US2060614A - Modulating carrier wave transmitter - Google Patents

Description

Nov. 10, 1936. w. T. DITCHAM I MODULATING CARRIER WAVE TRANSMITTER Filed Oct. 10, 1934 2 Sheets-Sheet 1 NOV. 1936. w. T. DITCHAM MODULATING CARRIER WAVE TRANSMITTER 2 Sheets-Sheet 2 Filed Opt. 10, 1954 Patented Nov. 10, 1936 UNITED STATES MODULATING CARRIER WAVE TRANSMITTER William Theodore Dit cham, Ghelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application October 10, 1934, Serial No..'747,673 In Great Britain October 13, 1933 7 Claims.

This invention relates to modulated carrier wave transmitters and more particularly to socalled series modulated transmitters, i. e. transmitters of the kind in which modulation is effected by applying modulating potentials to the input circuit of a thermionic tube or bank of tubes whose internal impedance is connected in series with the internal impedance of a thermionic tube or bank of tubes operating at the carrier frequency to be modulated.

From another aspect the invention may be regarded as relating to that type of modulated carrier wave transmitter wherein the carrier power output is varied (in addition to being modulated) in dependence upon modulation level. Transmitters of this kind have the advantage of improved efficiency and the present invention has for its object to provide an improved simplified and highly efficient transmitter of this kind.

For the sake of brevity in description the tube or bank of tubes operating at the carrier frequency and at which modulation is effected will be referred to simply as the higher frequency tube (although, as previously stated, there may be more than one high frequency tube) while similarly the tube or bank of tubes in series with the high frequency tube will be referred to simply as the modulating tube.

According to this invention a modulated carrier wave transmitter comprises a high frequency tube, a modulating tube having its internal impedance in series with the internal impedance of the high frequency tube, means for applying modulating potentials to the input circuit of the modulating tube as in the usual way, and means for rectifying modulating potentials and utilizing the rectified direct current potentials ob tained as part of the grid bias for the modulating tube, the whole arrangement being such that normally i. e. in the absence of modulating potentials, the carrier power will be low but the said carrier power will be increased due tothe variation in grid bias of the modulating tube when modulating potentials are applied.

The invention is illustrated in the accompanying drawings which show, diagramatically two forms of construction wherein the said invention is applied to series modulated transmitters in each of which modulation is effected at a high frequency amplifier stage.

In Figure 1 a modulator tube or modulating potential amplifier tube has its anode to cathode impedance connected in series with the anode to cathode impedance of a carrier wave relay tube or amplifying tube, both impedances being connected in series with a source of direct cur-- rent potential. Modulating potentials are applied to the control grid of the modulator tube and rectified modulating potentials are impressed on the direct current grid biasing circuit of the modulator tube to oppose the bias source therein. In Figure 2, amplifying tubes are interposed between the source of modulating potentials and the control grid of the modulator tube and additional amplifying and potential reversing tubes are interposed between the rectifier and the direct current circuit of the modulator tube.

Referring to Figure 1 modulating potentials derived from a land line or other source (not shown) are applied, for example through a transformer T1 to the grid of a first amplifier tube V1 whose anode circuit is resistance-capacity coupled by elements B1 C1 to the grid of a second amplifying tube V2 whose anode circuit is similarly resistance-capacity coupled by elements R2C2 to the grid of a modulating tube V4. The grid cathode circuit of the modulating tube V4 consists of a resistance RgS connected at one end to the grid, a resistance Rn shunted by a condenser C4, and a grid bias source E4, the source having its positive terminal connected to the cathode and the elements Rg3 Rg4 and B4 being in series. The anode of the tube V1 is connected through a condenser G3, a resistance Rg2 and a bias source B3 all in series, to the common cathode point of all the tubes so far described and a preferably adjustable tapping T upon the resistance RgZ is connected to the grid of a further thermionic tube V3 whose plate is connected through the primary of a transformer T2 to a source of anode potential (not shown). The secondary of this transformer is connected at each end to a rectifier D1 or D2 (for example metal rectifiers may be used) the further terminals of the rectifiers being connected together and to the grid side of the resistance R 4. The other side of this resistance Hg} is connected to a centre tapping point on the secondary of the transformer T2. Thus voltage from the anode of the tube V1 is applied; at a strength which can be adjusted by moving the adjustable tapping T to the grid of the tube V3 and, owing to the action of the rectifying circuit including D1 and D2 (which is a full wave rectifier circuit) there will be set up across the resistance R 4 a direct current potential substantially proportional to the level of the modulating potentials. The arrangement is such that the grid bias applied to the modulating tube V4 as a result of the voltage set up across the resistance Rg4 opposes the negative grid bias due to the bias source B4. The anode of the modulating tube V4 is connected through an inductance L to the electrical centre of the cathode of the high frequency tube V5 and the anode of the said tube is connected through an inductance L1 to the anode potential source as in the usual way, so that the modulating tube V4 and the high frequency tube V5 are in series across the source of anode potential. Carrier frequency potentials are applied to the grid circuit of the high frequency tube V5 as indicated and the anode circuit of this tube is associated with the usual tuned circuit 00.

The adjustments are such that in the absence of modulating potentials only a comparatively small potential exists across the high frequency tube V5 so that the carrier power and the input power will be low. When, however, signal voltages are applied to the grid of the tube V1 the negative grid bias upon the tube V4 will be reduced, and the carrier power produced in the output of the high frequency tube accordingly increased.

fne resistance Rg in the grid of the modulatng tube and the condenser C4 in shunt therewith constitute a tim control circuit, and the time control circuit should be designed to have a period (time constant) which is long compared to the time period of the lowest modulating frequency required to be transmitted.

In the modification shown in Figure 2 modulating potentials are applied through transformer T1 to the grid of a tube V1 whose output is amplified by the tube V2 in cascade therewith, and thence passed to the grid of a modulating tube V4 whose anode-cathode space is in series with that of a high frequency tube V5, as in the well known series modulation arrangement. The modulating potentials are also applied through a transformer T'2 to a full wave rectifier circuit including rectifiers D'1 D2. The rectified output is applied across a resistance R 4 shunted by a capacity 0'4, the combination being in series with a bias battery H4 in the grid circuit of an amplifier tube V4. The voltage set up across R'gi by a rectifier circuit opposes the bias due to the source B4. The anode of tube V4 is connected through a resistance, as shown, to a preferably adjustable tapping T1 on a resistance which is connected at one end to a source of anode potential (not shown) and at the other to the cathode point. The anode of tube V4 is also connected through a resistance R"g4 to the cathode point and through a bias source B"4 in series with a resistance R'gii to the grid of a tube V4. The anode of tube V4 is connected (at) through a resistance, as shown, to the anode source for the tube V4 (b) through a time control circuit R 4C4 to the cathode of the modulating tube V4 and (0) through a bias battery B4 in series with a resistance Rg3 to the grid of the tube V4.

In operation rectified modulating potentials from the circuit T'2 D'1 D'z reduce the negative bias on the grid of V4 which causes the positive voltage across this tube to decrease in value thereby increasing the effective negative bias on the grid of V"4 which in turn results in increasing the positive voltage across V4 and so reduces the effective negative bias on the grid of the modulator tube V4. This reduction in grid bias on the modulator tube results in an increase of the power at the high frequency tube V5. The resistance capacity combination Rg4C4 is arranged to have a time constant greater than the time period of the lowest required modulating frequency.

Although in the illustrated embodiments full wave rectifier circuits are used (and preferred) full wave rectification is not an essential feature and half wave rectification may be resorted to and effective operation still obtained.

What is claimed is:

1. In an improved modulation system, a thermionic relay tube having a control grid, an anode and a cathode, a circuit applying carrier waves to be modulated to the control grid and cathode of said tube, a modulator tube having an anode, a cathode and a control grid, a circuit connecting the anode-to-cathode impedances of said tubes in series with each other and with a source of direct current potential, a source of modulating potential connected to the control grid of said modulator tube, a source of direct current potential connected with the control grid and cathode of said modulator tube for maintaining said control grid negative relative to said cathode, whereby the impedance between the anode and cathode of said modulator tube is high in the absence of modulating potentials, and means for reducing the impedance of said modulator tube in the presence of modulating potentials, comprising a resistance connected in series with said last named direct current potential source, and a rectifier having input electrodes coupled to said source of modulating potentials and output electrodes connected to said resistance.

2. A modulation system as recited in claim 1 in which a thermionic amplifier is interposed between said source of modulating potentials and the input electrodes of said rectifier.

3. A modulating system as recited in claim 1 in which amplifying means is interposed between said source of modulating potentials and the control grid of said modulator tube.

4. A modulating system as recited in claim 1 in which said rectifier is of the full Wave type.

5. A modulating system as recited in claim 1 in which amplifying and signal potential reversing tubes are interposed between the output of said rectifier and said resistance.

6. A modulating system as recited in claim 1 in which amplifying devices are interposed between said source of modulating potentials and the control grid of said modulator tube, and in which other amplifying devices are interposed between said source of modulating potentials and the input electrodes of said rectifier.

'7. A modulation system as recited in claim 1 in which amplifying devices are interposed between said source of modulating potentials and the control grid of said modulator, and in which additional potential amplifying and reversing devices are interposed between the output of said rectifier and said resistance.

WILLIAM THEODORE DITCI-IAM.

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