Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
1997-07-28
2001-02-20
Lee, Benny T (Department: 2817)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C333S164000
Reexamination Certificate
active
06191735
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to time delay circuits and more particularly, to an apparatus for producing time delayed microwave signals for large instantaneous bandwidth systems to provide an antenna beam pattern which is substantially constant over the bandwidth of the system.
BACKGROUND OF THE INVENTION
Many modern electrically scanned antenna arrays for radar, communication and electronic countermeasures systems require large instantaneous bandwidth. Historically, electronically scanned antennas have utilized phase shifters in each element of the array to control beam position and direction. However, in moderate to large arrays, such a method results in a beam position which varies with frequency. This prevents instantaneous operation over a large portion of the bandwidth since the beam position will move away from the desired direction, or the beam pattern becomes distorted.
An alternative to phase shifters which can be used to scan the beam in a frequency independent manner is the use of true time delay circuits, whereby the time delay of a signal is varied rather than the phase. While this approach has been recognized, few practical implementations of this method have been developed. One such method involves the use of fiber optic delay lines whereby a microwave signal is carried on a lightwave whose time delay is varied. After the appropriate delay, the lightwave is detected and converted back to a microwave signal.
However, fiber optic delay lines for wideband microwave array antennas have several disadvantages. First, the microwave signal is modulated onto a lightwave at the input to the fiber (delay line) and then converted back (demodulated) to a microwave signal at the fiber output. These processes result in signal loss which can be as high as 20 to 30 dB. This signal loss must be made up by external amplifiers, which add complexity to the system. In addition, the optical detection process adds noise to the microwave signal which cannot be totally removed.
Some of the optical approaches utilize lasers, whose frequency is varied to provide the variable delay. This approach has limitations in the switching time. Whereas the desired switching time for large array communications is fast for example, one microsecond (e.g. 1 &mgr;sec); the achievable time in prior art laser switching devices is relatively slow on the order of 100 msec.
Furthermore, optical approaches tend to be expensive. Since many such devices are required in a typical array (100 to 1000), the cost for producing multiple fiber optic delay lines may be prohibitive.
The approach described here overcomes the shortcomings described above, because all of the time delay is accomplished with microwave circuitry alone, eliminating the need for optical fibers. By eliminating the need to convert from microwaves to light and back again, the large signal loss is eliminated. The approach described here uses microwave switches which are very fast, resulting in switching times of much less than 1 &mgr;sec. Finally, by the use of monolithic microwave integrated circuit (MMIC) technology and printed circuit transmission lines, the approach described here can be implemented at low cost.
SUMMARY OF THE INVENTION
The present invention provides a system for generating time delayed signals from an input microwave RF signal having a wide instantaneous bandwidth. The system comprises a first time-delay circuit having a plurality of conductive paths of varying lengths for the signal to be switchably connected and in response to a first control signal for time-delaying the input microwave signal in a controllable manner to produce a time-delayed microwave signal. A second time-delay circuit comprising a plurality of gallium arsenide-based (GaAs) monolithic microwave integrated circuit (MMIC) chips is coupled to the first time-delay circuit, where each MMIC chip has a plurality of conductive line segments of varying lengths switchably connected and selectable for time-delaying the input time-delayed microwave signal in a controllable manner to produce an output time-delayed signal corresponding to the input microwave RF signal shifted in phase by the corresponding time delay, whereby the plurality of output time-delay signals are radiated through antenna elements to form a desired beam pattern.
The second time-delay circuit also includes on each MMIC a digital phase shifter for shifting the phase of the incident microwave RF signal by a predetermined amount. Therefore, the integrated circuit chip (MMIC) has both time-delay means and digital phase shift means and is operable in a first mode to time-delay the input RF microwave signal in response to control signals to cause a corresponding shift in phase of the signal at an output of the circuit, and operable in a second mode to time-delay and phase delay the input RF microwave signal in response to control signals to cause a shift in phase of the signal at the output of the circuit corresponding to the sum of the phase shifts caused by each of the individual time-delays and digital phase delay.
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DiPaolo, Franco, “A Simple High Yield 6 to 18 GHz GaAs Monlithic Phase Shifter”Microwave Journal,Apr. 1997; pp. 92-104;.
Noste, R.J. et al., “Affordable MMIC Designs for Phased Arrays”Microwave Journal;Mar. 1987; pp. 141-147.
ITT Manufacturing Enterprises Inc.
Lee Benny T
McDermott & Will & Emery
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