US2514413A - Modulator - Google Patents

Description

July 11, 1950 PAwLEY 2,514,413

MODULATOR Filed Sept. 19} 12545 2 Sheets-Sheet 1 LOAD A-C POWER INPUT STORAGE O 0- DEVICE -A-C POWER LOAD INPUT gwua/wto'a MYRON G. PAWLEY y 1950 M. e. PAWLEY ,51

MODULATQR Filed Sept. 19, 1945 2 Sheets-Sheet 2 ILE=E in I o '2 '00 (VOLTAGE INDUCED m wmnms n) IIE=3 0| (THYRATRON PLATETO GATHODE VOLTAGE) (THYRATRON CRITICAL emu VOLTAGE) l I, HYRATRON FIRES ,1 AT THIS TIME o2 (THYRATRON GRID-TO- CATHODE VOLTAGE) THYRATRON FIRES AT THlS TIME [O4 (VOLTAGE APPLIED TO LOAD) MYRON G. PAWLEY @R MW energy in short intermittent bursts.

Patented July 131, 1950 MODULATOR Myron G. Pawley, Alexandria, Va.

Application September 19, 1945, Serial No. 617,413

6 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) This invention relates to pulse generators, and is particularly directed to providing simple apparatus suitable for employment as the modulater in an echo-ranging or other pulse-modulated radio system.

The function of the modulator component in such systems is to produce a series of highpower, short-duration voltage pulses for application to a high frequency radio transmitter, to cause the transmitter to emit radio frequency The invention herein described has a simpler circuit and requires less apparatus to accomplish the required result than pre-existing modulator systems, and is thus particularly adapted for use in echo-ranging systems wherein small size, light weight, and economy are important design considerations.

An object of this invention is to provide a simple, light weight modulator for echo-ranging systerns.

Another object of this invention is to provide a modulator, suitable for employment in pulsemodulated radio systems, which requires few components and is inexpensive.

The invention will be described with reference to the appended drawings, of which:

Figure 1 is a circuit diagram in schematic and block form, of an illustrative embodiment of the invention;

Figures 2, 3 and 4 are voltage-time graphs showing the variation with time of various voltages involved in the operation of the embodiment of the invention illustrated in Figure 1; and

Figure 5 is a diagram, in schematic and block form, of still another embodiment of the inven tion.

Referring to Figure 1, transformer i0 is a stepup power-transformer, of which primary winding l is connected to an A.-C. power source. In a typical application the power source might supply energy at 115 volts, 400 cycles per second. One side of high voltage secondary winding II is grounded; the other side is connected through inductor [2 to one terminal of energy storage device l3. Device I3 may be a simple condenser or it may be a pulse-forming network comprising capacitance and inductance. In either event, the impedance of device I3 is predominantly capacitive to currents of low frequency such as that of the power source. The other terminal of storage device I3 is connected to the cathode of grid-controlled gas tube 20 and is returned to ground through resistor l5. Gas tube 20 is of the type commonly called thyratron" and has the grid thereof connected to ground through limiting resistor I 7. Primary winding 2 of step-up pulse transformer 30 is connected between the plate of gas tube 20 and the junction of device I3 and inductor 12. The secondary winding 33 of pulse transformer 30 is connected across load device 25, shown in block form. In a practical application load 25 might be the power input terminals of a magnetron oscillator or other high frequency radio transmitter.

The secondary loop comprising winding l I and storage device I3 is adjusted to be resonant at the power frequency, and for this purpose it must contain a substantial amount of inductance. The required inductance is represented in Figure l as inductor 12. in practice inductor l2 may be a physical coil, it may be the leakage inductance of winding ll, if that be adequate, or it may be reflected inductance in the secondary loop produced by a physical inductor inserted in the primary circuit of transformer Ill.

The system operation may best be described with reference to Figures 2, 3, and 4. All these figures are graphs in Cartesian coordinates having time as abscissa and voltage as ordinate. The horizontal time scales are identical for the three figures, but the vertical scales, indicating voltage. are in no case calibrated numerically and no uniformity of scale from one figure to another is intended. Curve I00 on Figure 2 shows, for time reference purposes, the voltage across secondary winding ll, polarity taken with reference to the grounded side thereof. Figure 3 contains three curves showing the approximate conformation of the voltage waveforms for thyratron gas tube 20. Curve l0l represents the voltage at the plate of gas tube 20 relative to its cathode; curve 802 represents the voltage of the grid of tube 20 relative to cathode; curve I02 represents the voltage of the grid of tube 20 relative to cathode, and curve I03 represents the instantaneous value of the grid-cathode voltage required for ionization, the so-called firing voltage. Curve I04 on Figure 4 shows graphically the waveform of the high-pow ered output voltage pulses supplied to the load. On the time scale employed for Figures 2, 3, and 4, the duration of the output pulses-a few microseconds or less-is imperceptible. The positive polarity shown for the pulses in Figure 4 is arbitrarily chosen; either positive or negative pulses can be had at will by reversing the connections to either winding of pulse transformer 30.

During the positive half cycles of voltage across the secondary winding, as indicated by curve I00 starting at zero time, current fiows in a direction to charge storage device I3 in a polarity such that its negative terminal is that connected to the cathode of gas tube 20. Winding 3 of pulse transformer 30 has no drop across it except when gas tube 20 is conducting; consequently during this interval the plate of tube 20 becomes increasingly positive relative to its cathode, as storage device I3 charges. This voltage rise is shown graphically by curve IOI, starting at zero time.

During the same part of the cycle the charging current through resistor I5 raises the cathode of gas tube 20 above ground potential and thus produces a negative-grid-to-cathode bias voltage as shown by curve I02. At the instant marked t1, on the time scale, when storage device I3 is approximately at peak charge, the grid-cathode voltage of tube 20 rises to a point more positive than the gas tubes firing voltage. This phenomenon is represented graphically by the intersection of curves I02 and I03. At that instant, the gas in tube 20 ionizes and the tube becomes virtually a short-circuit. As a result the entire voltage built up across storage device I3 is suddenly applied to the primary winding 3 of pulse transformer 30. A voltage several times larger instantly appears across the secondary winding 33 and is applied to load 25. The pulse of voltage continues until storage device I3 has discharged virtually all its stored energy. When this has occurred gas tube 20 deionizes and becomes again an open circuit.

The waveform and duration of the output pulse produced are dependent upon the impedance of the load, the constants of the pulse transformer, and the characteristics of the storage device. The leading edge of the voltage pulse will in any event be very steep, and in most cases the trailing edge will likewise have a large slope. In general a pulse-forming network employed as a storage device will give a more nearly rectangular output pulse than will a simple condenser in the same role. In practical constructions output pulses having a duration of one microsecond or less have been obtained without difiiculty.

When the stored energy in storage device I3 has been dissipated in producing the output pulse and gas tube 20 has deionized, the voltage from the power transformer starts the negative part of its cycle (the interval t1 to t: on the time axes). Current flows around the secondary loop in a direction to charge storage device I3 negatively and to make the grid of gas tube 20 positive relative to its cathode. During this part of the cycle, the plate of tube 20 is negative, as shown by curve IN, and the gas tube grid voltage flattens out at a value slightly above zero relative to cathode. The voltage drop in grid-current limiting resistor I'I maintains the grid at approximately cathode potential during the entire part of the cycle in which there is a flow of grid current.

At time 152 the source voltage again reverses, and, aided by the stored charge'in device I3, begins to force current around the circuit in the original direction. This results in storage device I3 again becoming charged positively, developing a very high positive plate voltage on the gas tube, as before. At time is the grid voltage again rises above the critical value, the gas tube fires, and another output pulse is produced. This cycle of events continues indefinitely, one output pulse being produced for each cycle of source voltage.

An alternative embodiment of the invention is shown in schematic and block form in Figure 5. In this embodiment the power transformer I0 is designed to possess sumcient secondary leakage inductance to effect resonance in the secondary loop; hence no counterpart to inductor I2 of Figure 1 appears in Figure 5. The primary winding I2I of power transformer I I0 is, as in the previous embodiment, connected to an A.-C. power source. One side of secondary winding III is connected to the plate of grid-controlled gas tube I20 which is of the same type as tube 20 of Figure 1; the other side of winding III is grounded. Resistor H5 is connected between the cathode of tube I20 and ground; resistor I I1 is connected between the grid of tube I20 and ground. Storage device I I3, which may be a condenser or a pulse-forming network, is connected in series with primary winding I 23 of pulse transformer I30 between the plate and cathode of gas tube I20. Secondary winding I33 of the pulse transformer I30 is connected to load device I25, which may in practice be a high frequency radio transmitter.

The operation of this embodiment is identical in principle to that of the embodiment already described. The resonant charging path comprises winding II I, with its leakage inductance, resistor H5, storage device H3, and the primary winding I23 of the pulse transformer.

The voltage on the gas tube grid varies as a function of the charging current, just as in the other circuit, and the gas tube fires when the charge stored in device II3 nears its peak. The ionization of the gas tube impresses the entire voltage of storage device II3 across the pulse transformer primary, and the output pulse results.

The rearrangement of parts resulting in the storage device. charging through the pulse transformer primary does not appreciably affect the operation of the modulator or alter the voltage Waveforms, because the rate of change of current through winding I23 during the charging process is much too slow to produce any appreciable induced voltage across the load. The circuit of Figure 5 possesses the advantage over the first circuit described that no part of the pulse transformer is at a high potential above ground except during the short intervals when the output pulses are being produced. This may induce insulation difiiculties in practical installa- It will be understood that the embodiments of the invention herein shown and described are exemplary only and that the scope of the invention is to be determined by reference to the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In combination, an alternating-current power source; charge storage means and a resistor connected in closed series connection with the power source; a grid-controlled, gas-filled rectifier tube having a. cathode, an anode, and a grid; load means; means connecting the gas tube and the load means in a series connection with the storage means; and means connecting the grid and cathode of said tube across said resistor so that the charging current in the resistor is operative to bias the grid negatively relative to the cathode to maintain the gas tube nonconducting while the storage means is charging and to fire the gas tube when the charge in the storage means is maximum, thereby discharging the storage means into the load.

2. In combination, an alternating-current power source; inductance means, charge storage means, and a resistor connected in closed series connection with the power source; a grid-controlled, gas-filled rectifier tube having a cathode, an anode, and a grid; load means; means connecting the gas tube and the load means in a series connection with the storage means; and means connecting the grid to the power circuit at a point operative to cause charging current in the resistor to bias th grid negatively relative to the cathode to maintain the gas tube nonconducting while the storage means is charging and to fire the gas tube when the charge in the storage means is maximum, thereby discharging the storage means into the load.

3. In combination, an alternating-current power source; inductance means, charge storage means, and a resistor connected in closed series connection with the power source substantially resonant at the frequency of the power source to form a circuit; grid-controlled, gas-filled rectifier tube having a cathode, an anode, and a grid; load means; means connecting the gas tube and the load means in a series connection with the storage means; and means connecting the grid to the power circuit at a point operative to cause charging current in the resistor to bias the grid negatively relative to the cathode to maintain the gas tube non-conducting while the storage means is charging and to fire the gas tube when the charge in the storage means is maximum, thereby discharging the storage means into the load.

4. In combination, an alternating-current power source; a power transformer having primary and secondary windings; means connecting the primary winding to the power source; charge storage means and a resistor connected in closed series connection with the secondary winding; load means; a pulse transformer having primary and secondary windings; means connecting the load means to the secondary winding of the pulse transformer; a grid-controlled gas-filled rectifier tube having a cathode, an anode, and a grid; means connecting the gas tube in a series connection with the primary winding of the pulse transformer and the storage means; and means connecting the grid and cathode of said tube across said resistor so that the charging current in the resistor is operative to bias the grid negatively relative to the cathode to maintain the gas tube non-conducting while the storage means is charging and to fire the gas tube when the charge in the storage means is maximum.

5. In combination, an alternating-current power source; a power transformer having primary and secondary windings; means connecting the primary winding to the power source; inductance means, charge storage means, and a resistor connected in closed series connection with the secondary winding to form a circuit substantially resonant at the frequency of the power source; load means; a pulse transformer having primary and secondary windings; means connecting the load means to the secondary winding of the pulse transformer; a grid-controlled, gas-filled rectifier tube having a cathode, an anode, and a grid; means connecting the gas tube in a series connection with the primary winding of the pulse transformer and the storage means; and means connecting the grid to the power circuit at a point operative to cause charging current in the resistor to bias the grid negatively relative to the cathode to maintain the gas tube non-conducting while the storage means is charging and to fire the gas tube when the charge in the storage means is maximum.

6. In combination, an alternating-current power source; a power transformer having a primary winding and a secondary winding possessing substantial leakage inductance; means connecting the primary winding to the power source; a charge storage device; a pulse transformer having primary and secondary windings; resistance means; means connecting the storage device, the primary of the pulse transformer and the resistance means in a closed series connection in the order named with the secondary winding of the power transformer to form a circuit substantially resonant at the frequency of the power source; load means connected to the secondary winding of the pulse transformer; a grid-controlled, gas-filled rectifier tube having a cathode, an anode, and a, grid; means connecting the anode of the gas tube to the junction of the storage means and the secondary winding of the power transformer; means connecting the cathode of the gas tube to the junc- I tion of the resistance means and the primary winding of the pulse transformer; and means connecting the grid of the gas tube to the junction of the resistance means and the secondary winding of the power transformer.

MYRON G. PAWLEY.

No references cited.

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