US2448364A - Pulse generator - Google Patents

Description

Patented Aug. 31, 1948 PULSE GENERATOR Albert G. Ganz, New York, and Andrew D. Hasley, New Rochelle, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a

corporation of New York Application April 28, 1945, Serial No. 590,824

9 Claims. 1

This invention relates to pulse generating systems and more particularly to one in which an oscillator generates high frequency pulses corresponding to unidirectional rectangular pulses applied thereto.

The principal object of the invention is to reduce or prevent distortion of the envelope of the high frequency output pulse due to fluctuations in the input current to the oscillator.

In generating high frequency pulses a unidir..ctional pulse is applied to an oscillator, which be of the magnetron type, through a conling circuit which may include a transformer. Vhcn the impressed voltage reaches an assigned operating value the oscillator becomes conduciive and starts to oscillate. However, associatd with the coupling circuit there will be an in- ..uctance effectively in series with the load and capacitance effectively in shunt with the load. hese may produce a transient oscillation, upon the appearance of the unidirectional pulse, and

lay the operation of the oscillator. Furthermore, unless this inductance and capacitance are properly related the input current to the oscillator will fluctuate in value and cause a distortion of the envelope of the output pulse.

In accordance with the invention this distortion of the envelope is greatly reduced or entirely prevented b so proportioning the ratio of the inductance to the capacitance with respect to the impedance of the source that the transient current substantially reaches a maximum, and approximately the value drawn by the magi tron in the steady state condition, at the instant the impressed voltage reaches the assigned operating value. The required proportion may b obtained by properly designing the trans 1 .ler or by adding an inductance or capaci- .ce, if necessary.

' Le nature of the invention will be more fully from the following detailed descripand. by reference to the accompanying draw- .%s, in which like reference characters refer to similar or corresponding parts and in which:-

Fig. l is a schematic circuit of a high frequency pulse generating system in accordance ah the invention;

Fig. 2 an equivalent circuit of the system;

Tgs. u and 4.- show curves used in explaining he luv ion; and

Figs. 5 and 6 are equivalent circuits of other pulse generating systems embodying the inventj n The tangular pulse generator 10, a load device H including an oscillator which may be of the magnetron type, and a coupling circuit which includes a transformer l2. The series inductance Lx and the shunt capacitances Cx are elements which may be added, if required. When a unidirectional pulse reaches the assigned operating value E0, the magnetron will start to oscillate or fire. However, since the transformer l2 inevitably has leakage inductance, the voltage Ec impressed upon the load M will not rise to its full value immediately upon the appearance of the unidirectional pulse but will be somewhat delayed. Furthermore, since the load El and the coupling circuit will have considerable ca-- pacitance, the voltage rise will tend to be oscillatory in character. Consequently, after the application of the pulse, there will be a brief period during which the magnetron is non-conductive and during which a transient oscillation will take place in the coupling circuit. Unless this circuit is designed to control the transient properly, th energy stored in the transformer inductances and in the parasitic capacitances at the moment the magnetron fires may discharge through the magnetron and produce a serious distortion of the output pulse envelope.

For analytical purposes the system of Fig. 1 may be represented by the circuit shown in Fig. 2. The transformer l2 is represented by its leakage inductance LT and its output shunt capacitance CT, both referred to the transformer primary. The generator la is represented by a unidirectional electromotive force in series with a resistive impedance Rs, connected to the transformer through a switch S1. The load H is represented by its effective shunt capacitance CL and its operating resistive impedance R-L, both referred to the primary side of the transformer. The impedance R1. is connected to the circuit through a secondary switch S2. The element Cx represents the value of CK, also referred to the transformer primary. It is assumed that the switch SI is closed at the instant the voltage pulse E is applied to the transformer 82 and that the switch S2 is closed the instant the volttage Ec impressed upon the magnetron reaches the operating value E0. For analytical purposes the two series inductance IJI and Lx may be combined into a single inductance L, that is,

and. the three shunt capacitances into a single capacitance 0 having thevalue When the rectangular pulse E is applied but before the magnetron fires, that is, with SI closed and S2 open, the circuit of Fig. 2 behaves simply as a damped series resonant circuit. The input current I1 is oscillatory and rises toward a maximum determined by the circuit constants Rs, L and C. The voltage Ecacross the capacitance C also rises toward a final steady voltage E which it approaches in an oscillatory fashion. When the voltage Ec reaches the operating voltage E of the magnetron, the latter fires, that is, $2 is closed and the magnetron draws a steady state load current I2 given by If at the instant the magnetron fires the current I1 is not the same as 12, the excess or deficit has to be balanced by a change in the current through the capacitance C, which will be accompanied by a change in the voltage across the capacitance and a distortion of the output pulse envelope. However, if at the instant the voltage Eo reaches the operating value E the current 11 has reached its maximum value and is equal to the load current I2, then the sudden closing of the magnetron path will produce no disturbing transient. Furthermore, for the duration of the pulse, the voltage Ec will remain fixed at the value Eb, that is,

E RS'IRL (4) At any time t after SI has been closed but before S2 is closed the current I1 and the voltage EC are given by the expressions it may be shown analytically, starting with Equations 5 and 6, that if K has the value given by RL 6 R. K 1 where tang-1 1 (12a) then the current I1 will have risen to its maximum value and will be equal to I2 at the instant the voltage Eo has reached the operating value E0. As pointed out above, these are the desired conditions for an undistorted output pulse. The curve of Fig. 3 is a plot of the value of K for the range of ratios from 0.4 to 10, the range most likely to be encountered in practice. A logarithmic scale is used for both the ordinates and the abscissas. To determine a point on the curve, a value of K is as sumed and the corresponding value of iii is found from Equation 12.

For the particular case where Rn is equal to Rs, that is, when the load resistance as seen from the transformer primary terminals matches the resistance of the pulse source, the proper value of K, as given by Fig. 3, is 0.85. In Fig. 4 the dash line curves give the ratio of Eo to E and the solid line curves give the ratio of I1 to I2, plotted against abscissas which are proportional to the time t and are equal to t divided by Zx/LC. If K is equal to 0.85, the current I1 will reach its maximum value and be equal to the steady state load current 12 at the point i on the curve l5 at the instant the voltage Ec has reached the operating value 05152 at the point IS on the curve ii. If the switch S2 is now closed, the current will remain at the value I given by S L S and the voltage Eo at the operating value E0 given y for the duration of the pulse without being disturbed by undesired transients.

However, if K has any other value, transients will occur. For example, if K has the value 1.1, at the instant the voltage Eo reaches the value 05E at the point [8 on the curve I 9, the current 11 has progressed beyond its maximum and is larger than the desired value I2, as shown by the point 20 on the curve 2 I. Now, upon the closing of S2, the current I1 will approach the operating value I2 in an oscillatory manner as indicated by the dot and dash curve 2'2, thus introducing a disturbing transient.

In designing a particular pulse generating systom the values of the source resistance Rs and the load resistance R1. will usually be known in advance. The proper. value of K can be found from Fig. 3. The required ratio of C to L may then be found from Equation 11. However, C is made up of CL, CT and Cx' and L is made up of LT and Lx. The value of CL, the capacitance associated with the load, is ascertainable, and Lx and CK are building out elements which are only included when necessary. Therefore, the required ratio of the output shunt capacitance CT of the transformer to its leakage inductance LT can be determined. If it is found to be difiicult to design the transformer l2 to have the required ratio of CT to LT, then either a capacitance Cx' or an inductance Lx of proper value is added to the circuit of Fig. 2. The required value of (3:: in Fig. 1 may now be found by dividing CK by the impedance transformation ratio of the transformer. Also, the addition of either Cx or Lx may be found useful in adjusting a transformer to compensate for manufacturing variations.

The above description shows how the necessary conditions for constant current into and voltage across the load can be satisfied for the simple case of a series inductance L and shunt capacitance C. These considerations apply similarly to pulse generating systems for which the equivalent circuits are more complex such, for example, as those shown in Figs. 5 and 6,. The circuit of Fig. 5 com prises a series inductance L1 with a shunt capacitance C1 on the input side and a second shunt capacitance C2 on the output side. The circuit of Fig. 6 comprises two series inductances L2 and L3, an interposed shunt capacitance C3, and a second shunt capacitance C; on the output side. The proper values for the elements L1, C1 and C2 of Fig. 5 or the elements L2, L3, C3 and C4 of Fig. 6 to meet the necessary conditions may be obtained by trial and error or by the solution of a perfectly determinate set of simultaneous equations.

The equivalent circuit of Fig. 6 may, for example, represent the pulse generating system shown in Fig. 1 when the impedance transformation ratio of the transformer I 2 is so low that the shunt capacitance C3 on the primary side cannot be neglected. In this case the inductance L3 is the series inductance Lx, the inductance L3 is the leakage inductance LT of the transformer l2, and the capacitance C4 is the capacitance C as given by Equation 2.

If the inductance Lx in Fig. 1 is zero the equivalent circuit of Fig. 5 is obtained. In this case the capacitance C1 represents the sum of the input shunt capacitance of the transformer l2 and the shunt capacitance associated with the source I. The inductance L1 and the capacitance C2 represent the same quantities, respectively, as the elements L3 and C4 shown in Fig. 6.

What is claimed is:

l. A high frequency pulse generating system comprising a source of unidirectional pulses, a load device which becomes conductive and draws a predetermined current when the voltage impressed thereon reaches an assigned value, and a circuit coupling said source and said load device, said circuit including an inductance L efiectively in series with said load device and a capacitance C eifectively in shunt with said load device, said inductance and capacitance producing a transient oscillation on the appearance of a pulse and delaying the operation of said load device, and the ratio of L to C being so proportioned with respect to the resistance impedance Rs of said source that the transient current approximately reaches a maximum and approximately the steady state value of the load device current at the instant the voltage impressed upon said load device reaches said assigned value.

2. A system in accordance with claim 1 in which said coupling circuit includes a transformer.

3. A system in accordance with claim 1 in which said coupling circuit includes a transformer having an impedance transformation ratio chosen to match Rs to the resistive impedance of said load device.

4. A system in accordance with claim 1 in which said coupling circuit includes a transformer and an added series inductance.

5. A system in accordance with claim 1 in which said coupling circuit includes a transformer and an added shunt capacitance.

6. A system in accordance with claim 1 in which the resistive impedance looking into the input terminals of said coupling circuit is approximately equal to Rs.

'7. A system in accordance with claim 1 in which Rs, L and C have the relationship R =K C the resistive impedance RL looking into the input terminals of said coup-ling circuit, Rs, L and C have the relationship where K is a constant having a value approximately equal to 0.85.

9. A high frequency pulse generating system comprising a source of unidirectional pulses, a load device comprising a magnetron oscillator which becomes conductive when the voltage impressed thereon reaches an assigned value, and a circuit coupling said source and said load device, said circuit including an inductance L effectively in series with said load device and a capacitance C effectively in shunt with said load device, said inductance and capacitance producing a transient oscillation on the appearance of a pulse and delaying the operation of said load device, and the ratio of L to C being so proportioned with respect to the resistive impedance Rs of said source that the transient current approximately reaches a maximum and approximately the steady state value of the load device current at the instant the voltage impressed upon said magnetron oscillator reaches said assigned value.

ALBERT G. GANZ. ANDREW D. HASLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,103,362 Hansell Dec. 28, 1937 2,276,994 Milinowski Mar. 17, 1942 2,295,585 Lindqulst Sept. 15, 1942

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