Generator For Short-duration High-frequency Pulse Signals

Abstract

Very narrow pulses of high-frequency sine wave electromagnetic energy are formed by use of a gated or pulse modulated continuous-wave source feeding a transmission line network having a time-limited impulse response and adjusted to provide a series resonance with the leakage capacity of the source gating or modulating switch for the purpose of assuring that a maximum of the available energy is employed to form the output and that the output level is nulled before and after the generation of the high-frequency pulse.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to means for producing pulses of high-frequency or microwave sine wave electromagnetic signals of duration of the order of one nanosecond and more particularly to apparatus for producing such pulse signals having abrupt initial rise and terminal fall characteristics with substantially no energy emission before or following the pulse.

2. Description of the Prior Art

The literature describes several prior art attempts to devise systems for generation of very short pulses of high-frequency or microwave signals. Generally, one method has involved synthetic generation of such pulse signals by the excitation of a pulse-forming network with an abrupt step function voltage. Various networks have been used to transform the incident step wave to a rectangular impulse.

As use of increasingly higher carrier frequencies is made, especially at C-band and above, the output pulse waveform for such prior systems degrades with increasing severity. The degeneration is traced to the increasing production of higher order mode propagation at network junctions and to increasing losses and accompanying dispersion in the network transmission lines. A further barrier to successful extension of the technique is the generally inversely proportional relation between microwave generator output signal level and increasing frequency.

Systems for generating pulse outputs not requiring carrier phase synchronization with respect to the pulse envelope have also been realized. For example, a switch in series with a continuous wave oscillator may simply be rapidly opened and rapidly closed to produce the desired short duration impulse signal. However, the leakage capacities of available switches are finite and severely limit the off-on ratio of the output pulses. In addition, equally abrupt initial rise times and terminal fall times are desired for the pulse waveform and switches generally available at reasonable cost do not open and close at the same rates and therefore from undesirably asymmetric output pulses.

SUMMARY OF THE INVENTION

The present invention is an apparatus for generating pulse-modulated, high-frequency carrier signals having pulse durations of the order of a nanosecond and having sharply rectangular envelopes. The invention preferably employs excitation of TEM-mode electromagnetic wave propagation in a novel pulse-forming network having time-limited response characteristics in response to a gated pulse or step modulated high-frequency input wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit representation of one form of the invention.

FIG. 2 is an explanatory graph used in understanding FIG. 1.

FIGS. 3, 4, and 5 are graphs showing output waveforms generated by variously adjusting the circuit of FIG. 1.

FIGS. 6 and 7 are equivalent circuits used in explaining the operation of the circuit of FIG. 1.

FIG. 8 is, like FIG. 1, a schematic circuit representation of one form of the invention.

FIG. 9 is a partially schematic top view of one preferred form of the invention.

FIG. 10 is a partially sectioned top view of a presently preferred form of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The invention is a high-frequency transmission line circuit for generating impulse modulated signals having an impulse envelope duration of the order of a nanosecond. The generator employs a pulse forming network having a time-limited response driven by a source of step modulated high-frequency energy. Passive properties inherent in the network may be employed to cause sharp terminal fall of the impulse for production of a sharply rectangular pulse envelope. In order better to understand the novel signal generator, consider first the network of FIG. 1. A lossless two-wire transmission line 3, 31 having a surge or characteristic impedance Z.sub.0, extends a distance D between impedance reference planes 4 and 5. At the midplane 6 of the first or main transmission line 3, 3a is conductively attached to a two-wire stub lossless transmission line 7, 7a which extends a distance L from line 3, 3a and which is open-circuited at its outer terminal ends 8, 8a. Stub transmission line 7, 7a has a surge impedance Z.sub.a and forms a pulse-forming network 10 in cooperation with the main transmission line 3, 3a. Various known types of low loss, nondispersive transmission lines may be used to form lines 3, 3a and 7, 7a, including transmission lines that propagate traveling high-frequency or microwave signals in nondispersive modes, such as in the TEM mode. The normalized surge or characteristic impedance Z.sub.0 of transmission line 3, 3a may be 1 ohm, while that of line 7, 7a must have a characteristic impedance one-half that of line 3, or 0.5 ohms.

Networks 10 may be excited at its input terminals in plane 4 by an impulse or repetitive train of impulses furnished at said input by a known type of impulse generator 11 having an internal resistance indicated by resistor 12 of R.sub.g ohms. A normalized value of 1 ohm may be chosen for the parameter R.sub.g. An impulse supplied by generator 11 travels through pulse-forming network 10 to a resistive or terminating load 15 which may be a radiating antenna or other matched utilization device. Load 15 has a resistance R.sub.L which may also have a normalized value of 1 ohm.

It has been shown that when the network 10 is excited by such an impulsive signal, the output response a(t) of network 10 across a load resistor or termination 10 having a 1 ohm value is two impulses 20 and 21 of the same sign and area, as illustrated in FIG. 2. Impulse 20 is located in time D/c seconds after the time the input impulse reaches plane 4, the symbol c representing the propagation velocity of energy along transmission line 3, 3a. Impulse 20 and 21 are separated by a time interval T=2L/c seconds. Such properties of the network 10 are discussed by G. F. Ross in the paper "A New Microwave Phase Equalizer Network," found in pages 647 to 650 of the Nov. 1967 issue of the I.E.E.E. Transactions on Microwave Theory and Techniques.

Under the above impulse excitation conditions, network 10 can be spoken of as having a time-limited impulse response, the value a(t) being zero for all times t>D+T=D+(2L/c). The significance of such a type of time-limited response becomes evident upon examination of the output of pulse-forming network 10 for a step-modulated excitation having a nominal carrier frequency f centered at f.sub.o, where f.sub.o may take values such as

and where k is an integer. As shown in FIGS. 3, 4, and 5, the output of network 10 for t>T seconds is identically zero, regardless of the starting phase of the sinusoidal input impulse.

FIG. 3, for example, represents the situation for a signal of nominal center frequency f.sub.o =1/2T, wherein an output pulse is formed of duration T. The time period following the end of interval T represents a forbidden era in which the input wave cannot contribute energy to an output signal. FIG. 4, like FIG. 3, represents a pattern in which the exciting pulse starts with zero phase, but the value of f.sub.o is now 9/2T. The same forbidden region is represented by dotted line waveforms. FIG. 5 is similar, but with f.sub.o =3/2T and a starting phase other than zero and therefore represents a more general case. Again, the time period following the end of time interval T represents a forbidden era and there is no output signal.

When the value of k in the quantity (2k-1) 2T is not an integer, the step modulated response of network 10 for t>T is no longer zero and the output signal is no longer an impulse. On the other hand, for any integral value of k, an impulse output waveform of desired envelope shape appears across termination 15. Where k is an integer much greater than unity, there will be many high-frequency cycles present within the envelopes of the output impulse signal. The larger the value of k, the more closely spaced are the permitted output frequencies; e.g., for a given pulse duration T=2L/c seconds and k>>1, there is a number of discrete frequencies at which the equality f.sub.o T=(2k-1)/2 can be substantially satisfied.

The foregoing discussion of the operation of pulse-forming network 10 has been somewhat idealized in the sense that certain practical considerations have been neglected. Most important, particularly for frequencies above those lying in the X-band, the leakage capacity of the gating switch within generator 11 which controls the start and stop of the pulse emanating form signal source 11 has been neglected. According to the present invention, the effects of switch leakage are recognized and are beneficially used.

Reference may be had to FIG. 6 for analysis of the effects of leakage of the modulating switch 22. In FIG. 6, pulse forming network 10 is connected to the sine wave generator 11a when switch 22 is open through the generator's source resistance 12 and the leakage capacity of switch 22, which capacitance may be represented by a capacitor 23 of value C.sub.s. The output voltage a(t) from network 10 appearing across matched termination 15 is zero regardless of the values of C.sub.s and V.sub.s because of the effective short circuit imposed by stub transmission line 3, 3a at f=f.sub.o. However, the magnitude and carrier phase of the output pulse depends critically on the steady state voltage V.sub.s across leakage capacitor 23 at the time at which switch 22 is to be closed.

The above-mentioned relations may better be understood by reference to FIG. 7. Upon closure of switch 22, the effective resistance r.sub.s of the switch drops suddenly to zero, so the voltage across condenser 23 must drop to zero (the voltage between points A and B drops to zero). Such is equivalent to the abrupt insertion of a generator source 24 of equal potential V.sub.s ' and opposite sign in series with voltage V.sub.s.

As noted previously, the steady state output appearing across termination 15 is zero regardless of the value of V.sub.s. Thus, the response due to voltage E.sub.g in series with voltage V.sub.s is zero. On the other hand, the output response instantly after t=D/c must be due directly to the fact that V.sub.s '=V.sub.s. Thus, to maximize the pulse output voltage, the value of V.sub.s must be maximized, since it indeed determines the amplitude of output signal a(t).

A maximum value for factor V.sub.s may be achieved by making use of a series resonance effect, for example, when the steady state driving point impedance Z.sub.dp (FIG. 7) of pulse-forming network 10 is inductive and the following relationship is satisfied at frequency f=f.sub.o :

Z.sub.dp = X.sub.L = X.sub.s (1) In equation (1), Z.sub.dp is defined as the steady state sinusoidal driving point impedance as measured at the input port of the pulse forming network at any frequency f. The quantity X.sub.c is the capacitive reactance of the switch in its normally open mode at frequency f=f.sub.o. Third, X.sub.L is the inductive reactance presented at the input port of the pulse forming network when f=f.sub.o. The quantity Z.sub.dp is purely inductive and equal to X.sub.L only at f=f.sub.o. It is seen that the desired relationship between the respective line and switch reactances X.sub.L and X.sub.s and the factor Z.sub.dp is met by proper selection of the value of distance d (FIG. 1) between reference plane 4 and plane 6 where stub branch line 7, 7a forms its junction with line 3, 3a. This results in resonance between the network input and the leakage capacity C.sub.s of the switch 22 and results because the stub branch line 7, 7a junction represents an effective short circuit.

Under the above circumstances, the value of the steady state driving point impedance Z.sub.dp for network 10 is:

Z.sub.dp =jZ.sub.o tan .beta.d (2)

where:

.beta.=2.pi.f.sub.o /c

The required maximizing value d.sub.m of distance d is found from equations (1) and (2) as follows:

or:

If we define Q as:

Then:

d.sub.m =(1/.beta.) tan.sup..sup.-1 Q (3)

Here, the factor Q is the conventional quality factor of the resonating circuit. Under the above conditions, the voltage V.sub.s is given by:

V.sub.s =- jQE.sub.g (4)

By inspection of FIGS. 1 and 2, it is seen that the peak output during the era of the output pulse across termination 15 is:

a(t).sub.peak = V.sub.s /4 (5)

In general, the signal a(t).sub. peak closely resembles the pulse wave shown in FIG. 5.

It is seen from the analysis of FIGS. 1 to 7 that the invention may be practiced in any of several ways. For example, the unsynchronized step modulated sine wave generator 11 of FIG. 1 may be any convenient relatively stable high-frequency oscillator, and it may internally include a series connected controlling or gating switch 22 which may be either a single-pole, single throw mercury-wetted mechanical switch or a voltage controlled semiconductor switch of conventional nature. Switches suitable for such applications are illustrated, for example, in the pending G. F. Ross Pat. application Ser. No. 843,945, entitled "High Frequency Switch" and filed July 23, 1969, now U.S. Pat. No. 3,569,877, issued Mar. 9, 1971, as well as in the H. C. Maguire Pat. application Ser. No. 852,656, entitled "Coaxial Line Reed Switch for Fast Rise Signal Generator With Attenuation Means Forming Outer Section of the Line" and filed Aug. 25, 1969, now Pat. No. 3,564,277, issued Feb. 16, 1971, both inventions being assigned to the Sperry Rand Corporation.

FIG. 8 represents schematically the form of a two-wire transmission line system in a practical instrumentation of the invention and is further to be discussed in connection with the apparatus of FIG. 9 which is illustrated in the form of a coaxial transmission line system. FIG. 8 is significantly similar to part of FIG. 1, and corresponding parts are identified by similar reference numerals. For example, two-wire transmission line 3, 3a extends between impedance reference planes 4 and 5 and has branching from it a two-wire open-circuited transmission line 7, 7a of length L whose junction with line 3, 3a is located in reference plane 6 a distance d from plane 4, as previously defined. The value d is again adjusted substantially to the m d.sub.m called for by equation (3). The pulse forming network 10a of FIG. 8 now includes a second branching two-wire open circuited transmission line 17, 17a, also of length L and whose junction with line 3, 3a is located also in reference plane 6 coincident with the junction of branch line 7, 7a and at reference plane 6 a distance d.sub.m from reference plane 4. For one practical application, lines 3, 3a and branch lines 7, 7a and 17, 17a may all have characteristic impedances of 50 ohms. It is seen that network 10a, with a factor d.sub.m of proper value, will operate substantially in the same manner as will the arrangement of FIG. 1 when connected to impulse generator 11 and termination 15 of FIG. 1.

One practical embodiment of the invention is shown in FIG. 9 as having an internally impulse modulated generator 111 similar to generator 11 of FIG. 1 and coupled at a reference plane 104 to a coaxial transmission line network 110a whose output impulse wave a(t) is coupled at reference plane 105 to matched termination 115. For convenience, parts of network 110a corresponding to parts of network 10a of FIG. 8 have similar reference numerals with the factor of 100 added to them.

In FIG. 9, the network 110a is conveniently formed of readily available coaxial transmission line components to form a cross-shaped configuration. It includes a main coaxial transmission line 103, 103a whose center conductor 103 is connected to generator 111 and to terminal load 115. The opposite lead of generator 111 and of load 115 are grounded, as is the outer conductor 103a of line 103, 103a. The network is formed with radially or oppositely branching coaxial lines 107, 107a and 117, 117a. The latter are shown cut away at junction 125 lying in plane 106 to emphasize the joint connection at point 125 of inner conductors 107 and 117 to inner conductor 103 of the main transmission line 103, 103a. Outer coaxial conductors 107a and 117a, being electrically connected to outer conductor 103a as indicated by dotted lines, are also at ground potential. Again, the distance between planes 104 and 106, which planes correspond to reference planes 4 and 6, is adjusted substantially to the value d.sub.m defined above.

FIG. 10 represents a modification 110b of the network 110a of FIG. 9 having additional features desirable in certain applications. The structure is electrically equivalent to that of FIG. 9, but radially extending coaxial lines 107, 107a and 117, 117a, which may be in the from of flexible coaxial cables, are bent into a generally circular loop. Then, the respective open-circuited ends 123 and 123a of lines 107, 107a and 117, 117a are directly joined in conductive relation, as in the plane 126 of FIG. 10. It is understood that part of the loop lies above the plane of the drawing of FIG. 10.

Operation of FIGS. 9 and 10 is similar, since all signals again travel the same distances in both embodiments. Those signals which return to junction 125 in FIG. 9 from the stub lines along the same paths as they originally left it, return thereto in FIG. 10 along the opposite branch transmission line. In the apparatus of FIGS. 9 and 10, all transmission line elements may conveniently be commercially standard 50-ohm components. An additional advantage of the FIG. 10 structure is that the output pulse duration is dependent upon the length of a single continuous cable loop. FUrther, no leadage of energy can occur because of an imperfect open circuit 123 or 123a and counterflowing signals must travel over equal paths.

Claims

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than of limitation, and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

1. Apparatus for generation of short duration high-frequency time-limited pulses having abrupt initial rise and terminal fall characteristics comprising:

first high-frequency transmission line means having a predetermined characteristic impedance Z.sub.o and input and output means,

high-frequency sine wave oscillator means for supplying a train of high-frequency sine wave cycles at said input means,

switch means having, when nonconducting, a leakage capacity C.sub.s, for gating said oscillator means,

matched utilization means attached at said output means,

open-circuited branch transmission line means joined to said first transmission line means at a distance d.sub.m therealong from said input means,

said distance d.sub.m so adjusted as substantially to provoke resonance between said input means and the leakage capacitance C.sub.s of said switch.

2. Apparatus as described in claim 1 wherein said distance d.sub.m is adjusted so as to make the reactance of said first transmission line means substantially equal to the reactance of said switch means at resonance.

3. Apparatus as described in claim 1 wherein said distance d.sub.m is adjusted so as to make the driving point impedance of the network comprising said first transmission line means and said branch transmission line means substantially equal to the reactance of said switch means at resonance.

4. Apparatus as described in claim 1 wherein the quantity d.sub.m is defined substantially by the equation

where:

.beta.=2.pi.f.sub.o /c

and where f.sub.o is the operating frequency of said oscillator means and c is the velocity of wave propagation is said first transmission line means.

5. Apparatus as described in claim 1 wherein said open-circuited branch transmission line means joined to said first transmission line means comprises radially extending coaxial transmission line means open-circuited at ends remote from said first transmission line means.

6. Apparatus as described in claim 5 wherein said open-circuited ends of said radially extending coaxial transmission line means remote from said first transmission line are joined one to the other in direct conductive relation.