Responsive simultaneous frequency agile radar

BACKGROUND OF THE INVENTION

Field

of

the

Invention:

0001The present invention relates to radar and communication systems. More specifically, the present invention relates to radar and communication systems that are adapted to circumvent interference.

Description

of

the

Related

Art

:

0002Conventional radar systems operate over a narrow band at a single carrier frequency. As a result, these systems were susceptible to interference. Many schemes have been developed to address this problem, two of which are the frequency hopping and spread spectrum schemes. Frequency hopping involves the transmission of a block of data at one carrier frequency, then "hopping" to another carrier frequency and transmitting another block of data and so on.

0003Unfortunately, when frequency hopping is employed, it is difficult, if not impossible, to coherently add individual pulses together to improved the signal-to-noise ratio. While slow hopping may preserve the (phase) coherency of the pulses, slow hopping is considerably more susceptible to interference.

0004Conventional spread spectrum systems operate at a single carrier over a wide bandwidth. However, these systems create many intermodulation products. Thus, a considerable amount of power may be wasted in spurious sidebands. In addition, the coherency of the signals is uncertain.

0005Further, neither the conventional frequency hopping schemes nor the conventional spread spectrum schemes fully exploit the wide bandwidth capabilities of current traveling wave tube amplifiers.

0006Thus, a need exists in the art for a radar transmitter which is secure from interference, offers the performance advantages of phase coherency, is power efficient and is capable of taking advantage of the broad bandwidth of current traveling wave tube amplifiers.

0007F.E. Terman discloses in his book "Radio Engineering" (3rd Edition, Published 1947) the use of a method and circuit often referred to as the Armstrong system. In this system, the output of a balanced modulator, consisting of two side bands with carrier suppressed, is combined with an unmodulated carrier wave which differs in phase by 90 degrees from the carrier associated with the balanced modulator. This results in a frequency-modulated wave. With this system, one is able to obtain a substantially linear relationship between the modulating voltage and phase shift up to a modulation index of approximately 0.5.

SUMMARY OF THE INVENTION

0008The need in the art is addressed by the present invention, as specified in claims 1 and 3, which provides a system for transmitting a comb of radar signals. The invention includes a phase shifter for shifting the phase of an input carrier signal. A frequency modulation signal is adjusted in amplitude by an attenuator and input to a balanced mixer. The balanced mixer mixes the input carrier signal with the attenuated frequency modulation signal. A summer combines the output of the phase shifter with the output of the balanced mixer. The system of the present invention provides an inexpensive means of generating and receiving a complex of illuminating signals having a spread spectrum consisting of lines (teeth) of carrier frequencies with controllable power. The system is agile in that the power levels and the spectral positions of the teeth may be varied.

BRIEF DESCRIPTION OF THE DRAWINGS

0009Fig. 1 is a simplified illustrative implementation of the system of the present invention.

0010Fig. 2a is a power spectra showing that the three frequency bands output by the system of the illustrative embodiment may be of equal power.

0011Fig. 2b is a power spectra showing the three frequencies generated by the present invention with more power allocated to the center frequency and correspondingly less power allocated to the two sidebands.

0012Fig. 3a shows a one-line power spectra.

0013Fig. 3b shows the three line power spectra provided by the system of the present invention.

0014Fig. 3c is a graph of frequency versus time showing how the frequency of a one line carrier can be varied over time.

0015Fig. 3d is a graph of frequency versus time showing how the frequency of the three line power spectra, of the present invention, can be varied with time.

0016Fig. 3e is a graph of frequency versus time showing how the frequency of the sidebands might be varied over time with respect to a center carrier frequency which has a constant frequency.

0017Fig. 3f is a graph of frequency versus time showing the result of sweeping the sidebands and the carrier frequency simultaneously.

0018Fig. 3g is a graph of frequency versus time showing the result of sweeping the sidebands only.

0019Fig. 3h is a graph of frequency versus time showing the result of sweeping the carrier and the upper sideband only.

DESCRIPTION OF THE INVENTION

0020Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings.

0021A simplified illustrative implementation of the system 10 of the present invention is provided in Fig. 1. The system 10 includes a 90 degree phase shifter 12, a balanced mixer 14, a first attenuator 16, a second attenuator 18 and a summer 20. An input carrier signal from an exciter (basic oscillator) or frequency reference unit (FRU) is input to the 90 degree phase shifter 12 and the balanced mixer 14. The phase shifter 12 shifts the phase of the input carrier signal and produces the center line of the output signal. The balanced mixer 14 suppresses the carrier signal and generates first and second sidebands. While two sidebands, three lines total, are probably optimum, those skilled in the art will recognize that more than two sidebands can be produced without departing from the scope of the present invention.

0022The balanced mixer 14 receives a frequency modulated (FM) signal through the first attenuator 16. A particularly novel aspect of the present invention is that the modulation index of the balanced mixer is controlled by the input FM signal. The modulation index of the balanced mixer 14 determines the spectral position of the sidebands produced by the balanced mixer 14. The first attenuator 16 provides coarse control of the relative amplitude of the sidebands produced by the balanced mixer. The output of the balanced mixer 14 is input to the second attenuator 18. Shown as a step attenuator, the second attenuator 18 provides fine control of the amplitude of the sidebands.

0023The summer combines the sidebands from the attenuator 18 with the phase shifted carrier from the phase shifter 12. The 90 degree phase shifting of the input carrier is effective to allow the sidebands to be summed with carrier without cancellation and with minimum spurious outband products. The spectral output of the summer 20 is a multiple frequency radar comb which approximates an FM signal. As shown in Fig. 1, the center line of the output signal is the phase shifted carrier and the left and right lines are the sidebands. The attenuators 16 and 18 are designed to provide sidebands of amplitudes which equal the amplitude of the center carrier. Thus, a significant feature of the present invention resides in the ability to control the allocation of available power between a number of simultaneously employed frequencies. This is illustrated in Figs. 2a and 2b. Fig. 2a shows that the three frequency bands output by the system of the illustrative embodiment may be of equal power. Fig. 2b shows the three frequencies with more power allocated to the center frequency and correspondingly less power allocated to the two sidebands.

0024In sum, the system 10 generates two sidebands from the carrier, phase shifts the sidebands and injects the sidebands into the carrier to provide a carrier with two sidebands instead of the single center carrier. The system 10 generates the desired frequency spectrum at low power levels. The output of the summer 20 is input to a traveling wave tube (TWT) not shown.

0025Fig. 3 illustrates the waveform flexibility afforded by the system of the present invention over a conventional system. With respect to power spectra, Fig. 3a shows a one-line power spectra while Fig. 3b shows the three line power spectra provided by the system of the present invention. Fig. 3b shows that some power may be generated in spurious sidebands 22 and 24. However, judicious control of the modulation index should be effective to mitigate the production of such outbands.

0026With respect to carrier frequency agility, Fig. 3c shows how the frequency of a one line carrier can be varied over time, while Fig. 3d shows how the frequency of the three line power spectra, of the present invention, can be varied with time.

0027Fig. 3e demonstrates line separation agility by showing how the frequency of the sidebands might be varied over time with respect to a center carrier frequency which has a constant frequency.

0028Figs. 3f, 3g and 3h show FM modulation of the carrier frequency, variable sideband spacing and simultaneous variation of both parameters, respectively. That is, these figures show that the system of the present invention allows the sidebands and the carrier frequencies to be swept in frequency. Thus, Fig. 3f shows the result of sweeping the sidebands and the carrier frequency simultaneously. This is achieved by sweeping the input carrier frequency. Often referred to as "linear frequency modulation", Fig. 3g shows the result of sweeping the sidebands only. This is achieved by sweeping the FM signal input to the balanced mixer 14. Fig. 3h shows the result of sweeping the carrier and the upper sideband only.

0029Thus, the present invention provides multiple bands which may be powered, swept or hopped independently.