Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1995-09-14
2001-11-06
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S076290
Reexamination Certificate
active
06313620
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to RF receivers and, in particular, the invention relates to a detector system for identifying the frequency of an RF signal.
BACKGROUND OF THE INVENTION
Multiple channel receivers are capable of receiving signals over various bands of the frequency spectrum. When an incoming signal having a known frequency is to be received, a receiver that is tuned to the particular channel corresponding to the signal's frequency can receive the signal. A difficulty arises, however, when the signal to be received is of an unknown frequency. In such a case, the receiver must determine the incoming signal's frequency in order to then tune to the appropriate channel and receive the signal. Unfortunately, it is difficult to determine the specific frequency channel which yields the best signal quality. This difficulty occurs because signals may be detected in more than one channel due to imperfect out-of-band rejection of the channels adjacent to the one yielding the best reception quality, or the application of pulse modulated signals to the receiver which are prone to cause detections in different channels. Consequently, the receiver may be tuned to an inappropriate adjacent channel, resulting in imperfect signal reception, signal distortions, and general delays in data and/or voice reception. Thus, it is important to have a device which could determine the specific frequency channel over which a signal exhibits its largest amplitude. With this capability, a channelized receiver tuned to this specific frequency channel will receive the signal with minimized distortion.
It is known in the art to identify the frequency of an RF signal in a channelized receiver. An example of one patent that discloses such technique is U.S. Pat. No. 4,301,454, issued Nov. 17, 1981, entitled “Channelized Receiver System” by D. E. Bailey. The Bailey patent discloses a receiver which identifies the frequency channel in which an unknown signal is received by comparing the amplitude of a filtered signal having a largest amplitude with that of a filtered signal having a next largest amplitude. The receiver splits a received RF signal into at least two portions and applies each portion to a bank of filters. Each bank is comprised of filters defining alternate portions of a search bandwidth. Within each filter bank, the signal portions are applied to individual filters via circulators. When the frequency band of a particular filter corresponds to the signal's frequency, the signal is bandpassed through the filter. Where the signal frequency does not correspond to the filter frequency band, the signal rebounds back to the circulator which directs the signal to the next filter, where this process is repeated and continued.
The signal portions which pass through each filter bank are then applied to a voting logic device. The voting logic device determines the specific filters in each group having the largest amplitude output signals and compares the amplitude relationship that exists between these determined signals. When the filters having the largest output signals have pass bands defining contiguous portions of the search bandwidth, the signals are used by a computer preprogrammed with each of the filter pass band characteristics to calculate the RF signal frequency. The computer, which makes the calculations by determining the difference between the output signals, then generates a tuning signal to be used to tune a narrowband receiver to an appropriate frequency channel.
The teaching of Bailey is exemplary of the complexity of conventional approaches for determining the frequency of a received RF signal.
It is one object of this invention to provide for a simplified system of small physical size which quickly and accurately determines the frequency of a received RF signal for use in a channelized receiver, and that is suitable for receiving both continuous wave and intermittent (e.g., pulsed) transmission signals.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the object of the invention is realized by a method, and by an apparatus for accomplishing the method, for use in a channelized receiver for determining the frequency of an RF signal by: (1) sequentially applying a received RF signal to sequentially selected filters; (2) comparing the magnitude of the filtered signal to the magnitude of an unfiltered RF signal, (3) generating an output signal to a correlator when the filtered signal and unfiltered signal magnitudes are substantially equal; and (4) correlating the output signal to the bandpass frequency of the selected filtering means to determine the frequency of the RF signal.
In a preferred embodiment of the invention, individual ones of a plurality of filters have predetermined frequency pass bands that are sequentially selected by an input multiplexer in response to a control signal. A received RF signal is sequentially applied to each one of the selected filters and the filtered output signal is then routed to a first detector via an output multiplexer. The unfiltered RF signal is simultaneously applied to a second detector. The first and second detectors detect the magnitudes of the filtered output signal and the unfiltered RF signal, respectively. Each signal is then simultaneously applied to a subtractor which subtracts the magnitude of the filtered output signal from the magnitude of the unfiltered RF signal to produce a resulting difference signal. The resulting difference signal is then compared to a magnitude of a composite reference level with a comparator such that the comparator generates an output signal to a digital logic block when the respective magnitudes of the filtered and unfiltered signals are substantially equal. The magnitude of the filtered output signal is also compared to a threshold reference level magnitude via a second comparator such that the second comparator generates an output signal to the digital logic block when the magnitude of the filtered signal is greater than the magnitude of the threshold reference level. When the digital logic block receives appropriate output signals from both the first and second comparators, an output signal is reported to a correlator, such as a data processor, which determines the frequency of the RF signal by correlating the output signal to the band pass frequency of the selected filter.
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patent: 4301454 (1981-11-01), Bailey
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patent: 4556982 (1985-12-01), Dunn
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patent: 5129100 (1992-07-01), Caporizzo et al.
patent: 5157348 (1992-10-01), Deveau
patent: 5251332 (1993-10-01), Hansen
Gorham Kenneth Duane
Richardson David Livingstone
Anderson Terry J.
Hoch, Jr. Karl J.
Metjahic Safet
Nguyen Vincent Q.
Northrop Grumman Corporation
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