Abstract:
A method, and apparatus for accomplishing the method, for determining the frequency of an RF signal that operates by sequentially applying a received RF signal to sequentially selected individual filters having predetermined frequency pass bands and comparing the magnitudes of the filtered and unfiltered RF signals. The frequency of the RF signal is determined as being within the predetermined frequency pass band of a filter that outputs a largest magnitude output signal relative to the magnitude of the RF signal. Signals of insufficient magnitude are not processed by the system.

Description:
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&#39;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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: 
     FIG. 1 is a block diagram of a preferred embodiment of a frequency detection system for a channelized receiver in accordance with the teaching of this invention; 
     FIG. 2 a  illustrates the frequency response characteristics of a typical filter used in the channelized receiver of FIG. 1; 
     FIG. 2 b  illustrates the frequency response characteristics of a plurality of filters used in the channelized receiver of FIG. 1; 
     FIG. 3 is a timing diagram for the sequential selection of filters wherein each of the plurality of filters is selected in sequential order; 
     FIG. 4 a  is a diagrammatical representation of the frequency spectrum of a typical continuous wave input RF signal; 
     FIG. 4 b  is a diagrammatical representation of the amplitude versus time of a typical continuous wave input RF signal; 
     FIG. 5 a  is a diagrammatical representation of the frequency spectrum of a typical pulse wave input RF signal; 
     FIG. 5 b  is a diagrammatical representation of the amplitude versus time of a typical pulse wave input RF signal; 
     FIG. 6 a  illustrates a diagram of signals designated Vnb and Vwb, which emanate from the first and second detectors, respectively, in response to the continuous wave input signal shown in FIG. 4 b;    
     FIG. 6 b  is a diagram representing the output signal of the digital logic block in response to the application of the signal represented in FIG. 6 a  to the Signal Processing Section; 
     FIG. 7 a  illustrates a diagram of the signals Vnb and Vwb, which emanate from the first and second detectors in response to the pulsed wave input signal shown in FIG. 5 b;  and 
     FIG. 7 b  is a diagram representing an output signal of the digital logic block in response to the application of the signal represented in FIG. 7 a  to the Signal Processing Section. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An example of a detector system for determining the frequency of a received RF signal for a channelized receiver  10  is shown in FIG.  1 . In this preferred embodiment, the system  10  is comprised of a first multiplexer  16  with an input  14  connected to an RF signal input  12 , and a plurality of outputs  18 , each of which is connected to an individual filter  20 , of predetermined frequency pass band. The output of each individual filter is connected to one of a number of individual inputs  22  of a second multiplexer  24 . Both the first and second multiplexers  16 ,  24  are controlled by a control device, such as a microprocessor  32 . A first detector  28 , designated as a “narrowband” detector, is connected to the output  26  of the second multiplexer  24 . A second detector  30 , designated as a “wideband” detector, is connected to the RF signal input  12 . A Signal Processing Section  34  is connected to the outputs of the first and second detectors  28 ,  30 . This Signal Processing Section  34  comprises a subtraction device  44 , a comparator # 1   50 , a comparator # 2   42 , and a digital logic block  58 . Comparator # 2   42  is connected at its first input  40  to the output of the first detector  28 . The second input  38  of comparator # 2   42  is connected to a predetermined threshold level  36  which will be described below. The subtraction device  44  is connected to subtract the output signals of the first and second detectors  28 ,  30 . The output of the subtraction device  44  is connected to a first input  46  of comparator # 1   50 . Comparator # 1   50  is connected at its second input  48  to a predetermined composite threshold level  52 , which will be described below. The outputs of comparator # 1   50  and comparator # 2   42  are connected to a digital logic block  58 , which has an output  60  connected to the microprocessor  32 . The digital logic block  58  may be implemented with any device known in the art for providing an output which depends upon logic levels present at the device&#39;s inputs, including but not limited to TTL, or CMOS circuits. 
     The first multiplexer  16  is controlled by a predetermined sequence of signals received from the microprocessor  32  to sequentially step through a number of channels over time, selecting one channel M over a selection time period. Each channel corresponds to a filter  20  connected to one of the first multiplexer&#39;s  16  respective channel outputs  18 . A graphical representation of the channel and associated filter selection over time is shown in FIG. 3. A diagram of the characteristics of a typical filter  20  is shown in FIG.  2 . 
     When an RF signal, which can be an unmodulated continuous wave signal or a pulse modulated wave signal (see FIG.  4  and FIG. 5, respectively), of unknown frequency is received by the RF signal input  12  of the detector system  10 , the signal is routed to two places. First, in response to the control signal received from the microprocessor  32 , the multiplexer  16  selects a channel over a time sufficient to capture the received RF signal, and routes the RF signal to the filter  20  connected to the output of selected channel M. The selected filter  20  then passes the portion of the RF signal that has a frequency which matches the band pass frequency of the selected filter  20  to the “narrowband” detector  28  via the second multiplexer  24 . Similar to the first multiplexer  16 , the second multiplexer  24  is controlled to select the same channel M as that selected by the first multiplexer  16  over the same time period, thus allowing it to receive and retransmit the filtered signal output from a currently selected filter  20 . 
     The second place the unfiltered RF signal is routed is to a “wideband” detector  30 . The application of the unfiltered RF signal to the “wideband” detector  30  occurs simultaneously with the application of the bandpassed RF signal to the “narrowband” detector  28 . When the “wideband” detector  30  and the “narrowband” detector  28  detect the presence of a signal at their respective inputs, each emits an output signal Vwb and Vnb, respectively. The detectors  28 ,  30  have characteristics such that their respective output signals Vwb and Vnb are linearly proportional to the power of their respective input signals. When a continuous wave RF signal is received by the channelized receiver  10  and input to the RF signal input  12 , the amplitude of signal Vwb is continuous over time since Vwb represents the detected unfiltered RF signal. The amplitude of signal Vnb, however, changes per each time period due to the sequential application of the continuous wave input signal to each filtering means over that same time period. When a pulsed wave RF signal is received by the channelized receiver  10  and input to the RF signal input  12 , the peak amplitudes of each pulse signal Vwb are continuous amplitude over time since Vwb represents the detected unfiltered RF signal. The peak amplitudes of each pulsed signal Vnb, however, changes per each time period due to the sequential application of the pulse input signal to each filtering means over that same time period. 
     It should be noted that the “narrowband” detector  28  and the “wideband” detector  30  may both be implemented with the same type of detector (e.g., a diode detector), and that their designations as to narrow and wide bandwidth refer to their respective input signals. 
     Output signals Vwb and Vnb are then applied to the Signal 
     Processing Section  34 . Within the Signal Processing Section  34 , the signals are subtracted by the subtraction device  44 , such as a differential amplifier, and the resulting difference signal is then applied to comparator # 1   50  where it is compared with the composite threshold level  52 . The composite threshold level  52  is preferably composed of an offset level  54  and a calibration level  56 . The offset level  54  is adjusted so that comparator # 1   50  changes state when the amplitude of signal Vnb is substantially equal to that of signal Vwb. The calibration level  56  is adjusted to compensate for any RF gain or loss that may occur between the wideband and narrowband channels. 
     The output of comparator # 1   50  changes state when a relationship exists between the resulting difference signal and the composite threshold level  52  which is substantially equivalent to a relationship predefined by a characteristic of comparator # 1   50 . The predefined relationship is defined as the one between the signals (the composite threshold level magnitude  52  and the resulting difference signal magnitude) applied to the two inputs  46 ,  48  of comparator # 1   50 , which causes the output of comparator # 1  to change state. The predefined relationship is set such that the output of comparator # 1   50  changes state when signals Vnb and Vwb are substantially equal, and hence, the resulting difference signal magnitude substantially equals zero. 
     The output signal Vnb of the “narrowband” detector  28  is also applied to comparator # 2   42  where it is compared with the predetermined threshold level  36  to determine whether the narrowband signal has sufficient amplitude to warrant further processing. When signals with amplitudes less than the amplitude of the predetermined threshold level  36  are applied to comparator # 2   42 , no signal is generated by the detecting system  10  to the microprocessor  32  via digital logic block output  60 . Thus, the predetermined threshold level is set to a magnitude which will prevent the unwanted processing of signals with insufficient amplitudes, such as those derived from electrical noise. The output of comparator # 2   42  changes state when the narrowband signal exceeds the predetermined threshold level  36 . 
     The outputs of comparator # 1   50  and comparator # 2   42  are connected to a digital logic block  58  which provides an output signal to the microprocessor  32  when the digital logic block  58  receives appropriate signals from both comparator # 1   50  and comparator # 2   42 . Comparator # 1   50  and comparator # 2   42  must both indicate a response for there to be valid detection. The microprocessor  32  then correlates the digital logic block output signal with the selected filter bandwidth to determine the frequency band over which the RF signal has its largest amplitude. See FIG.  6  and FIG. 7, respectively, for diagrams of the detected signals Vnb and Vwb, and the digital logic block output responses for the continuous wave and pulse wave input signal applications shown in FIG.  4  and FIG. 5 respectively. 
     It should be particularly noted that this invention is capable of channelizinq received signals of durations shorter than 200 nanoseconds and bandwidths exceeding 100 megahertz. 
     While the invention has been particularly shown and described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention. By example, the duration or dwell time for the selection signal (FIG. 3) is adjusted to ensure that sufficient energy is captured from the received signal. When receiving pulsed signals, the dwell time should take into account the pulse repetition rate of the received signal. Also by example, it is within the scope of this invention to incorporate the digital logic block  58  into the microprocessor  32 , and to thus connect the outputs of comparators # 1  and # 2 , either directly or through level shifting circuitry, to input ports of the microprocessor.