Correlation detector for FM signals

Abstract: Apparatus for detecting FM signals, such as from a radio altimeter on an aircraft (12), contains a phase detector (24) which has as inputs the received signal (16C) and a delayed (22) replica of the input signal. The output of the phase detector (24) is filtered in a filter bank (26), the outputs of which feed a logic circuit (28). The logic circuit (28) selects the output with the largest signal level. If the selected output has a level exceeding a threshold (30), the presence of an FM signal is indicated.

Description: This invention relates to apparatus for indicating the presence of a source of a radio frequency signal having a time-varying frequency, the apparatus comprising an antenna coupled to supply a received radio frequency signal to an FM receiving circuit including filtering means for selecting a band of frequencies. FM signals are well suited for measuring the distance from a transmitter to an object from which the FM signal reflects. Signals in which the frequency increases or decreases linearly ("chirped signals") are particularly well suited for these functions. For this reason, FM signals are sometimes used in altimeters on aircraft. Also, FM signals are used in radars which are often on aircraft or other moving bodies. It is sometimes desirable to determine when an aircraft is in the vicinity. For example, in a battlefield scenario, it can be very important to detect all aircraft in the area. The FM signals emitted by altimeters or radars on aircraft can indicate the presence of the aircraft. However, a problem results when FM altimeter or FM radar signals are used to detect aircraft. These signals are generally very directional and are not always directed at the device trying to detect the aircraft. For example, an altimeter signal is transmitted directly at the ground below the aircraft. A device trying to detect the aircraft would receive only the side lobes of the FM signal. Thus, the received FM signal would be at a relatively low level. Detecting relatively low level signals, especially in noisy environments, can be difficult and requires a high sensitivity receiver. Existing systems use what is called a "walk-through receiver" to detect FM signals. The walk-through receiver contains a relatively narrow bandpass filter. As the FM signal changes in frequency, its frequency will, for some period of time, fall in the passband of the filter. The output of the filter will be a pulse having a non-zero value when the FM signal has a frequency in the passband of the filter. Ordinary pulse detection techniques are applied to the output of the filter to detect the pulses which signify the presence of an FM signal. The walk-through receiver has a drawback in that only a portion of the energy in the FM signal appears in the output pulse. In effect, the signal level is attenuated. This attenuation reduces the sensitivity of the receiver and makes detection of an FM signal less likely. With the foregoing background in mind, it is an object of this invention to provide a higher receiver sensitivity to detect FM signals. According to one aspect of the present invention, an apparatus of the kind defined hereinbefore at the beginning is characterised in that the FM receiving circuit includes means for coupling a received- radio frequency signal through a pair of signal channels to two inputs of a mixer with one of the said channels providing a delay relative to the other channel whereby the mixer mixes the received signal with a delayed replica of the received signal, and the mixer produces an output signal having a frequency component related to the frequency difference between the mixed signals when the received signal and the delayed replica thereof have different frequencies during a common time interval, and means, including the said filtering means, coupled to the output of the mixer for so detecting a plurality of frequency components of the output signal as to indicate the presence of the source of the radio frequency signal. According to another aspect of the invention, there is provided a method of indicating the presence of a source of a radio frequency signal having a time-varying frequency, the method comprising receiving the radio frequency signal at an antenna, coupling the received signal to an FM receiving circuit, and filtering to select a band of frequencies in the receiving circuit, characterised by the steps of dividing the received radio frequency signal into two parts and delaying one part relative to the other part, mixing the delayed part with the other part to produce an output signal having a frequency component related to the frequency difference between the delayed part and the said one part, and so detecting a plurality of frequency components in the output signal as to indicate the presence of the source of the radio frequency signal. A preferred embodiment of this invention takes the form of a receiver for detecting aircraft by identifying signals from FM altimeters. In this receiver the received signal is split into two channels. The first channel contains a delay element. Both channels are fed to a phase detector, the output of which is fed to a filter bank. The outputs of all the filters in the filter bank are fed to a logic circuit which selects the filter output with the largest signal level and passes that output to a threshold circuit. If the signal level exceeds a predetermined threshold, the threshold circuit signals the detection of an FM signal. Brief Description of the Drawings The invention may be better understood by reference to the following more detailed description and accompanying drawings in which FIG. 1 shows a block diagram of a receiver fabricated according to the invention; FIG. 2A shows a frequency versus time plot of the inputs to the phase detector of the circuit in FIG. 1; and FIG. 2B shows a frequency versus time plot of the output of the phase detector in FIG. 1. Description of the Preferred Embodiments FIG. 1 shows a block diagram of a receiver 10 designed to detect the presence of an aircraft 12. Aircraft 12 contains a radio altimeter (not shown) which operates by transmitting a signal 16A towards the ground 14. Signal 16A reflects from the ground and the altimeter determines the height of aircraft 12 above ground 14 by measuring the back and forth propagation delay of signal 16A. Most known radio altimeters use FM signals for signal 16A. In particular, chirped signals are used. Thus, receiver 10 detects the presence of aircraft 10 by detecting the presence of a chirped signal. As shown in FIG. 1, signal 16A is directed from aircraft 12 to ground 14. Signal 16A is not necessarily received by receiver 10. However, the altimeter (not shown) on aircraft 12 invariably transmits sidelobe signals such as signals 16b and 16c. At least a portion of sidelobe signals 16b and 16c will be received by receiver 10 when aircraft 12 is sufficiently close to receiver 10. Thus, receiver 10 will most likely detect the sidelobe signals 16b or 16c. Sidelobe signal 16c impinges on antenna 18 of receiver 10. Here, antenna 18 is some antenna of known construction. Preferably, antenna 18 has a beam pattern with broad angular coverage. The received signal is amplified by amplifier 20. Amplifier 20 is of known construction. The amplified signal is split into two paths. One path runs directly to input 21a of phase detector 24. The second path runs to input 21b of phase detector 24 through delay element 22. Delay element 22 is any substantially non-dispersive delay element of known construction. Delay element 22 delays the signal by an amount T d . The signals at inputs 21a and 21b are shown in FIG. 2A. FIG. 2A shows a received waveform 202. Waveform 202 sweeps from a frequency f min to f max in T seconds. These frequency sweeps are repeated at regular intervals. The type of signal received will dictate the numeric values of the parameters of signal 202. The operating characteristics of the circuit elements in FIG. 1 are selected accordingly. For example, if T had a value of 10ms and the difference between f max and f min were 100MHz, a value of 60ns would be an appropriate value of T d . In general T d is selected to be approximately as given by Eq. 1: T d = 1 2 1 (f max -f min ) Signal 202 is applied to input 21a of phase detector 24. Signal 204 is a replica of signal 202, but delayed by an amount T d . Signal 204 is fed to input 21b of phase detector 24. Phase detector 24 is a phase detector of known construction. Phase detector 24 may also be thought of as a mixer. The output of phase detector 24 is a signal with a frequency equal to the difference between the signals at inputs 21a and 21b. FIG. 2B shows the output of phase detector 24 for input signals 202 and 204. As seen in FIG. 2B, the major portion of the output of phase detector 24 consists of segments 250a, 250b, 250c... which are of constant frequency f DIF . The output of phase detector 24 is fed to filter bank 26. Filter bank 26 is here an FFT processor and an analog to digital converter of known construction. The filtering properties of FFT processors are well known. However, other techniques for fabricating filter bank 26 could be employed. Filter bank 26 has a plurality of outputs. Each output represents the cumulative strength of signals in a particular frequency band averaged over some time window (called the "integration time of the FFT"). The signal of FIG. 2B will produce an output in the band containing the frequency f DIF . The output signal from phase detector 24 will also produce signals on outputs of filter bank 26 corresponding to other frequency bands. However, these other signals will be at a lower average level because the signal of FIG. 2B has those frequencies for much shorter periods of time. The outputs of filter bank 26 are fed to logic circuit 28 which selects the output with the largest magnitude. With filter bank 26 implemented with an FFT processor, the outputs of filter bank 26 will be digital words. Thus, logic circuit 28 can be constructed using conventional digital logic circuit design techniques. The output of logic circuit 28 indicates the level of the largest output of filter bank 26. This level is provided to threshold circuit 30. If the level of the largest signal exceeds a predetermined threshold, threshold circuit 30 produces an output indicating a chirped signal has been detected. This output may be used by some utilization device (not shown) which may interpret the output as indicating the presence of an aircraft. Having described one embodiment of the invention, it will be apparent to one of skill in the art that various modifications can be made without departing from the invention. For example, the threshold used by threshold circuit 30 could be set adaptively. In other words, the threshold could be increased if many RF signals and noise are detected. Conversely, if the RF environment is relatively free of noise and other signals, the threshold can be lowered. As another modification, logic circuit 28 could be modified to sequentially pass to threshold circuit 30 several of the largest signals from filter bank 26. If several aircraft are present, several signals might exceed the threshold set by threshold circuit 30 and the OUTPUT could indicate the presence of several aircraft. The disclosed system could also be modified to provide more information about the detected chirped signal. As shown in FIG. 2B, signal 202 of FIG. 2A produces a signal out of phase detector 24 with mainly a frequency f DIF . A little thought will reveal that f DIF is the increase in frequency of signal 202 in time T d . The value of f DIF thus depends on how fast the frequency of signal 202 is changing (sometimes called the "chirp slope"). Certain radio altimeters or FM radios will use different chirp slopes and thus produce different values of f DIF . By determining in which frequency band the largest signal falls, the chirp slope of the signal can be determined. Knowing the chirp slope of the signal which produced an OUTPUT signal may allow some utilization device to identify the source of the chirped signal. Along the same lines, logic circuit 28 could be modified so that the largest signal was selected from one of a few selected frequency bands corresponding to chirp slopes of interest.

Claims: Apparatus for indicating the presence of a source of a radio frequency signal having a time-varying frequency, the apparatus comprising an antenna (18) coupled to supply a received radio frequency signal to an FM receiving circuit including filtering means (26) for selecting a band of frequencies, characterised in that the FM receiving circuit includes means (20,22,21a,21b) for coupling a received radio frequency signal through a pair of signal channels to two inputs of a mixer (24) with one of the said channels (20,22,21a) providing a delay relative to the other channel (20,21b) whereby the mixer (24) mixes the received signal with a delayed replica of the received signal, and the mixer (24) produces an output signal having a frequency component related to the frequency difference between the mixed signals when the received signal and the delayed replica thereof have different frequencies during a common time interval, and means (26,28,30), including the said filtering means (26), coupled to the output of the mixer (24) for so detecting a plurality of frequency components of the output signal as to indicate the presence of the source of the radio frequency signal. Apparatus according to claim 1, characterised in that the said one channel includes a delay element (22). Apparatus according to claim 2, characterised in that the means for detecting the frequency components comprises a filter bank (26) fed by the output of the mixer (24) and having a plurality of outputs each of which provides an indication of the level of a respective range of frequency components in the said output signal, logic circuitry (28) coupled to the outputs of the filter bank (26) and adapted to select that one of the levels indicated at the outputs of the filter bank (26) which is the highest present, and producing an output indicative of the selected level, and a threshold circuit (30) coupled to the logic circuitry (28) to produce an output indicative of the presence of the source of the radio frequency signal when the output from the logic circuitry represents a level above a predetermined threshold. Apparatus according to claim 3, characterised in that the filter bank (26) comprises an analog to digital converter arranged to convert the output signal of the mixer (24), and an FFT processor arranged to process the digital output from the converter. Apparatus according to claim 1, characterised in that the said one channel includes a delay element (22), the mixer comprises a phase detector (24) having one input coupled to the antenna and the other coupled to the delay element (22), the means for detecting the frequency components comprises a filter bank (26) having an input coupled to the phase detector (24) and a plurality of outputs, and logic means (28,30) for selecting the largest signal level amongst those output by the filter bank (26). Apparatus according to claim 5, characterised in that the logic means includes thresholding means (30) for producing an output indicative of a signal from the said source when the selected level exceeds a predetermined threshold. A method of indicating the presence of a source of a radio frequency signal having a time-varying frequency, the method comprising receiving the radio frequency signal at an antenna (18), coupling the receiving signal to an FM receiving circuit, and filtering to select a band of frequencies in the receiving circuit, characterised by the steps of dividing the received radio frequency signal into two parts and delaying (22) one part relative to the other part, mixing (24) the delayed part with the other part to produce an output signal having a frequency component related to the frequency difference between the delayed part and the said one part, and so detecting (26,28,30) a plurality of frequency components in the output signal as to indicate the presence of the source of the radio frequency signal. A method according to claim 7, characterised in that the step of detecting a plurality of frequency components comprises selecting (26,28) from a predetermined set of frequency bands the frequency band in which the output signal has frequency components with the greatest magnitude, and indicating (30) the presence of the source of the radio frequency signal when the magnitude of the frequency components in the selected frequency band exceeds a predetermined threshold.

Inventors: GORDON BRUCE E, LOWENSCHUSS OSCAR, GORDON, BRUCE E., LOWENSCHUSS, OSCAR
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1991
Main classification: G01S7
IPC codes: G01S13, G01S7