Microwave tube modulator

Abstract: Provided is a modulator having two amplifying stages for supplying high-voltage pulses to the primary winding of a transformer connected to a microwave transmitter tube. The first stage uses field-effect transistors connected in a common source ccnfiguration to amplify input pulses to drive the second stage, which uses field-effect transistors connected in a common gate configuration to drive the transformer. The circuit configuration provides a large voltage swing across the transformer primary and a short transistor switching time.

Description: This invention relates to a microwave tube modulator. A number of devices have been designed to provide pulses for use in microwave modulation applications. Some of these use field-effect transistors (FET's) connected in parallel, and conventional designs use parallel FET's connected in a common source configuration. Such a configuration places comparative limits on the maximum voltages available at microwave tube modulator outputs and does not minimize the effects of drain-to-gate (Miller) capacitance, which adversely affects FET switching speeds. See, for example, US Patent Nos. 3,374,406; 3,374,407; 3,426,255; 3,509,567; 3,525,861; 3,544,399; 3,562,672; 3,582,975; 3,828,324; 3,914,555; 3,967,217; 3,995,178; 4,010,450; 4,071,777; 4,095,251; 4,143,387; 4,221,044; 4,235,010; 4,247,903; 4,262,296; 4,304,042; 4,328,470; 4,409,594; 4,425,527; 4,662,058; 4,679,050; 4,679,299; 4,683,387 and 4,689,802. Also EP-A-0 174 164 discloses a microwave tube modulator comprising an input terminal for receiving electrical input signals, an amplifier arrangement which comprises first and second amplifiers and a first coupling device for coupling an output signal of said first amplifier to an input of the second amplifier, and a storage device comprising at least one capacitor for storing electrical energy and arranged to couple electrical energy to one of said amplifiers, each amplifier comprising a plurality of field-effect transistors each having source, gate and drain terminals. The microwave tube modulator of the present invention is characterised in that the field effect transistors of the first amplifier are connected in common source configuration with said input terminal coupled to their gate terminals, the field-effect transistors of the second amplifier are connected in common gate configuration, said first coupling device comprises a transformer having primary and secondary windings, said primary winding being connected in series between said storage device and the drain terminals of the field-effect transistors of the first amplifier to couple electrical energy from the storage device to said first amplifier, and said secondary winding being connected to the source terminals of the field-effect transistors of the second amplifier, and a second coupling device is arranged to couple a feedback current, representative of the output signal of the first amplifier, to the field-effect transistors of the first amplifier. Modulators in accordance with the present invention are capable of supplying high-voltage pulses having short rise and fall times to the primary windings of appropriately selected transformers connected to a large number of different microwave transmitter tubes such as magnetrons, pulse cavity oscillators, cross field amplifiers and travelling wave tubes. The preferred embodiment of modulator in accordance with the present invention has a first stage using a number of parallel field-effect transistors (FET's) connected in a common source configuration to amplify input pulses for driving a second stage using a number of parallel FET's connected in a common gate configuration for driving a microwave transmitter tube transformer. The preferred embodiment of the present invention also comprises fail-safe means for ensuring the survival and continued operation of the modulator in the event of the failure of one or more FET's: this is an extremely advantageous feature if the embodiment is to be used in remote or aircraft installations. Modulators in accordance with the invention are capable of a relatively high switching speed. Modulators in accordance with the invention require a relatively low operating voltage. Modulators in accordance with the invention can be matched using appropriate output transformers, to a large number of microwave transmitter tubes. Modulators in accordance with the invention can supply high-voltage pulses having different pulse widths. Modulators in accordance with the invention can be made relatively small. Modulators in accordance with the invention are easy to produce and maintain. In the drawings, in which like reference characters indicate corresponding parts in all the figures: FIG. 1 shows, schematically, the microwave tube modulator; and FIG. 2 shows, schematically, a preferred combination of microwave tube modulator units connected to supply high power pulses to the primary circuit of a transformer connected to a microwave transmitter tube. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION With reference to FIG. 1 , shown is a schematic representation of a microwave tube modulator having a first, or driver, stage 10 and a second, or primary circuit, stage 12 . An input transformer 14 inductively connects a pulse source (not shown) to the first stage 10 of the modulator, and a coupling transformer 16 inductively connects the output of the first stage 10 to the second stage 12 . The first, or driver, stage 10 of the modulator has, in the embodiment shown, three field-effect transistors (FET's) 18 , 19 and 20 connected in parallel in a common source configuration. The source terminal of each of the three driver stage transistors 18 , 19 and 20 is connected through one of three associated resistors 22 , 23 and 24 respectively to the common terminal 26 and is grounded. Each of the resistors 22 , 23 and 24 is a fuse resistor and will open the source circuit of its associated FET if that transistor fails and draws excessive current. This action will effectively remove the defective transistor from the modulator circuit, leaving the overall circuit to function at reduced power but otherwise in a relatively normal manner. The secondary winding of the input transformer 14 is electrically shunted by a resistor 27 , one end of which is connected to the common terminal 26 and is grounded. Pulses induced in the secondary winding of the input transformer 14 are applied through resistors 28 , 29 and 30 to the gate terminals of the three driver stage FET's 18 , 19 and 20 respectively. The gate terminals of the FET's 18 , 19 and 20 are connected through resistors 32 , 33 and 34 respectively to one end of a feedback winding 36 of the coupling transformer 16 , the other end of the winding 36 being connected to ground. The drain terminals of the FET's 18 , 19 and 20 are connected directly to one end of the primary winding 38 of the coupling transformer 16 , and the opposite end of the primary winding 38 is connected to a source of positive voltage (as at 40 ), 300 volts in the embodiment shown. The second, or primary, stage 12 of the modulator has, in the embodiment shown, twelve power FET's (indicated by consecutive reference numbers 44 through 55 ) connected in parallel in a common gate configuration. The secondary winding 42 of the coupling transformer 16 is connected at one end to the common terminal 26 and is grounded. Pulses induced in the secondary winding 42 of the transformer 16 are applied to the source terminal of each of twelve primary stage transistors (indicated by consecutive reference numbers 44 through 55 ) through twelve associated resistors (indicated by consecutive reference numbers 56 through 67 respectively). Each of the resistors 56 through 67 is a fuse resistor and will open the source circuit of its associated FET if that transistor fails and draws excessive current. This action will effectively remove the defective transistor from the modulator circuit, leaving the overall circuit to function at reduced power but otherwise in a relatively normal manner. Also connected to the source of positive voltage 40 is one side of a capacitor bank 68 used to store primary stage electrical charges. The other side of the capacitor bank 68 is connected to the common terminal 26 and is grounded. The gate terminal of each of the twelve primary stage FET's (indicated by consecutive reference numbers 44 through 55 ) is connected respectively to one of twelve resistors (indicated by consecutive reference numbers 70 through 81 ) the other end of which is connected to the common terminal 26 and is grounded. The resistors ( 70 through 81 ) minimize parasitic oscillations in the transistor gate circuitry. The drain terminal of each of the twelve primary stage FET's ( 44 through 55 ) is connected to an output terminal 82 . A microwave tube (not shown) connected to the secondary winding of a suitable output transformer (also not shown) may be coupled to the modulator by connecting the primary winding of the transformer to the output terminal 82 . The microwave tube modulator just described may be advantageously combined if more power is required than can be provided by a single unit. A typical arrangement having four units (indicated by consecutive reference numbers 84 through 87 ) connected in parallel is shown in FIG. 2 . Their combined outputs are connected to the primary winding of an output transformer 88 that has a high secondary-winding-to-primary-winding turns ratio selected to match the voltage and current requirements of the microwave tube to be connected to the secondary winding of the transformer. The transistors used in the embodiment shown may be n-channel, enhancement-mode, metal-oxide silicon, field-effect transistors. An enhancement-mode type of FET is constructed so that it effectively has no channel until an appropriate gate voltage is applied. A direct-current operating condition is established by applying a positive voltage to the drain. As shown, the substrate is connected to the source. Both are effectively at ground potential, and channel electrons are attracted to the positive drain. A voltage applied to the gate controls the current flow. When a negative voltage is applied to the gate, positively charged holes from the p-type substrate are attracted toward the gate, effectively narrowing the conducting channel and reducing the source-drain current. When a positive voltage is applied to the gate, the positively charged holes are repelled, effectively widening the conducting channel and increasing the source-drain current. Prior to the application of an input signal, the capacitor bank connected between the source of positive voltage 40 and ground is charged. A positive input signal is applied through resistors 28 , 29 and 30 to the gates of transistors 18 , 19 and 20 , making them positive with respect to their associated sources and enabling current to flow through the current channel between the source and drain of each respective transistor. This completes a path enabling electrical energy stored in the capacitor bank 68 to flow as current through the primary winding 38 of the coupling transformer 16 . The feedback winding 36 of the coupling transformer 16 provides feedback current through resistors 32 , 33 and 34 to the respective gates of the transistors 18 , 19 and 20 of the driver stage 10 . The feedback acts to stabilize the gain of the driver stage 10 . The primary-winding-to-secondary-winding turns ratio is high, and a large current is induced into the secondary winding 42 . Since the gates of the power transistors 44 through 55 are positive with respect to their sources, the large current induced in the secondary winding 42 will flow through the current channel between the source and drain of each respective transistor and be applied to the primary winding of the output transformer 88 shown in FIG. 2 . This cycle is repeated for each positive pulse applied to the input of the modulator. During the operation of this configuration, the source terminals of the power FET's (indicated by consecutive reference numbers 44 through 55 ) may be driven to a potential of approximately minus twenty volts; therefore, the drain terminals of the FET's can be driven to a potential of zero volts, providing 300-volt pulses across the primary winding of the output transformer. Typical pulse voltage requirements for microwave transmitter tubes range from 15 to 35 kilovolts. The embodiment of the microwave tube modulator represented in FIG. 2 requires only 300 volts of direct current for its operation and will supply a current of 2600 to 3000 amperes to the primary winding of the output transformer 88 and provide sufficient power to operate a 250 kilowatt magnetron. The common gate configuration of the power transistors in the second, or primary, stage of the modulator minimizes the effects of drain-to-gate (Miller) capacitance and thereby increases the switching speed of the power FET's over that obtainable by transistors having a common source configuration. The power FET's shown have switching times that are less than 100 nanoseconds.

Claims: A microwave tube modulator comprising an input terminal for receiving electrical input signals, an amplifier arrangement which comprises first and second amplifiers (10, 12) and a first coupling device (38,42) for coupling an output signal of said first amplifier (10) to an input of the second amplifier (12), and a storage device (68) comprising at least one capacitor for storing electrical energy and arranged to couple electrical energy to one of said amplifiers, each amplifier (10,12) comprising a plurality of field-effect transistors each having source, gate and drain terminals, characterised in that the field-effect transistors (18, 19, 20) of the first amplifier (10) are connected in common source configuration with said input terminal coupled to their gate terminals, the field-effect transistors (44-55) of the second amplifier (12) are connected in common gate configuration, said first coupling device (38,42) comprises a transformer having primary and secondary windings, said primary winding (38) being connected in series between said storage device (68) and the drain terminals of the field-effect transistors (18,19,20) of the first amplifier (10), to couple electrical energy from the storage device (68) to said first amplifier (10), and said secondary winding (42) being connected to the source terminals of the field-effect transistors (44,55) of the second amplifier (12), and a second coupling device (36) is arranged to couple a feedback current, representative of the output signal of the first amplifier (10) to the field-effect transistors (18,19, 20) of the first amplifier. A microwave tube modulator according to claim 1 characterised by an input coupling device (14) coupling said electrical input terminal to the gate terminals of the field-effect transistors (18,19,20) of said first amplifier (10). A microwave tube modulator according to claim 2, characterised in that said input coupling device (14) comprises an electrical transformer. A microwave tube modulator according to any preceding claim, characterised in that said second coupling device (36) comprises a feedback winding on said electrical transformer of said first coupling device (38,42). A microwave tube modulator according to claim 4, characterised in that the turns ratio of said primary winding to said secondary winding is high. A microwave tube modulator according to any preceding claim, characterised in that said storage device (68) comprises a plurality of electrical capacitors. A microwave tube modulator according to claim 6, characterised in that said storage device (68) comprises said plurality of electrical capacitors connected in parallel. A microwave tube modulator according to any preceding claim, characterised in that said field-effect transistors (18-20,44-55) are n-channel, enhancement-type, metal-oxide silicon, field-effect transistors. A microwave tube modulator according to any preceding claim, characterised by a current-sensitive device (22-24,56-67) electrically connected in series with the source of each field-effect transistor, for protecting the modulator from excessive electrical current flow through said field-effect transistors by any field-effect transistor being subjected to or causing excessive current flow between its source and its drain. A microwave tube modulator according to claim 9, characterised in that each said current-sensitive device (22-24,56-67) comprises a fuse resistor. A microwave tube modulator according to any preceding claim, characterised in that said first amplifier (10) has three said field-effect transistors (18-20) connected in common source configuration and said second amplifier (12) has twelve said field-effect transistors (44-55) connected in common gate configuration. A microwave tube modulator according to any preceding claim, characterised in that said microwave tube modulator forms a unit (e.g. 84) connected to at least one other such unit (85-87) for cooperatively providing electrical output power equal to the cumulative outputs of said units.

Inventors: EHRENHALT DAN, GILES GEORGE RODNEY, EHRENHALT, DAN, GILES, GEORGE RODNEY
Applicants: SIERRA TECHNOLOGIES INC, SIERRA TECHNOLOGIES, INC.
Application year: 1988
Main classification: H03C1
IPC codes: H03K5, H03K17, H03C1, H03K3