Abstract:
A background rejection circuit uses the harmonic content of an angularly narrow target to produce a signal. The signal is used to modulate an externally derived audio signal to produce a target indication signal free of background noise.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the field of electronics. More specifically, this invention pertains to the field of electro-optics. In greater particularity, the invention pertains to an electronic detector of infrared energies. By way of further characterization, by without limitation thereto, the invention shall be described as it pertains to any audio circuit for use in a missile guidance system. 
     2. Description of the Prior Art 
     It is known in the missile electronics arts to use a audio signal derived from a detected target to indicate target acquisition or various other missile readiness perimeters. Generally, these signals are derived from the missile signal obtained from the target itself. That is, incoming radiation from the target is modulated by a suitable reticle and pre-photocell optical system such that a characteristic modulation is impressed upon the signal for processing purposes. This signal is then amplified and used to indicate the presence of such a target. A difficult problem exists in a relatively narrow angular target in the presence of background radiation. 
     Prior attempts to solve this problem have utilized various bucking circuits or separate background and target modulators such that the target signal is indicated by characteristic frequencies or other characteristics imposed upon the target by its position within the field of view of the missile or its position on the reticle. 
     SUMMARY OF THE INVENTION 
     The present invention uses the harmonics generated by a signal of small angular dimensions to indicate the presence of a target in a background of other radiation. A bandpass filter filters this target energy limiting it to the harmonic content produced by such a small target interacting with the reticle as an initial target signal. This signal is detected and fed to a summing amplifier where it is mixed with a guidance control system audio signal to produce an output in the presence of a target signal. The output of the summing amplifier is connected to a spin frequency filter to develop a sinusoidal output of the desired audio frequency. This output is fed to a multiplier where it modulates a second audio signal to produce an enhanced audio output which is indicative of the acquisition of a target signal. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an electronic circuit to produce an audio signal upon the reception of a target having predetermined angular dimensions. 
     A further object of this invention is to provide an electronic circuit for the production of enhanced audio signal. 
     A still further object of this invention is to provide an electronic circuit for use in a missile guidance system to indicate the presence of a target signal. 
    
    
     These objects, as well as other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elementary schematic of the invention in an operational environment; 
     FIG. 2 is a block diagram of the background rejection circuit of FIG. 1; and 
     FIG. 3 is a schematic diagram of the circuit of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, radiation from an area containing the target is passed to the interior of a missile by means of an optical systems indicated at  6  as a simple convex positive lens. The pre-photocell optical system  7  focuses the radiation from the target area upon a reticle  8  positioned along the optical axis from pre-photocell optics  7 . Thus, lens  6  and pre-photocell optics  7 , together, comprise an acceptance optical system. By relative movement of pre-photocell optics  7  and reticle  8 , together with the motion of the missile, the image of a target appearing in the field of view of lens  6  is caused to be chopped or otherwide interrupted by the action of reticle  8 . This chopping action is conventional in the art and may be affected by movement of either pre-photocell optics  7  or reticle  8 . 
     The chopped optical signal is detected by a photocell symbolically indicated at  9 . The output of the photocell is connected to missile target detection and guidance electronics as is conventional in the art and also connected to a background rejection circuit  11 . 
     Because the target is of small angular dimensions its image is abruptly occulted by reticle  8 , whereas radiation from objects having a relatively large angular extent pass characteristic radiation patterns which are relatively unaffected by the fine line structure of reticle  8 . 
     As is well understood by those in the electronics arts, the abrupt turning off and on of the illumination from the small angular target as seen by photocell  9  produces an electrical output which is rich in harmonic content of the interruption frequency whereas the relatively more continuous illumination afforded by objects of large angular dimensions in the background produce little high frequency harmonics. The invention circuit utilizes the presence of these high frequency harmonics to indicate the presence and give some idea of the relative size of the target. 
     In order to aid operational personnel to position the target within the acceptance field of the acceptance optical system an audio tone is conventionally utilized to indicate the presence of such a target. Background rejection circuit  11  serves this purpose of accentuating the signal produced by the small angle target while leaving the background, or non-target, radiation relatively unaffected. 
     Referring to FIG. 2, a block diagram indicates a preferred fashion for accomplishing these desirable objectives. As shown, the alternating current from photocell  9  is connected to the input of a bandpass filter  12 . Bandpass filter  12  is a sharply tuned, narrow-band filter which passes only the high frequency harmonics produced by the small angle target. The output of filter  12  is connected to a demodulator circuit  13 . Demodulator circuit  13  produces an output which is coupled to a summing amplifier  15 . The output from demodulator  13 , although previously used to generate an audio signal to be used as a target acquisition signal, has a low amplitude because of the relatively small amount of power in the high frequency harmonics when the target is weak. Therefore, the amplification of this signal by itself produces little background rejection because of the noise always present with this type of device. This is because the harmonic content of a weak signal is low. 
     To overcome this problem, an internal audio signal from the guidance portion of the missile is also fed to amplifier  15  by means of a suitable demodulator  14 . The guidance system audio, abbreviated GCS audio, is conventionally present in many missile systems but, for the purpose of this application, need be considered only as a source of audio frequency electrical energy. In practice, the frequency is commonly derived from a commentator like signal associated with the relative movement of the pre-photocell optics  7  and reticle  8 . Of course, other sources of audio energy may be utilized. 
     For strong signals, the harmonic content is high and the detected signal provides the necessary enhancement, a modulating signal. 
     Amplifier  15  has a threshhold adjustment such that the GCS audio signal fed to it by demodulator  14  is insufficient, by itself, to produce an output. However, in the presence of a signal from demodulator  13 , an output is produced. Of course, such an output is present only when a target having the necessary high frequency harmonics is processed. 
     The output of summing amplifier  15  is further processed by a spin frequency filter  16  which is a synchronous filter utilizing the up and down guidance control signals from the guidance and control section of the missile to produce a relatively smooth sinusoidal wave having a frequency associated with the reticle-optical spin rate. This frequency is termed “spin frequency” and is a characteristic of most modern electro-optically guided missiles. The spin frequency output, which is readily recognized by operational personnel familiar with the missile electronics, is fed to a multiplier  17  which may be conceptionalized as a non-linear modulator along with an audio energy signal indicated as AGC audio to produce a modulated audio tone output which may be further amplified and processed by a suitable amplifier  18 . Thus, the system provides a circuit producing the familiar tone of the spin frequency and produces such a tone without the annoying presence of background generated tonial information such that it may be clearly recognized by operational personnel when at a low level and in high noise environment. 
     Because the enhanced target audio signal is unnecessary once guidance has been obtained, that is the missile has “locked-on” the target, the circuit of FIG. 2 has been constructed to receive a gyro cage sensing signal from the guidance and control system of the missile to cut off the passage of the enhanced audio signal when the gyro guidance signal is in the condition that it is tracking the target. This provides an operational convenience and an audible indication to operational personnel that “lock-on” has occured. 
     Referring to FIG. 3, a schematic representation of an actual implementation of the circuit of FIG. 2 is illustrated. As shown, active filter  12  comprises two operational amplifiers  121  and  122  with frequency limiting feedback circuity to provide for the requisite narrowpass-band. The output is capacitively coupled to an operational amplifier  123  for amplitude and impedance control. The output of amplifier  123  constitutes the output of filter  12  and is connected to a diode demodulator  13 . 
     The output of diode  13  is connected to summing amplifier  15  which includes an operational amplifier  151  together with a resistive signal integrator comprising resistors  152 ,  153 ,  154 , and  155 . The GCS audio signal is connected via diode  14  to resistor  152  which is in series with resistor  153  to constitute a voltage divider, the output of which is capacitively coupled to amplifier  151 . Similarly, resistor  154  is connected in series with resistor  153  and provides a similar voltage dividing function for the output of demodulator diode  13 . The gain of the amplifier is controlled by the appropriate selection of the values of the resistors including resistor  155  in the feedback control circuit of amplifier  151 . 
     The output of summation amplifier  15 , as previously explained, is connected to a spin frequency filter  16 . Spin frequency filter  16  is an active filter and includes operational amplifiers  165  and  166  which are connected in series. The output is fed to amplifier  165  by means of a conventional resistance coupling network and a plurality of solid state devices indicated at  161 ,  162 ,  163 , and  164 . These transistors serve as synchronizing inputs for the guidance signals such as the right-left reference signal connected to transistors  163  and  164  and the up-down reference signals connected to transistors  161  and  162 . As shown, the polarity construction of the transistor pairs associated with the right-left guidance signal and the up-down guidance signal are opposite to allow for each pair to process signals having positive and negative voltage polarities. Of course, if a single polarity were used for this purpose, a single transistor having the requisite construction could provide for this bias control. 
     Multiplier  17  which receives the output of spin frequency filter  16  includes an operational amplifier  171  which is connected as a pulse width modulator. The output of amplifier  171  is connected to an operational amplifier  172 , via a transistor  173 , to provide for a polarity inversion and necessary impedance matching. Amplifier  172  is connected as a conventional difference amplifier and receives an AGC audio signal on one terminal and the output of pulse width modulator  171  to produce an output in accordance with the average difference of the voltage on these two terminals in the well understood fashion. This signal then corresponds to a tone modulation of the AGC audio signal in the pattern determined by the output of pulse width modulator  171 . 
     The output from multiplier  17  is connected to an amplifier  18  which includes an operational amplifier  181  connected as a straight amplifier and a plurality of impedance matching and gain controlling transistors  182  and  183 . Transistors  182  and  183  serve to provide the necessary audio power and impedance matching characteristics for utilization by the system in the well understood fashion. 
     When “lock-on” is obtained, the output from circuit  11  is squelched by means of a bias control system. This control system selectively applies a bias to operational amplifier  166  and spin frequency filter  16  and to pulse width modulator  171  and multiplier  17 . This switching is performed by a plurality of transistors  22 - 23  in response to a cage sense signal connected to the gates thereof. 
     Transistor  21  is used to change the level at which the carrier amplifier (front end) AGC operates. The audio circuit requires a reduction or limiting of the AGC audio signal, which is the output of the front end. When the seeker is caged, the cage sense signal is low. This turns transistor  21  on and biases the AGC so as to limit AGC audio at a lower level than normal. This replaces, and is more accurate than, an earlier circuit which used a diode clipper circuit to limit AGC audio. When the seeker is uncaged, transistor  21  is turned off and the AGC operates normally. 
     When the seeker is uncaged, modulation of missile audio is undesirable, so the modulating signal is shorted out by transistor  22 . At the same time transistor  23  turns on. This allows the AGC control signal (a DC level) to control the volume of missile audio so a hotter target, which produces a higher level at AGC control, will produce a higher volume at the missile audio output. 
     The aforegoing circuit shows how the desired objects of the invention may be implemented in a conventional missile environment by the utilization of conventional components and circuity techniques. As will be recognized by those versed in the art, such as circuit may be miniaturized such that it occupies a minimal space in the compact guidance system of a modern aerial missile. Of course, using well understood engineering tradeoffs, circuit modifications may suggest themselves to those versed in the art. 
     The foregoing description taken together with the appended claims and drawings constitute a disclosure such as to enable a person skilled in the electronics and aerial missile arts and having the benefit of the teachings contained therein to make and use the invention. Further, the structure herein described meets the objects of invention and generally constitutes a meritorious advance in the art unobvious to such a worker not having the benefit of these teachings.