canadian Patent 203,986, September 14, 1920. It is distinguished from other well known oscillator circuits by a capacitive coupling between its output and input circuits as constituted, in the particular circuit illustrated, by condensers 15 and 16.

The frequency is determined by the natural frequency of the circuit comprising these condensers and inductance 17. The condensers 18 and 19 play no part in determining the frequency of the oscillation produced by the oscillator. They prevent the fiow of direct current from the source 11 through certain portions of the associated circuit, whereby this current is caused to traverse the space path within the tube. These blocking condensers are accordingly designed to have large capacity as compared with that of condensers 15 and 16.

The oscillator described above is coupled to the power amplifier AP by connecting together the cathodes and grids, respectively, of the two devices. The power amplifier can accordingly be considered as coupled to the oscillator through the series combination of condensers 16 and 18.

The space current source 11, resistance 13 and low frequency choke coil 12 are related to the modulator M and power amplifier AP in the same manner as the corresponding elements are related to the modulator and oscillator in the system of Fig. 1.

The high frequency choke coil 20 is inserted between the plates of the modulator and power amplifier to prevent reaction from the high frequency circuits to the low frequency circuits, that is, to the circuits associated with the modulator. Coil 20 therefore insures that the modulating function will not be complicated by the conditions in the high frequency circuits. This coil should accordingly have high impedance for the carrier frequency, but low impedance for the modulating frequencies.

It is desirable for the most eflicient operation 'of the system of Fig. 2 to make the oscillator 01 oversize. This results in a reduction of the reaction of the power amplifier on the oscillator which would otherwise occur to a substantially greater extent on account of the shunting effect of certain of the amplifier tube impedanoes.

An over-size oscillator is distinguished from a normal size oscillator by a design which results in a greater amplitude of current in its output circuit than would be required by its normal operation in the system.

With the larger amplitude of current sumcient voltage is impressed on the grid of the power amplifier tube, when using a larger coupling capacity supplied by condensers 16 and 18 than would be necessary if a smaller size oscillator were used. The grid-filament and grid-plate resistance and capacity of the power amplifier therefore have a much smaller effect on the frequency of the oscillator, because of the relatively smaller reactance in parallel with them. This results in a great constancy of frequency in the oscillator and therefore in the system as a whole.

' This expedient is of value, for example, where it is desired to simplify the transmitting circuits, as by substituting a single-stage for a multi-stage power amplifier. A simplification of this kind tends to result in greater frequency variations in the system. Since such simplification means a reduction in the power of the set, the expedient is useful in securing substantially as great a constancy of frequency in small power transmitting sets as can be achieved by other means in large power sets.

Because of the economy of space and power, this expedient is especially applicable on board ship. In a typical instance, its use made possible the requirement of only four tubes for the transmitting circuits and only two sizes of tube were found necessary, the speech amplifier requiring a four-watt tube and the modulator, oscillator, and power amplifier each requiring a 150 watt tube.

Of course, other types of oscillator than that illustrated, including the one illustrated in Fig. 1, could be used in the system of Fig. 2, and the over-size principle could be analogously applied, with similar advantage, if either of these other types were used. However, the particular type of oscillator shown has been found to be preferable for the purpose, and particularly it has been found that greater stability of frequency may be secured by its use.

It will be obvious that the general principles herein disclosed may be'embodied mother organizations greatly difierent from those illustrated without departing from the spirit of the invention. Accordingly, it is to be understood that this invention is notlimited to the arrangements or specific details disclosed but only by the scope of the appended claims.

What is claimed is: .1

1. In combination, a variable impedance device having a space discharge path and an electrostatic control element, a second device having a space discharge path, a common source of space current for said devices, means independent of the variable impedance device for producing alternating current variations in the discharge path of the second device as in accordance with the constant current modulation principle, and a resistance in series with said common source and a tap connection from said resistance to said control element.

2. In combination, a variable impedance device having a space discharge path and an electrostatic control element, a second device having a space discharge path, means for varying the impedance of said variable impedance device, a common source of space current for said devices, means independent of the variable impedance device for producing alternating current variations in the space discharge path of the second device, and a resistance and a condenser in series-multiple relation with said common source, said condenser having low impedance for currents whose frequency corresponds to the fre-- quency of the variations of said impedance.

3. In combination, a variable impedance device having an anode, a discharge electrode and a variations in the space current of the second device, and means connecting said control electrode with a point in said common circuit which is negative with respect to the discharge electrode of the variable impedance device.

4. The combination specified in claim 3 characterized by means for impressing a modulating wave between the discharge and control electrodes of said variable impedance device, and in that said alternating variations are of carrier frequency.

5. In combination, a variable impedance device having an anode, a discharge electrode and a control electrode, a second device having an anode and a discharge electrode, a common circuit including a uni-directional source of space current for the anode-discharge-electrode paths of said devices and a constant current choke coil, means for connecting said paths in parallel with each other and in series with said common circuit, means independent of the variable impedance device for producing alternating current variations in the space current of the second device, a resistance connected between the negative terminal of the space discharge source and the discharge electrodes of said devices, and means connecting said control electrode with a point in said resistance.

6. The combination specified in the next preceding claim, characterized by means for impressing a modulating wave between the discharge and control electrodes of said variable impedance devices, by "a constant current choke coil in said common circuit, and in that said alternating variations are of carrier frequency.

7. A variable impedance device comprising a discharge electrode, an anode and a control electrode, a. circuit including a uni-directional space current source connected to said discharge electrode and anode, a resistance in said circuit between the negative terminal of the source and the discharge electrode, a condenser shunting said resistance, and having an impedance at the frequency at which the device works which is small as compared with that of the resistance, and means connecting the control electrode to a point in said resistance.

8. An oscillator tube in combination with a variable impedance tube having a grid, arranged in shunt thereto with respect to a common source of space current for said tubes, a resistance and a constant current choke coil in series with said common source, and a tap connection from a point in said resistance to said grid.

9; A variable impedance device comprising a discharge electrode, an anode, and a control electrode, a circuit including a uni-directional space current source connected to said discharge electrode and anode, a resistance in said circuit between the negative terminal of the source and the discharge electrode, a circuit including a condenser shunting said resistance, and means connecting the control electrode to the closed circuit comprising said resistance and shunting circuit at such a point, and the reactance of said condenser being such for alternating current flowing between said point and said discharge electrode, that there is a difference of potential which is small as compared with the normal direct current potential drop across said resistance.

10. The combination of a space discharge repeater having an impedance control electrode, impedance means included in the output circuit of said repeater for impressing a desired negative potential upon said control electrode, and a circuit including a condenser connected in shunt to said impedance means. y

11. In an electrical system, the combination with an oscillator tube having a power-supply circuit, of a modulator tube also having a powersupply circuit, said power-supply circuits being partially in common including the supply source and a constant current choke coil, said modulator tube having a grid-filament circuit including a modulating device and at least a part of the common portion of said power-supply circuits, said part including a grid biasing device.

12. The combination with an electron discharge device having a cathode, an anode, and a grid, of

an external circuit connecting the cathode and grid and having intercalated means to impress upon said grid an actuating potential, a second external circuit connecting said cathode and anode and having a portion in common with said first external circuit, and an aperiodic loop in series in the common portion of said circuits to oppose variation of the anode current.

13. The combination with an electron discharge device having a cathode, an anode and a grid, of an external circuit connecting the cathode and the grid and having intercalated therein means for impressing an actuating potential upon said grid, a second external circuit connecting said cathode and anode, said external circuits having a common portion, and means in said common portion whose impedance varies inversely with the frequency to oppose low frequency variations in the anode current or potential.

14. The combination with an electron discharge device having a cathode, an anode, and a grid, of an external circuit connecting the cathode and the grid and having intercalated therein means for impressing an actuating potential upon said grid, a second external circuit connecting said cathode and anode, said external circuits having a common portion, and means controlled by the anode current in said common portion whose impedance varies inversely with the frequency to oppose low frequency variation in the anode current.

15. The combination with an electron discharge device having a cathode, an anodefand a grid, of an external circuit connecting the cathode and grid and having intercalated therein means for impressing upon said grid an actuating potential from an external source, a second external circuit connecting said cathode and anode, a common portion of said circuits having two branches constituting a series loop circuit, and a resistance and a capacitance respectively in said branches, said loop circuit serving to oppose variations in the anode current or potential.

16. In a circuit including a vacuum tube comprising a cathode, a grid and an anode, connections for energizing the said elements, a portion of said connections being common, means in said common portion for opposing oscillations due to electrical interaction comprising a loop circuit containing resistance and capacity, the reactance of the former being greatly in excess of that of the latter at operating frequencies.

17. The combination with an electron discharge device having a cathode, an anode, and a grid, of an external circuit connecting the oathode and grid and having therein means to im-- indicating device and having a portion in common with said first circuit, and an aperiodic loop circuit in series with the common portion of said circuits whereby low frequency variations in the anode current are opposed.

18. In a radio system, two vacuum tube devices, a source of direct current potential connected to the plate circuits of said vacuum tube devices in parallel, a condenser connected in parallel to said two plate circuits and connections from the grid circuit or one of said vacuum tube devices to a point of intermediate potential in the plate circuit or the other or said vacuum tube devices,

RAYMOND A. nErsma.

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