Abstract:
A circuit for compensating for frequency modulation on a radar signal is disclosed. The disclosed circuit includes an adaptive narrow band filter in the feedback path of a phase-locked detector, such filter having a bandwidth which is narrower than the frequency range of the signal being compensated, and means for detecting the amplitude of such signal. 
     The invention herein described was made in the course of, or under a contract or subcontract thereunder, with the Department of Defense.

Description:
BACKGROUND OF THE INVENTION 
   This application pertains generally to signal processing circuitry in a radar receiver and particularly to an adaptive narrow band filter capable of locking on and tracking a signal having a frequency range wider than the bandwidth of such filter. 
   It is known in the art that a continuous wave radar may be made capable of ranging on targets by appropriate modification of the waveform of the signal transmitted by such a radar. One known way of accomplishing such an end is periodically to frequency modulate a continuous wave carrier signal with a sinusoidal waveform so that a comparison between the instantaneous frequency of the sinusoidal waveform and the frequency of echo signals may be made to indicate the approximate range of each detected target. A radar with such coding of the transmitted signal will be referred to hereinafter as an FM-CW radar. 
   In the semiactive missile guidance system shown in the copending patent application entitled “Adaptive Semiactive Missile Guidance System and Elements Therefor”, Ser. No. 579,291, inventors Donald S. Banks, George R. Spencer and James Williamson, filed on May 20, 1975 and assigned to the same assignee as this invention, an FM-CW radar is used as a control radar to illuminate both a target and a guided missile in flight to allow the latter to process radar signals to derive guidance signals. In the processing of echo signals on the guided missile final demodulation is accomplished by applying the downconverted echo signal to a phase detector along with a reference signal from a voltage-controlled oscillator. The frequency of the voltage-controlled oscillator is controlled through a feedback circuit incorporating a filter (which has a relatively wide passband under some conditions and relatively narrow passband under other conditions) to null the output of the phase detector. When the filter has a relatively wide passband, i.e. wider than any frequency modulation present on the signal being demodulated, the frequency modulation on the down-converted echo signal is passed to the voltage-controlled oscillator with practically no change in amplitude or phase. The reference signal out of the voltage-controlled oscillator then is caused similarly to vary, ultimately then to null the output of the phase detector. A different situation obtains, however, when the filter has a relatively narrow passband, i.e. narrower than the frequency modulation on the signal to be demodulated. In that case the frequency modulation on the down-converted echo signal is blocked, more or less, or shifted in phase by the filter. Then, because the reference signal out of the voltage-controlled oscillator is not modulated with a replica frequency modulation on the downconverted echo signal, the output of the phase detector cannot be nulled. That is to say, the frequency modulation on the downconverted echo signal appears at the output of the phase detector as an error signal at the repetition frequency of the FM-CW radar. 
   An attempt was made in the referenced system to solve the problem being discussed. The approach taken was to detect the frequency modulation on the downconverted echo signal and then, after filtering, to apply the resultant (along with the signal out of the filter in the feedback circuit) to the voltage-controlled oscillator. While, in theory, the just-outlined approach should be satisfactory, it has not so proved to be. The difficulty experienced in practice derives from the fact that proper filtering of the detected frequency modulation is almost impossible to achieve under all conditions. That is, when the signal-to-noise ratio between the downconverted echo signal and noise accompanying such signal is low, there is no practical way of reducing the noisiness of the detected frequency modulation to an acceptable degree. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is a primary object of this invention to provide an improved circuit for eliminating the effect of frequency modulation of echo signals in an FM-CW radar. 
   Another object of this invention is to accomplish the fore-going when the signal-to-noise ratio between echo signals and noise in an FM-CW radar is low. 
   The foregoing and other objects of this invention are attained generally by detecting, in a detecting arrangement incorporating a phase-locked loop to provide adaptive narrow band filtering, the frequency modulation on the radar signal transmitted directed from an FM-CW radar on a mother aircraft to a guided missile in flight, then modifying the amplitude and phase of such detected frequency modulation to form a replica of the frequency modulation on the echo signal from a target (as such echo signal is received at the guided missile), and applying such replica to a voltage-controlled oscillator which furnishes a reference signal to a phase detector in a phase-locked loop for demodulating the downconverted echo signal, thereby to eliminate the effect of frequency modulation of such echo signal under almost all conditions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention reference is now made to the following description of the accompanying drawings wherein: 
       FIG. 1  is a generalized sketch showing a guided missile in flight toward a target, such missile and target being illuminated by an FM-CW radar; 
       FIG. 1A  is a block diagram of the electronics in the guided missile of  FIG. 1 , the diagram being intended to show how the major elements of the electronics are arranged to eliminate the effects of frequency modulation of the radar signal from the FM-CW radar; and 
       FIG. 2  is a block diagram showing how the frequency modulation on the radar signal transmitted directly from the control radar ( FIG. 1 ) may be demodulated and adjusting the phase and amplitude of the demodulated signal may be changed to produce a replica of the frequency modulation on the echo signal from the target. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a control radar  10  (here an FM-CW radar) carried on a mother aircraft  12  is shown to be illuminating a guided missile  14  and a target  16  (here an aircraft). Taking the phase of the frequency modulation being transmitted by the control radar  10  at any instant in time as a reference, it will be observed that the phase shift of the transmitted radar signal received at the guided missile  14  is a function of the propagation delay of such signal in traveling from the control radar  10  to the guided missile  14 ; similarly, the phase shift of the echo signal received at the guided missile  14  is a function of the sum of the propagation delay of the transmitted radar signal in traveling from the control radar  10  to the target  16  and the propagation delay of the echo signal in traveling from the target  16  to the guided missile  14 . It will also be observed that the amplitude of the transmitted radar signal received at the guided missile  14  is greater than the amplitude of the echo signal until such missile enters the terminal phase of its flight. With the foregoing in mind, it will be obvious that the missile electronics  18  must, if the frequency modulation on the transmitted radar signal is to be used, be adapted to shift phase and change amplitude to form a replica of the frequency modulation on the echo signal. 
   Referring now to  FIG. 1A , it should first be noted that the representation of the missile electronics  18  in the FIGURE has been limited to a showing of only the elements which are required for an understanding of this invention. Thus, the missile electronics  18  shown in  FIG. 1A  consist of a rear receiver  20 , a front receiver  22 , a local oscillator  24  (common to the front and rear receivers  22 ,  20 ), a computer  26  and an arrangement designated “amplitude and phase adjuster  28 ”. 
   The rear receiver comprises a rear antenna  31  for receiving the transmitted radar signal from the control radar  10  ( FIG. 1 ). Such receiver signal is downconverted by heterodyning in a rear first mixer  33  with a first local oscillator signal from the local oscillator  24 . The downconverted signal, i.e. a signal at a first intermediate frequency, is passed, through a rear first I.F. amplifier  35 , to a rear second mixer  37 . A signal having a frequency representative of the tuning error of the front receiver  22  is produced by an error signal generator  39 . Reference is here made to the referenced application for the constructional details of this generator. The again downconverted signal (now at a second intermediate frequency) is passed, via a rear second I.F. amplifier  41 , to a rear demodulator  43 . Again, the constructional details of the rear demodulator  43  are shown in the referenced application. Suffice it to say here that the output of the rear demodulator  43  is (with the front receiver  22  inoperative) a direct current signal having an amplitude and sense indicative of the tuning error of the local oscillator  24  plus an alternating component determined by the frequency modulation on the transmitted radar signal (as such signal is received by the rear antenna  31 ). The output of the rear demodulator  43  is applied as indicated to an L.O. driver  45  and to the computer  26 . A portion of the signal out of the rear demodulator  43  is passed to the computer  26 . As described in detail in the referenced application, computer  26  is responsive to the output of the rear demodulator  43  to produce, inter alia, programmed inputs to the L.O. driver  45  as required. A portion of the output of the L.O. driver  45  is passed to the local oscillator  24  (which, as described in detail in the referenced application comprises a voltage-controlled oscillator in circuit with an electronically tuned filter as the frequency determining element for such oscillator) to determine the first local oscillator frequency. It will be now appreciated that if, as is the case here, the bandwidth of the various elements just described is sufficiently wide to accommodate the frequency modulation on the output of the rear demodulator  43 , ultimately the frequency of the local oscillator  24  will be changed to null the alternating component in the output of the rear demodulator  43 . In other words, the output of the L.O. driver  45  will have a component corresponding to the frequency modulation on the transmitted radar signal as received by the rear receiver  20 . Further, especially at the beginning of the flight of the guided missile  14  ( FIG. 1 ), the signal-to-noise ratio is very high because of the proximity of the rear receiver  20  to the control radar  10  ( FIG. 1 ). 
   The portion of the output of the L.O. driver  45  which is passed to an amplitude and phase adjuster  28  (described in detail in connection with  FIG. 2 ) is converted therein to a replica of the phase modulation on the echo signal as received by the guided missile  14  ( FIG. 1 ) and applied, when required, to an oscillator driver  49  in the front receiver  22 . 
   The front receiver, as shown, consists of a front antenna  51 , a front mixer  53  and a front I.F. amplifier  55  to produce one input signal to a phase detector  57 . The second input to the latter then is derived from a reference oscillator  59  (here a voltage-controlled oscillator). The particular frequency of the reference oscillator  59  is determined by the output of the oscillator driver  49  which, in turn, is controlled, as shown, by the amplitude and phase adjuster  28 , a dual-band filter  61  and control signals from the computer  26 . 
   The characteristics of the dual-band filter  61  (referred to the frequency modulation on the echo signal) here are such that: (a) while the echo signal is being acquired, the bandwidth of the dual-band filter  61  is greater than the bandwidth required to accommodate the frequency modulation on the echo signal; and, (b) while the echo signal is being tracked, the bandwidth of the dual-band filter  61  is less than the bandwidth required to accommodate the frequency modulation on the echo signal. In the former situation, the alternating portion of the output of the phase detector  57  is passed through the dual-band filter  61  to the oscillator driver  49  without any substantial phase shift or attenuation. In the latter situation, however, significant phase shift and attenuation is suffered by the alternating portion of the output of the phase detector  57  in passing through the dual-band filter  61 . If the effect of noise were to be neglected, the amplitude and phase adjuster  28  would be required only when a target is being tracked. As noted hereinbefore, however, the effect of noise cannot be neglected when the signal-to-noise ratio at the input of the dual-band filter  61  is small, i.e. near unity. In practice, then, the amplitude and phase adjuster  28  is required except when the guided missile  14  ( FIG. 1 ) is in the terminal part of its flight. 
   Referring now to  FIG. 2 , it may be seen that the portion of the output of the L.O. driver  45  ( FIG. 1A ) which is applied to the amplitude and phase adjuster  28  is first passed through a bandpass filter  62 . That element has a passband to accept the frequency modulation on the transmitted radar signal from the control radar  10  ( FIG. 1 ), regardless of any nominal shift in the rate at which the transmitted radar signal is modulated during operation. As a result, then, the bandpass filter  62  is operative to increase the signal-to-noise ratio of the signal to be processed. The output of the bandpass filter  62  is connected, after passing through an amplifier  67 , to a limiter  66  and also to a phase detector  68 . The output of the limiter  66  (which output is indicative of the instantaneous phase of the frequency modulation on the transmitted, radar signal) is applied to a phase detector  70 . The second input to the phase detector  70  is derived from a voltage-controlled oscillator  72 . The frequency of the latter is adjusted, through a dual-band filter  74 , to null the output of the phase detector  70 . In operation the passband of the dual-band filter  74  is controlled by a filter selection circuit  76  (which may be a differential amplifier having a reference voltage, E R , and a signal indicative of the amplitude of the transmitted radar signal). The dual-band filter  74  may be of the type shown in the referenced application with the passbands typically 1 Hz and 10 Hz. 
   The output of the voltage-controlled oscillator  72  is also passed, through a 90° phase shifter  78 , to the phase detector  68 . The output of the latter then (which output is indicative of the deviation of the frequency modulation on the transmitted radar signal) is passed through a low pass filter  80  and an amplifier  82  to a multiplier  84 . The second input to the latter is taken from the voltage-controlled oscillator  72  through a phase shifter  86 . The output of the multiplier  84  then is a signal of the same frequency and phase as the residual frequency modulation whose amplitude varies with the deviation of the frequency modulation on the transmitted radar signal. 
   A multiplier  88  is responsive to the output of the multiplier  84  and a signal indicative of the range between the guided missile  14  and the target  16  ( FIG. 1 ). Such range indicative signal is formed by the computer  26  ( FIG. 1A ) by conventional processing of the outputs of inertial instruments (not shown). The output of the multiplier  88  is connected, through a switch  90  (actuated as shown by a switch actuator  92  when required), to the oscillator driver  49  ( FIG. 1A ). 
   Having described a preferred embodiment of this invention, it will now be apparent to one of skill in the art that changes may be made without departing from the concepts of this invention. It is felt, therefore, that this invention should not be restricted to its disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.