Improved Switching Circuitry For Semiconductor Diodes

Abstract

A driver amplifier for p-i-n diodes in the phase shifters of a phased array antenna, such amplifier being arranged so as to produce, in response to a command signal from a beam steering computer, either a forward-bias or a back-bias signal for such diodes, the particular bias signal produced by such amplifier being delayed by substantially the same length of time after application of a command signal.

Description

BACKGROUND OF THE INVENTION

This invention pertains generally to phased array antennas for radar and particularly to antennas of such type suing semiconductor diode phase shifters to collimate and direct a beam of microwave energy.

It is known in the art that a matrix of so-called semiconductor diode phase shifters may be used selectively to adjust the phase of microwave energy passing to, or from, individual antenna elements in a phased array. Such diode phase shifters are operative, in accordance with a program determined by the parameters of the particular array and the desired deflection angle of a beam of microwave energy, to change the length of the electrical path of the microwave energy between each antenna element and a source (or detector) of such energy.

When relatively large amounts of microwave energy are to be passed through a semiconductor diode phase shifter, it is common practice to use so-called p-i-n diodes as the switching element in such a phase shifter. Unfortunately, however, the characteristics of p-i-n diodes and associated elements are such that the length of time required to switch from a forward-bias to back-bias condition is longer than the time required to switch in the opposite direction. It follows therefore, in view of the fact that provision must be made to prevent the propagation of microwave energy during the time in which any of the p-i-n diodes is switching, that it is necessary to inhibit generation of microwave energy for a relatively long period of time whenever it is desired to change beam direction.

Attempts have been made to reduce the time required to operate p-i-n diodes by using driver amplifiers with output stages having special characteristics. That is, it is known to provide, in the output stages of driver amplifiers for p-i-n diodes, power transistors having low storage and fall times so that only the delays inherent in the switching of p-i-n diodes are experienced. It has been found, however, that the types of power transistors required are extremely expensive and that, even with the best of existing power transistors, marked improvement cannot be attained.

SUMMARY OF THE INVENTION

Therefore, it is a primary object of this invention to provide improved circuitry for operating p-i-n diodes in semiconductor phase shifters for microwave energy.

Another object of this invention is to provide improved circuitry as just mentioned, such circuitry being adapted to operate with conventional, relatively inexpensive components.

These and other objects of this invention are attained generally in a driver amplifier for p-i-n diodes by providing, in such an amplifier, delay means operative on the control signals to a greater degree when the p-i-n diodes are to be driven from their back-bias to their forward-bias conditions, the amount of delay of such control signals being substantially equal to the delay inherent in such p-i-n diodes when being driven from their forward-bias to their back-bias conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is now made to the following description of the accompanying drawings, in which:

FIG. 1 is a greatly simplified sketch showing the relationship of a drive amplifier according to this invention in relation to a radar system; and

FIGS. 2 and 3 are sketches of the waveforms appearing at the output of driver amplifiers in response to command signals from a beam steering computer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, it may be seen that the contemplated radar system includes a controller 10, a beam steering computer 12, a plurality of driver amplifiers 14, . . . 14n, a matrix (not numbered) of phase shifters 18 . . . 18n, and a transmitter/receiver 20. The just recited elements, except for the drive amplifiers 14 . . . 14n, are conventional in construction and operation. Thus, the controller 10 simply produces beam steering command signals for the beam steering computer 12 and synchronizing pulses for the transmitter/receiver 20. It is noted, however, that, if it is desired to permit microwave energy to be propagated only after the phase shifters 16 . . . 16n have completed any required change in response to a change in the beam steering command signal, the beam steering command signal line and the synchronizing signal line should be interlocked in any convenient fashion. Such interlocking should inhibit the transmitter/receiver 20 for a short period of time, say 1.5 microseconds, after any change in the beam steering command signal to permit the phase shifters 16 . . . 16n to be forward or back-biased as required. It is also noted that, for convenience here, each phase shifter 16 . . . 16n has been shown as a three bit phase shifter. It is not, however, essential to the invention that a three bit phase shifter be used.

Referring now to the exemplary driver amplifier 14, it may be seen that each such amplifier includes a delay circuit 21, an amplifying section 23, a pair of output power transistors 25, 27, a discharging transistor 28 and associated elements to be described. Suffice it to say here, however, that the various elements combine in the steady state with a logic "one," i.e. approximately +2 volts, on the input line from the beam steering computer 12 so that output power transistor 25 is conducting and output power transistor 27 is cut-off. Conversely, in the steady state with a logic "zero," i.e. approximately zero volts, on the input line from the beam steering computer 12, both output power transistors 25, 27 are cut-off. On other words, in the first steady state, the corresponding p-i-n diodes 29, 29a are ultimately connected, via limiting resistors 31, 31a, cable 33, inductor 35, output power transistor 25 and diode 37 to a forward-biasing current source, -E.sub.f. In the second steady state, the p-i-n diodes 31, 31a are connected, as indicated, through a resistor 39 (having a resistance on the order of 270 kilohms), the inductor 35, cable 33 and resistors 31, 31a to a back-bias current source, +E.sub.b.

The delay circuit 21 is made up of a resistor 43, a capacitor 45 and a transistor 47. With the arrangement shown, it is obvious that, in the steady state, the transistor 47 is cut-off when a logic "zero" is applied to the resistor 43 from the beam steering computer 12. When the output signal from the latter is changed to a logic "one," it is equally obvious that the voltage on the emitter electrode (not numbered) of the transistor 47 rises in accordance with the time constant of the integrating circuit (resistor 43 and capacitor 45). It follows, therefore, that the threshold voltage for conduction by transistor 47 is reached only after some period of time, say 2 microseconds, has elapsed after the change of a logic "one." When transistor 47 becomes conducting, amplifier 23 produces a positive going signal which is coupled through a diode 49 and a resistor 51 to the base electrode (not numbered) of the output power transistor 25. It is noted that the positive going signal out of the amplifier 23 is blocked from the base electrode (not numbered) of a discharging transistor 28 by a diode 55. The latter transistor is biased into its cut-off condition. The positive going signal on the base electrode of the output power transistor 25 is sufficient to cause the latter to conduct, thereby producing a forward-bias signal (via inductor 35, cable 33 and resistors 31, 31a) to the p-i-n diodes 29, 29a.

It us noted that the current path for forward-biasing the p-i-n diodes 29, 29a initially includes a capacitor 59 (which capacitor is charged to a voltage substantially equal to -E.sub.ff when output power transistor 25 starts to conduct). As current is drawn from capacitor 59, its voltage changes approaching the forward bias voltage, -E .sub.f, to permit diode 37 to conduct. Thereafter, the current path includes the forward bias current source, -E.sub.f. The operation of capacitor 59, as just described, helps discharging the cable 33 and improves rise time of the forward-bias signal at the p-i-n diodes 29, 29a.

When the output signal from the beam steering computer 12 changes from a logic "one" to a logic "zero," the capacitor 45 may discharge, at least partially, through the transistor 47 because that element is still conducting. The time constant of the combination of the capacitor 45 and the emitter-base circuit of the transistor 47, as long as the latter is conducting, is relatively short as compared to the time constant of the combination of resistor 43 and capacitor 45. Consequently, the discharge of capacitor 45 occurs at a more rapid rate than its charge. In a practical application, then, transistor 47 cuts off within a few tenths of a microsecond after a logic "zero" is received from the beam steering computer 12. When transistor 47 ceases to conduct, amplifier 23 produces a negative going signal. Such signal is blocked by diode 49 but is passed by diode 55 so as to appear on the base electrode (not numbered) of discharging transistor 28. The emitter circuit for the latter is then completed, via capacitor 59, a diode 60 and a resistor 61 so that it conducts momentarily (until capacitor 59 is charged). During this time, say a few tenths of a microsecond, transistor 28 conducts, the storage charge of the output power transistor 25 is removed, thereby causing that element to cut-off more quickly than would be the case if discharging transistor 28 were not in the circuit. Because the current through the inductor 35 cannot change instantaneously, the cut-off of output power transistor 25 causes the inductor 35 to produce, for a short period of time, a voltage "spike" on the base electrode (not numbered) of the output power transistor 27, which voltage spike is sufficient to turn that transistor on. A relatively high back-current "spike" is, consequently, passed from back-bias source, +E.sub.b, through resistor 41, output power transistor 27, cable 33 and resistors 31, 31a to p-i-n diodes 29, 29a. Such a current spike is effective rapidly to deplete the storage charge of the p-i-n diodes thereby changing them to their back-bias condition. As noted hereinbefore, when output power transistor 27 reverts back to its cut-off condition after, say, 2 microseconds, back-bias is maintained on the p-i-n diodes through the path from the back-bias source, +E.sub.b, which includes resistor 39.

Having described an embodiment of a driver amplifier according to this invention, reference is now made to FIGS. 2 and 3 of our driver amplifier. Thus, in FIG. 2 the upper set of curves shows that, in response to a negative-going signal, S.sub.F.sub.-B, from a beam steering computer (such signal being a command signal to switch p-i-n diodes in a phase shifter from a forward-bias to a back-bias condition), some time elapses before the voltage, V.sub.D, reaches a desired steady state back-bias level. With a positive-going signal, S.sub.B.sub.-F, from a beam steering computer (such signal, illustrated in the lower set of curves in FIG. 2, being a command signal to switch p-i-n diodes in a phase shifter from a back-bias condition to a forward-bias condition), the requisite change in level of the voltage across the p-i-n diodes takes place almost coincidentally with the command signal. It is apparent, therefore, that the generation of microwave energy in a radar system according to the prior art must be inhibited for an interval equal to the interval between application of a command signal and completion of the slower switching direction. Typically, such as interval is in the order of 3 microseconds.

Referring now to FIG. 3 it may be seen that, according to our invention, switching from a forward-bias to a back-bias condition is accomplished in substantially the same manner (as shown in the upper set of curves in FIG. 3) as in the prior art. Application of a positive-going signal, S.sub.B.sub.-F, from a beam steering computer, i.e. a command signal to switch p-i-n diodes in a phase shifter from a back-bias to a forward-bias condition, has, however, no immediate effect on the output signal from our driver amplifier. After a period of time (which period is approximately the same as the time taken to switch the p-i-n diodes from a forward to a back-bias condition) our circuit then operates. Obviously, therefore, the generation of microwave energy need be inhibited for an interval, typically 1.5 microseconds, as indicated by the vertical dashed lines.

It will be appreciated by those of skill in the art that reducing the period of time during which generation of microwave energy is inhibited in order to change the condition of phase shifters in a phased array antenna is important when a large number of targets is being tracked. It will also be appreciated that our contemplated circuitry permits such reduction without making it necessary to use relatively fast acting (and, therefore, expensive) semiconductor elements. It is felt, in view of the foregoing, that this invention should not be restricted to its disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.

Claims

What is claimed is:

1. In a phased array antenna assembly for microwave energy, such assembly incorporating a matrix of antenna elements, each one of such elements having associated therewith a semiconductor-diode phase shifter to change, in accordance with binary control signals, the phase of microwave energy to and from each one of such elements, separate circuitry for selectively forward-biasing and back-biasing the semiconductor diodes in each semiconductor-diode phase shifter, each such separate circuitry including:

a. first circuit means, including a first resistor, for connecting the semiconductor diodes in an associated semiconductor-diode phase shifter to a terminal;

b. second circuit means, including a second resistor, for connecting a source of back-bias voltage to the terminal;

c. third circuit means, including a first transistor, for connecting a source of forward-bias voltage to the terminal; and

d. fourth circuit means, including a time delay circuit for connecting a source of binary control signals to the first transistor to switch that element from its conducting state to its nonconducting state in accordance with the binary control signals, the time delay of the time delay circuit also varying in accordance with the binary control signals.

2. Circuitry as in claim 1 having, additionally:

a. a second transistor, such transistor having its collector electrode connected through a third resistor to the source of back-bias voltage, its base electrode connected to the terminal and its emitter electrode connected to a junction in the first circuit means; and

b. inductor means, disposed in the first circuit means between the terminal and the junction and responsive to the binary control signals, for momentarily biasing the second transistor into its conductive state during each period of time when the first transistor is changing from its conducting to its nonconducting state.

3. Circuitry as in claim 2 having additionally: fifth circuit means, including a third transistor, the emitter electrode of such transistor being connected to the base electrode of the first transistor, the collector electrode of such transistor being connected to a discharging load and the base electrode of such transistor being in circuit with the fourth circuit means and the discharging load, the third transistor thereby being biased into its conducting state only during each period of time when the first transistor is changing from its conducting to its nonconducting state.