FM modulation technique for producing frequency rejection bands

Details FM modulation technique for producing frequency rejection bands FM modulation technique for producing frequency rejection bands

1988-05-20

2001-05-22

Hellner, Mark (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

With particular circuit

For frequency modulation

C342S159000

Reexamination Certificate

active

06236353

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to modulation techniques and more particularly to a technique for forming a null at virtually any desired frequency within the spectrum of a transmitted FM signal pulse for eliminating therefrom noises or interferences on reception which, if not eliminated, would decrease the energy level of the signal pulse.
CROSS REFERENCES TO RELATED APPLICATIONS
This invention is related to copending application entitled METHOD AND APPARATUS FOR SUPPRESSING NARROW BAND INTERFERENCE FOR A PULSE COMPRESSION RADAR SYSTEM by the same inventors and assigned to the same assignee as the instant invention, filed on May 20, 1988.
BACKGROUND OF THE INVENTION
It is well known by an artisan in the radar waveform design art that increased radar target resolution can be achieved by transmitting a linearly or non-linearly stepped FM signal pulse (toward a target) and processing its received echo by passing it through a network which processes linear (or non-linear) delay characteristics, with respect to frequency, so as to compress the received energy into a narrow high amplitude time pulse. Environments do exist, however, for radar operation wherein narrow bandwidth interference sources are active at frequencies within the same bandwidth as that of the stepped FM pulse. These interferences are received with the desired echo, generally at much larger amplitudes than the echo. Thus, when a combination of the received interference(s) and desired signal is processed by the linear delay network, the resulting compressed time pulse may become distorted, so much so as to become unrecognizable and therefore undetectable, due to the interferences.
To correct this problem, a current method involves the implementation of a narrow band elimination filter in the radar receiver prior to pulse recompression so that the unwanted interference frequencies are eliminated prior to passing the received echo through the linear delay network. Less perturbation to the ideal echo results and the received compressed pulse is restored to a shape which at least is detectable, rather than being totally unrecognizable.
Yet the use of band elimination filter in the frequency spectrum of the stepped FM pulse also results in the removal of energy from the desired signal. This leads to an intolerable decrease in the amplitude of the desired echo, thereby leading to a situation where the radar receiver often fails to detect these weak echoes.
SUMMARY OF THE INVENTION
The present invention technique utilizes non-linear frequency modulation of the transmitted stepped FM pulses so that controlled frequency elimination bands will be inherently present in the FM pulse signal spectrum. Consequently, the use of band elimination filters in the radar receiver to remove interference superposed on the FM pulses would not remove energy from the desired signal echo and, therefore, the available received signal power would not be decreased.
To achieve this end, a natural null is produced in the frequency spectrum of the of interest signal pulse at the frequency at which rejection is to take place. By thus providing a null at that frequency, the remaining energy of the signal pulse would then be redistributed to the remaining areas of the signal spectrum so that when the signal spectrum is filtered by a band elimination filter, only the interference, and not the energy ordinarily associated therewith, would be eliminated from the spectrum, since the interference is located in the null position of the signal spectrum. As a consequence, all available energy for the desired echo remain intact and the echo can easily be detected by a radar receiver.
It is thus an objective of the present invention to provide for a technique which would produce a null in the frequency spectrum of a FM signal for matching an interference superposed thereon before the same is to be rejected by a band elimination filter.
It is another objective of the present invention to provide for a frequency spectrum of a signal pulse which would retain its energy, despite the filtering of interference superposed thereon.
It is yet another objective of the present invention technique to provide for a frequency spectrum of a signal pulse which results in a desired echo signal that is both recognizable and easily detected by a conventional radar receiver.
The above-mentioned objects and advantages of the present invention will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings:


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