US2169859A - Carrier reduced modulator - Google Patents

Description

Aug. 15, 1939. G. 1.. USSELMAN CARRIER REDUCED MODULATOR Filed June 26, 1937 ELECTROSTATIC sol/R05 or news ENERGY INVENTOR 6.1. USSELMAN BY ATTORNEY SIGNAL /4./-T SOURCE HF l SOURCE OF WAVE ENERG Y Patented Aug. 15, 1939 UNITED STATES PATENT OFFICE CARRIER REDUCED MODULATOR Application June 26, 1937, Serial No. 150,499

3 Claims.

This application includes a new and improved means for producing sideband modulation with the carrier reduced or suppressed, the modulation being amplitude or phase and amplitude.

In modulators of the carrier reduced or suppression type known heretofore such as shown in Carson Patents a l-"1,343,306 and #1,343,307, two balanced tubes have been used in the modulator. In the present invention by the use of a novel circuit arrangement I accomplish sideband modulation by the use of a single tube in which modulation the carrier is reduced or entirely suppressed. In a modification, I accomplish sideband modulation accompanied by phase modulation with carrier reduction or suppression.

In describing my invention reference will be made to the attached drawing wherein,

Figures 1 and 2 illustrate the two modifications of the modulators described hereinbefore.

In Figure 1 amplitude sideband modulation With reduced carrier is accomplished.

In the arrangement of Figure 2 phase and amplitude modulation of sideband energy with reduced carrier is accomplished. 1

In both figures wave energy to 'be modulated is supplied from a high frequency source of wave energy HF to a balanced relaying circuit L2 Z1 Z2 and from the adjacent points of impedances Z1 Z2 by way of impedance Z3 to the control grid 8 of an electron discharge tube V1 wherein the wave energy from the two paths are combined, repeated, or amplified. Grid bias for the control grid 8 is supplied by Z3 from a. source B2. The anode ll] of V1 is connected in an alternating current circuit C L3 and is supplied by potential from a source B1. The screen grid electrode of the tube may also be supplied by a potential from a source B1. By-passing condensers are connect ed between the screen grid and cathode and between the low potential end of the alternating current circuit C1 L3 and the cathode. The sideband energy and reduced carrier is supplied from L3 to an inductance L4 in a load circuit A. Modulator tubes V2 V3 have their anode to cathode impedances shunting the impedance Z1 Z2 respectively. The anode to cathode direct current circuits of V2 V3 are both completed through reactor X, source B3 andground. The alternating current anode to cathode circuits of V2 V3 are completed through Z1 Z2, the grid to cathode impedance'of tube V1 and impedance of Z3 in parallel, and ground G. Modulating potentials are supplied to the grids l4 and 16 of tubes V2 V3 in phase opposition from the secondary of transformer T, the primary of which is coupled to asource of modulating potentials AF.

In the modification of Figure 2 the impedances Z1 and Z2 are capacitive and inductive in character respectively. The impedances Z1 Z2 are 5 coupled with the terminals of L2 by blocking condensers B0. In this arrangement phase and ampiitude modulated sidebands with reduced or suppressed carrier are produced.

In Figure 1, tubes V2 V3 are the audio fre- 1 quency signal amplifier tubes. Tube V1 and circuit L2, Z1, Z2, and Z3 is the sideband modulator circuit in which the carrier frequency energy to be modulated fiows. Circuit HF and L1 is the carrier frequency energy source. An electrostatic shield S is placed between coils L1 and L2 to prevent undesirable electrostatic coupling between the carrier frequency source and the modulator circuits. Reactor X allows the center point of coil L2 to float at carrier frequency, that is, to be above ground carrier frequency potential, This reactor X, as stated above, allows the direct current circuit to flow through the tubes V2 V3. The reactor X also insures a steady flow of current from source B3 in that changes in direct current and/or potential therefrom are opposed by the impedance of X, this improves the modulation characteristic of the entire arrangement because when the current in one tube, say V2, is reduced a certain amount (by modulation) the current in V3 is compelled to increase by a like amount.

The operation of the modulators may be best described by referring to Figure 1. Assume all cathodes are heated and. all grids and anodes are supplied with proper potentials. Carrier frequency energy is supplied from source HF through coupling inductances L1 and L2 and from the latter through impedances Z1 Z2 to the grid of the tube V3 and to impedance Z3. If no audio signal or controlling potentials is present in transformer T the two sides of the circuit L2 Z1 Z2 are balanced relative to ground by the equal impedances to ground of tubes V2 V3. We have assumed here that the impedances Z2 Z1 are balanced relative to ground to which they are connected through Z3 and the control grid and cathode impedance of V1 in parallel. When signal potentials or controlling potentials are impressed from transformer T to the control grids of V2 and V3 then circuit L2 Z1 Z2 is unbalanced alternately, one side being at high and the other side being at relatively low radio frequency potential with respect to ground because of the relative change in impedances of V2 V3. This results in the combination of the carrier wave and the signal wave, which is impressed on the grid 8 of tube V1. The result is that tube V1 repeats or amplifies the sideband energy impressed on the grid 8 with the carrier wave energy reduced by circuit L2 Z1 Z2.

Sideband energy with reduced or suppressed carrier appears in the circuit C L3 and may be supplied to the load A. If resistance is used for impedance Z3 then tube V1 must be of the type that will pass anode current without the presence of rectified current in grid I.

By replacing the similar impedances Z1 and Z2 of Figure 1 by dissimilar impedances Z1 and Z2 as in Figure 2, a phase shift of the wave energy supplied by each branch to grid 8 with respect to the wave energy from HF is accomplished and phase as well as amplitude modulation of the sidebands supplied to C L3 is produced with reduced or suppressed carrier energy. In this case, a resistance is used at Z3 to supply the necessary impedance.

In modifications, the tube V1 may be replaced by triodes or tubes having a different number of electrodes. In the case triodes are utilized it may be desirable to neutralize the effect of grid to anode capacity in a well known manner.

In both modifications the circuit C L3 may be connected to said load by Way of filters, amplitude limiters, or amplifiers, one or more of which may be used between the modulator stage V1 and the antenna.

I claim:

1. In a modulation system, a source of wave energy to be modulated, a combining tube having a control grid and a cathode, a reactance coupled to said source of wave energy, impedances of different character coupling said reactance to the control electrode of said tube, a connection between the cathode of said tube and a point of symmetry on said reactance so that both sides of said circuit are substantially balanced, and means for unbalancing said circuit in accordance with signal frequencies, whereby sideband energy, the amplitude and phase of which varies in accordance with signal oscillations, and carrier energy of relatively small amplitude appears in the output circuit of said tube.

2. In a modulation system, a source of Wave energy to be modulated, a combining tube having a control electrode and a cathode, a reactance coupled to said source of wave energy, an inductive reactance and a capacitive reactance coupling the respective terminals of said first reactance to the control electrode of said tube, a connection between the cathode of said tube and a point of symmetry on said first reactance so that both sides of said circuit are substantially relatively balanced, and means for relatively unbalancing said circuit in accordance with signal frequencies, whereby sideband energy, the amplitude and phase of which varies in accordance with signal oscillations, and carrier energy of relatively small amplitude appears on the control electrode of said tube.

3. In a signalling system, a source of wave energy of carrier wave frequency to be modulated, an electron discharge tube having a control grid, a cathode and an anode, an output circuit coupled with the anode and cathode of said tube, a reactance coupled to said source of wave energy, an inductive reactance coupling a point on said first reactance to said control grid, a capacitive reactance coupling a point on said first reactance to said control grid, means coupling a point on said first reactance to the cathode of said tube, said last point being selected so that said reactance is substantially electrically symmetrical with respect to said cathode, a pair of electron discharge devices each having input and output electrodes, circuits connecting the output electrodes of said tubes in series with said first named reactance, means connecting a point of electrical symmetry on said last named circuit to the cathode of said first named tube, a source of modulating potentials, and means coupling said source of modulating potentials substantially in phase opposition to the input electrodes of said pair of tubes whereby sideband energy modulated in phase and amplitude and carrier wave energy of relatively small amplitude is produced in said output circuit.

GEORGE L. USSELMAN.

Images