US2793304A - Modulator - Google Patents

Description

y 1957 s. T. MARTIN 2,793,304

MODULATOR Filed March 24, 1955 2 Sheets-Sheet 1 ANODE CURRENT ANODE CURRENT NO DAMPING I00 onus r: 4|? ,Ujl f INVERSE INVERSE} I ENERGY}' I ENERGY 5B -E 1 0.3 4/15EC. o 0.1 0.2 551.4 1sec.

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INVENTOR, STUART r MART/Al szfa wg ATTORNEY MODULATOR Stuart T. Martin, Burlington, Vt., assignor to the United States of America as represented by the Secretary of the Army Application March 24, 1955, Serial No. 496,633

8 Claims. (Cl. 307-106) This invention relates to modulators and more particularly to line type radar modulators utilizing hydrogen thyratrons.

A characteristic of hydrogen thyratrons is the inverse anode voltage, viz; that which appears immediately after the pulse, usually because of mismatch, and that which appears during the interpulse interval, associated with charging circuits.

It has been determined experimentally that inverse voltage appearing immediately after a pulse of forward current, viz; the inverse voltage appearing during the first 100-150 millimicroseconds immediately succeeding the current pulse, causes the liberation of a large amount of energy at the anode resulting in excessive heating of the anode material. he magnitude of the effect of the inverse voltage appearing immediately after forward conduction is directly proportional to the ioivvard current existing prior to the application of the inverse voltage and it is proportional to the square of the inverse voltage applied. The main cause of such anode heating are ions drawn to the anode from the ionized hydrogen gas. The effects of the heating are detrimental, limiting the maximum energy that the tube can switch.

In most radar modulator circuits, it is desirable to have this inverse voltage since the latter permits the thyratron to deionize and recover before the anode voltage is reapplied. The density of the ions decays rapidly due to the normal deionization process. If the application of the desired inverse voltage can be delayed for periods on the order of 100 millimicroseconds, the ion density will have decayed to a point where the desired inverse voltage can be applied without damage to the tube or heating of the anode.

It is known that when a thyratron discharges a transmission line, the complete discharge of the line takes a finite time. Such time is determined by the length of the line and the velocity with which an electric wave travels therealong. Thus, the discharge does not cease until the electric wave has traveled from one end of the transmission line to the other end and then back again.

It is, accordingly, the primary object of the present invention to provide a hydrogen thyratron line type modulator wherein means are provided to modify the wave reflected by the transmission line.

It is a further object to provide a line type modulator utilizing a hydrogen thyratron wherein means are provided to delay the application of inverse voltage to the thyratron anode.

In accordance with the present invention, there is provided in a line type modulator comprising a gaseous discharge tube having a cathode, a control grid, an anode, and a transmission line in the output circuit of the tube with its input end connected to said anode, the modulator providing an output current pulse, the output end of the line reflecting an electric wave; an impedance element at the output termination of the transmission line for modifying the shape of the electric wave reflected by the hire States Patent 'ice transmission line without significantly affecting the shape of the output current pulse.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

in the drawings,

Fig. l is a schematic of a pulse line modulator utilizing a thyratron.

Fig. 2 is a group of curves indicating the relationship of inverse voltages and reverse current to the main current pulse in the modulator of Fig. 1.

Fig. 3 is an embodiment of the present invention.

Figs. 4 and 5 are groups of curves showing the relationship of inverse voltage and reverse current to the main current pulse in the device of Pig. 3; and

Fig. 6 is a graph showing details of power dissipated during the inverse voltage period as shown in Figs. 2, 4 and 5.

Referring now to Fig. l, the thyratron having at least an anode, a cathode and a control grid is shown connected in series with a uniform transmission line 101 whose characteristic impedance is Z0, a small inductance 102 and a load resistance 104. The input end of the transmission line lt i connected to the anode of the thyratron is designated as A and its free end is designated as B. Initially the capacitance of the transmission line is charged with voltage, V0.

When thyratron 1% becomes conducting, the voltage at the input end A of line 161 is reduced from its original value, V0, to a value determined by the voltage drop developed across load resistance 104 and the characteristic impedance of line 101 in series. This sudden change in voltage is not felt instantaneously at every point of the line. instead, it takes a certain time, determined by the velocity of propagation of an electric wave, for the effect felt at any particular point. Thus, the line discharges by the traveling of a wave along it from A toward B. This wave subtracts its voltage from the original voltage, V0, leaving a residue behind which is the voltage appearing across the load during the main portion of the pulse of current which flows as a result. The wave travels from the end A to the end B of the transmission line, is reflected, and proceeds back toward end A. As the reflected wave goes back toward the thyratron, it subtracts from the residue left behind on the first traversal down the transmission line. When the reflected wave has reached the point A again, the voltage of thyratron falls still further and the tube in general is extinguished.

if load resistance 164 is less than the characteristic impedance Z0 of the transmission line, then the fall of voltage occurring at the instant of conduction and traveling along the transmission line is more than half of the original voltage. If it is assumed that the fall is two thirds of the original voltage, as the wave travels from A to B, it leaves behind one third of the original voltage. After reflection, the Wave travels from B to A and is still two thirds of the original voltage which, subtracting from the one third voltage left, leaves a negative voltage whose value is one third of the original voltage but whose polarity is directed in the opposite sense. Thus, after this reflected wave has reached point A, the anode of the thyratron becomes negative. it is this ideal negative voltage which is referred to as the mismatch inverse voltage.

At the instant that conduction has ceased in the thyratron, a dense plasma consisting of equal numbers of positive hydrogen ions and electrons is left in the tube. The number of ions decays rapidly immediately after current has ceased and later on at a somewhat slower rate. The result is that when a strong negative voltage is applied to the anode of tube 100 instantly after the forward flow of current has ceased, a heavy current of positive ions is drawn to it.

The inverse voltage appearing on the thyratron after conduction has ceased is fairly complex if the load is inductive, even though the pulse network is a perfect transmission line. it consists of several parts as follows:

(a) The impedance mismatch voltage.

(b) An additional voltage, determined by the circuit inductance, which increases the inverse voltage. This voltage will appear periodically beginning approximately a pulse length after cessation of current flow and there after with a period which is exactly the pulse length. These maxirna will be present even though the mismatch voltage is zero.

(c) A periodically recurring voltage which makes the inverse voltage less than the mismatch inverse voltage. This voltage is primarily determined by the current which fiows in the reverse direction (collection of positive ions by the anode). In certain cases, it is possible that this voltage will cause the inverse voltage to swing positive for a short interval of time. This voltage also appears approximately one pulse length after current flow has ceased and thereafter with a period of precisely one pulse length.

(d) A very short spike of voltage which may appear a few millimicroseconds after the end of the current pulse and which may cause the inverse voltage to be greater than the mismatch inverse voltage. This voltage is caused by the flow of reverse current in the circuit impedance.

The power dissipated during the appearance of inverse voltage is all localized in the first 50 to 100 millimicroseconds, since the flow of reverse current is substantially over in that interval. All of this power is dissipated in the anode. After this interval, the power dissipated at the anode is negligible.

The voltage spike appearing at the end of the current pulse is the most important part of the inverse voltage. This spike is largely determined by the manner in which the reverse current decays, for a given inductance and resistance in the circuit. The primary source of the re verse current is the positive ions left in the decaying plasma in the thyratron. The current would not appear, however, if no inverse voltage were applied to the thyratron. Thus, the ideal voltage (impedance mismatch) and the total positive ion charge in the tube, together determine the current during the inverse interval.

, Referring now to Fig. 2, there is shown the results of measurements of voltage and current flowing in the reverse direction in a conventional network as shown in Fig. 1. The network of approximately 50 ohms impedance was operated into a suitable load, such that, at approximately 90 amperes peak forward current at 8 kilovolts, the inverse voltage was approximately 4,000 volts. A total energy dissipation at the anode of about 1,800 microjoules was measured by directly multiplying current and voltage to obtain power and integrating the result. This took place in the sharply rising spike, at whose maximum, the power expended at the anode amounted to 70 kilowatts. The dissipation of this power during the first 150 millimicroseconds is shown at curve D of Fig. 6.

Fig. 3 shows a circuit similar to that of Fig. 1 but with an impedance comprising the series combination of resistance 106 and capacitance 168 placed at the one end of the pulse forming network. Figs. 4 and 5 show current pulse and inverse voltage measurements with the circuit of Fig. 3. The value of capacitance 108 in both Figs. 4 and 5 was 417 micromicrofarads, the value of resistance 1-06 being 100 ohms in Fig. 4 and 200 ohms in Fig. 5. As seen in curve E of Fig. 6, with resistance 106 equal to 100 ohms, the energy dissipation at the anode was 560 microjoules, and as shown in curve F of Fig. 6, with resistance 106 equal to 200 ohms, the energy dissipation at the anode was 360 microjoules. It is to be observed in comparing the curves of Figs. 4 and 5 4 to that of Fig. 2 that the leading edge of the current pulse is not changed appreciably by the inclusion of the impedance at the end of the line. Thus, the only element of power dissipation which has been altered by the changes is that corresponding to the flow of reverse current during the application of inverse voltage.

From the Figs. 2, 4 and 5, it is to be seen that whether or not the negative voltage is applied suddenly or gradually to the anode of thyratron 1% depends entirely on the shape of the reflected wave. If the end B of transmission line 161 has no circuit element connected across it, the shape of the wave arriving at A will be determined primarily by the manner in which current began to flow at the instant that forward current pulse ceased (the thyratron was inoperative). If the end B of transmission line is not a perfect reflector, the shape of the reflected wave is modified, and no longer solely determined by the manner in which current began to flow. Thus the reflection can be controlled by inserting an electrical impedance element, whether linear or non-linear does not matter, at the end B of the transmission line. In this way, the shape of the reflected wave can be altered in a manner more or less independent of the shape of the incident wave. By this means, the inverse voltage can be applied independently of the other circuit parameters.

While there has been described what is, at present, considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a line type modulator comprising a gaseous discharge tube having a cathode, a control grid, an anode and including a transmission line in the output circuit of said tube with its input end connected to said anode, said modulator providing an output current pulse, the output end of said line reflecting an electric wave; an impedance element connected across the output end of said line for modifying the shape of the electric wave reilected by said line without significantly affecting the shape of said output current pulse, the magnitude of said impedance being sufficient to delay for about 50 millimicroseconds the application of inverse voltage to said anode.

2. In a line type modulator as defined in claim 1 wherein said impedance element comprises a series combination of a capacitance and a resistance.

3. A line type modulator comprising a gaseous discharge tube having at least an anode and a cathode, and a pulse forming line having one end connected to the anode-cathode circuit of said tube, whereby upon ionization of said tube said line is discharged and an inverse voltage wave is reflected from the other end of said line and applied to said anode, and impedance means connected to said other end of the line to reduce the amplitude of the initial cycle of said inverse voltage wave, whereby the amount of reverse current drawn by said anode due to the positive ions in the decaying plasma is reduced.

4. A line type modulator comprising a gaseous discharge tube having at least an anode and a cathode, a pulse forming transmission line having one end connected to the anode-cathode circuit of said tube, whereby upon the momentary application of a keying pulse to the discharge path of said tube said path is rendered conducting, causing said line to be discharged and an inverse voltage wave reflected from the other end of said line to be applied to said anode, and impedance means connected to said other end of said line to reduce the rate of rise of said inverse voltage wave, whereby there is a reduction in the amount of reverse current drawn by said tube due to the ions in the decaying plasma.

5. A line type modulator comprising a hydrogen thyratron tube having an anode, a cathode, and a control electrode, a pulse forming transmission line having one end connected to the anode-cathode circuit of said tube whereby, upon impression of a pulse upon said control electrode the gas in said tube is ionized and said line is discharged through said tube, an inverse voltage wave being reflected from the other end of said line and applied to said anode, and means comprising a resistance and reactance connected in series across said other end of the line to reduce the rate of rise of said inverse voltage wave, whereby the amount of reverse current drawn by said anode due to the positive ions in the decaying plasma is reduced.

6. A line type modulator including a gas discharge tube and a delay line having one end connected to the anode circuit of said tube, wherein the pulsing of said tube causes ionization of the gas therein and initiates a discharge of said line, generating a wave which travels to the other end of the line and is reflected toward the anode in the form of an inverse voltage wave, said inverse voltage wave causing said tube to draw inverse current due to positive ions in the decaying plasma, and reactive impedance means connected across the other end of said line to delay the application of said inverse voltage until the positive ion density in said plasma is reduced, thereby reducing the amount of inverse current drawn by said tube.

7. A modulator as set forth in claim 6, wherein the magnitude of said impedance means is sufiicient to produce a delay of at least millimicroseconds.

8. A modulator as set forth in claim 7, wherein said impedance means comprises a 417 micromicrofarad condenser connected in series with a resistor of at least ohms.

Fundingsland June 8, 1954 Krienen Sept. 27, 1955

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