CA1097754A - Wide instantaneous dynamic range proportional signal processor - Google Patents
Wide instantaneous dynamic range proportional signal processorInfo
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- CA1097754A CA1097754A CA268,113A CA268113A CA1097754A CA 1097754 A CA1097754 A CA 1097754A CA 268113 A CA268113 A CA 268113A CA 1097754 A CA1097754 A CA 1097754A
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Abstract
Abstract of the Disclosure A proportional processing technique having a wide instantaneous dynamic range and providing a significant increase in instantaneous dynamic range over presently known proportional processing methods. These improvements are made possible by the use in accordance with this invention of a logarithmic amplifier in each channel of a pair of channels relatable to the same sensing plane. Each logarithmic amplifier is preferably arranged to operate substantially at the midpoint of its operating characteristic, and the pair of channels may be orthogonally related to another pair of channels, such that suitable guidance commands can be derived and furnished for example to a missile. Each logarithmic amplifier produces signal outputs of nanosecond duration which are converted into signals of at least sufficient duration to provide sufficient time for comparison of the channel output signals thereby produced. The channel output signals are compared to produce a steering command output that has a polarity indicative of the channel having the higher output signal level, and has a slope representing volts versus target bearing that is independent of the input signal level thereby inherently accomplishing normalization. The steering command output is limited to a preselected voltage level below a non-linear region of the output.
Description
1~3~77S4 Background of the Invention It is important to realize that in the operation of a laser seeker, two types of signal level variations exist. One of these is the variations of the average input signal level with range, and the other is a change in instantaneous signal level due to scintillation, foreground objects, etc.
Laser Seeker signal processors conventionally employed for proportional tracking utilize linear signal amplification of a type which limits the total instantaneous dynamic range to approximately 20 db, or -10 db about the average pulse amplitude. However, the scintillation in the reflected laser energy from a target caused by missile and illumlnator aiming motion can cause pulse to pulse amplitude variations exceeding 20 db. The resulting saturation or dropping of pulses will reduce the data rate and degrade guidance accuracy. In addition, terrain masking can occur, which is responsible for creating false pulses and preventing a large percentage of the energy from reaching the target.
In several instances, during field tests of laser illumLnated tactical tArgets, a 25 db variation from pulse to pulse was observed due to scintillation and terrain masking. Under these conditions the 20 db instantaneous dynamic range of a conventional proportional processor will cause pulses to be lost with the resultant degradation in accuracy.
Further, with conventional processing equipment, it is possible to get a series of returns, such as from foreground bushes or other objects, which will have the effect in signal processors of limited dynamic range of causing the signal processor to track the false return. It will be seen that l~q~75~
if a false pulse arrives earlier than the true pulse and is of a higher amplitude, if the false pulse is greater than 1/2 of the instantaneous dynamic range than the true pulse, then the system may lock upon the false target. If this type of situation is to be avoided, the signal processor must have a wide instantaneous dynamic range.
Summary of the Invention According to the invention there is provided a method of processing two detector output signals related to a common sensing plane so as to produce a normalized output signal related to the two detector output signals, comprising applying the detector output signals to separate signal channels each containing a logarithmic amplifier in order to produce signals representing the logarithms of the detector output signals;
converting signal outputs of nanosecond duration from each logarithmic amplifier into signals of at least sufficient duration to provide sufficient time for comparison of the channel output signals thereby produced; comparing the channel output signals to produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the input signal level thus inherently accomplishing normalization; and limiting the steering command output to a preselected voltage level below a non-linear region of that output.
The invention also provides a proportional signal processor for processing two detector output signals related to a common sensing plane by the method of the invention, comprising ., 10"7754 detector means for sensing the bearing of a target; at least one pair of channels connected to the detector means to receive detector output signals related to a common sensing plane which output signals represent target position; a logarithmic amplifier in each signal channel each connected to receive one of the detector output signals and to produce a signal related to the logarithm of a wide range of levels of the received detector output signal; sample and hold circuit means in each signal channel for converting pulse type signal outputs of nanoseconds duration into signals of at least sufficient duration to provide sufficient time for comparison of the signals in the two channels; difference amplifier means connected to receive the outputs of said channels and operable to compare the outputs and produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the detector output signal level thus inherently accomplishing normalization;
and limiter means for limiting the output of said difference amplifier means to a preselected voltage level below a non-linear region of that output.
The invention thus provides a signal processor having an instantaneous dynamic range of - 30 db, which enables my system to distinguish true target returns from false ones.
Whereas previous proportional type signal processors were of limited capability, the present invention makes possible an instantaneous dynamic range of 60 db or greater in a proportional system, and accurate tracking information will be provided when - 3a -10~7754 pulse to pulse variations are as great as + 30 db from the average signal level. The instantaneous dynamic range of my invention is to a large extent determined by the dynamic range capability of a logarithmic amplifier utilized in the amplification arrangement of each channel of my device. However, even a logarithmic amplifier may not by itself have sufficient dynamic range to cope with signal variation due to scintillation when this is superimposed with variation due to range closure changes.
Accordingly, I use an AGC arrangement that enables the amplification arrangement to operate about the middle of its linear range, which makes it possible with a 60 db logarithmic amplifier to handle a pulse to pulse variation equivalent to the square root of 1,000, which is about 31.6 to 1.
- 3b -It is to be noted that normalization previously obtained only by the use of additional circuitry is inherently accomplished in accordance with the present invention by taking the quotient of the logarithm of the up and down channels, which produces a steering command voltage whose slope is independent of signal level.
As should now be apparent, the distinct advantage of extremely wide dynamic range is thus obtained by the utilization of the logarithmic amplifies, while at the same time, circuit complexity and cost are reduced inasmuch as normalization of the guidance signal is inherent in the signal processing utilized in accordance with this invention, thereby eliminating the discrete normalization circuits previously needed. A further cost reduction results from the fact that matched gains or tracking in the video amplifiers is required between only two channels rather than necessitating a matching between four channels as required in present proportional processors.
Thus, wide instantaneous dynamic range in accordance with this invention, in conjunction with last pulse logic, uill allow my signal processor to track the true target even though the true target is 30 db lower than an earlier false pulse.
It is therefore a primary object of the present invention to provide a signal processor having extremely wide dynamic range.
It is another object of this invention to provide a signal processor for use in conjunction with laser illuminators and the like for proportional tracking, which provides an instantaneous dynamic range of 60 db or greater.
It is yet another object of this invention to provide a signal processor having increased dynamic range obtained with reduced circuit complexity.
It is still another object of this invention to provide a signal processor of reduced complexity and cost, made possible because the normalization of the guidance signal is inherent in the signal processing technique utilized.
10~7 7~4 These and other objects, features and advantages will be more apparent from a study of the drawings in which:
Figure 1 is a somewhat simplified block diagram illustrating my novel signal processor in conjunction with a more or less conventional quad-rant detector;
Figure 2 is a graph which shows the steering command voltage produced by the present invention as a function of target motion about the optical system boresight axis; and Figure 3 is a graph which shows the relation between the received target energy level and the sample and hDld output level as controlled by the AGC system.
Detailed Descrip ion T~rning now to Figure 1, I have there revealed a typical e~bodi-ment of my signal processor 10, which is shown operatively associated with a detector 12. The detector is disposed in such a position that light from, for example, a laser illuminated target 14 is imaged thereon as a defocused spot by means of a suitable optical system, represented here by a lens 16.
The detector 12 may be a four quadrant PIN diode, utilizing quadrants iden-tified as A, B, C, & D, reading in a clockwise direction. As will be under-stood, my signal processor may be used as an intrinsic part of a cccker head of a missile, for example, but is obviously not to be so limited. As an example, the detector may relate to use with a non-optical arrangement, such as an RF direction finder in which the incoming signal is detected by four directional antennas. Thus, an emkodiment of my invention can use either a quadrant type detector as illustrated, or can use four related but separate detectors.
My signal processor will be explained in conjuction with a pair of channels related to the same sensing plane, such as with the channels con-cerned with the derivation of up-down commands. The channels shown in Figure 1 provide a pitch guidance signal proportional to the vertical displace-ment of the defocused spot frcm the center of the detector, and in accordance with the teachings of the present invention, pitch error is equal to Log (A+B) m mus Log (C+D). However, it is to be understood that the signal processor for the orthogonally related channel is essentially identical, except that of course the yaw error is equal to Log (A+D) minus Log (B+C). The processor for the left-right channels therefore does not need to be separately treated here.
It will De seen in Figure 1 that the output signals from quadrants A, B, C and D are delivered to respective preamplifiers 18, 20, 22 and 24, each of which has a bandwidth of 25 megacycles. Incorporated in each pre-amplifier is a diode attenuator network that makes gain control possible. The signal handling range of each preamplifier is 60 db, with the AGCing of the diode attenuators in a manner described hereinafter affording an additional 90 db of gain control.
Figure 1 further reveals that the outputs of preamplifiers 18 and 20 are summed in a linear summing amplifier 26, the bandwidth of which is 35 megacycles and the gain of which is unity. Simiarly, the outputs of preamplifiers 22 and 24 are summed in a linear summing amplifier 28, the characteristics of which are identical to those of summing amplifier 26.
The outputs from summing amplifiers 26 and 28 are respectively applied to logarithmic amplifiers 30 and 32, the gain charac eristics of which may for example be logarithmic over a 60 db dynamic range. The utilization of logarithmic amplifiers is of key importance to my signal processor in that the function they provide to the circuit to a large extent makes possible the wide dynamic range capability of my device, but the log amps per se are not a part of my invention, and may for example be of integrated circuit construc-tion, such as are obtainable from Texas Instruments and others. Thus, the use herein of amplifier means including log amps makes possible the ampli-fication of wide range of signal levels.
The output from log amps 30 and 32 is resepctively connected to sample and hold circuits 34 and 36, where the short duration pulses, such as 15 nanosecond pulses from a laser, may be stretched to hold a constant value between consecutives pulses. The sample and hold circuits are prefer-ably known devices of a two stretch type, that serve to stretch the pulses from the nanosecond to the millisecond region in accordance with conventional practice.
Steering ccmmands are developed by now taking the difference of the tWD sample and hold outputs, this being accomplished by connecting the sample and hold devices 34 and 36 to a difference amplifier 40, the output of which is linear over an angular region of the detector 12 corresponding to 2/3 of the radius of the defocused spot. The output ~rom the difference amplifier is delivered to a limiter 42 in order to provide a steering ccmman~
at output 44 of constant amplitude heyond the 2/3 radius point. Figure 2 reveals the clamping of the signal at an appropriate location to provide a constant amplitude steering ccmmand beyond the linear region.
The outputs of the sample and hold circuits 34 and 36 are also applied to an OR gate 46 and thence to AGC 48 in order to develop an ACG
voltage. A hold off bias is applied to the OR gate so that the average signal level rises 30 db above threshold (1/2 of the logarithmic range of the ampli-fiers prior to the development of an ACG voltage). After the AGC threshold is reached, the AGC output voltage is fed from 48 kack to the four preampli-fiers 18, 20, 22 and 24 as shown in Figure 1 in order to maintain the output of the sample and holds at a constant value. This AGC is effective over an additional 90 db of dynamic range.
The AGC arrangement serves to hold the average signal strength in the middle of the log amp dynamic range by suitably changing the gain of the preamps. This gain change can be accomplished for example by the use of a diode attenuator netw~rk, as previously mentioned. Typically, if a 60 db instantaneous dynamic range is required, a 30 db threshold w~uld be employed in the AGC. No AGC would be developed through the OR gate 46 until the stronger of the tWD channels exceeds 30 db above threshold~ The AGC w~uld then be applied to the linear amplifiers to maintain the average pulse amplitude at the midpoint of the 60 db log amp dynamic range. Pulse to pulse variations of - 30 db cound thus occur without affecting the accuracy of the proportional tracking signal.
The instantaneous dynamic range of my design is dictated by the ratio of the main loke to side loke energy of the target illuminator. With present day state of the art laser illuminators, an instantaneous dynamic 1~97~S4 range of greater than - 30 db might well result in the processor tracking fal æ target created by side lobe energy. An A~C system is therefore highly desirable in conjunction with the log amp dynamic range, to cover the 120 db total dynamic range required by most laser _eekers. However, a total log amplifier range of 120 db is possible with the present state of the art, and if u æd, would elimLnate the need for the AGC arragnement.
In operation, energy reflected from the target 14 is received through the optical system and imaged onto the four quadrant detector 12.
The signal from each quadrant is amplified in a linear manner by the respec-tive preamplifiers 18 through 24. The signals from the A and B quadrants are summd in the summing amplifier 26, applied to the log amplifier 30, and the pulse output from the log amplifier representing the logarithm of the (A + B) sum signal is then stretched for one int~r pulse period in the sample and hold circuit 34. Similarly, the signals from the C and D quadrants are amplified in the preamps 22 and 24, c~mbined in the summing amplifier 28, and applied to the log amplifier 32. The output of the log amplifier repre ænting the logarithm of the (C + D) sum signal is then stretched in the sample and hold circuit 36 for one inter pulse period.
The difference in the sample and hold outputs is then taken in the difference amplifier 40, coupled through the limiter 42, which then pro-duces at 44 the steering command for the up/down channel. As the missile closes on the target, the average signal level at the output of the sample and h~lds 34 and 36 will increase and when it reaches a point 30 db akove thres-hold, the biased diodes in the O~ gate 46 will couple a signal to the AGC 48, which will ~e fed back to the preamps 18 thorugh 24 in such a manner as to ~. ~
10~7754 maintain the larger of the sample and hold outputs at a constant amplitude.
Most importantly, therefore, the output of the sample and holds is held con-stant at the midpoint of the logarithmic range of the log amps, despite range closure.
The signal may therefore vary on a pulse-to-pulse basis by a factor of + 30 db from the average value once the AGC threshold has keen reached, without loss of signals. This enables, through utilization of last pulse logic, my seeker to develop accurate guidance information, even in the presence of scintillation that produces large pulse-to-pulse signal variations, or even in the presence of terrain masking, which produces false signal returns which may be as much as 30 db greater than the true target return.
Last pulse logic develops the steering information from the last signal energy which exceeds the threshold sensitivity of the system. ~ach pulse which exceeds the threshold is processed and stored in the sample and hold circuits. A succeeding pulse discharges the steering informations~ored in the sample and hold from the previous pulse, and therefore produces a steering ccmmand from the last pulse only.
Inherent also in the processing technique in accordance with this invention is the implementation of the steering commands so that the steering ocmmand voltage which is proportional to the angle between the optical axis and t~rget bearing remains constant over a range of signals of - 30 db about the average value.
The normalization technique inherent in this implementation is accomplished by taking the quotient of the logarithm of the up and down chan-:lOg7754 nels, which produces a steering command voltage slope is independentof signal level. Significantly, as taught herein, the numker of parts required in order to obtain normalization is appreciably reduced from the number re-quired in the usual normalization procedures, which necessitated taking A + B, subtracting C + D, and then dividing the sum of A + B + C + D.
My signal processing technique is capable of use in other applica-tions, such as in a monopulse R. F. direction finder. In an RF direction finder each of the quadrants of the detector would be replaced by an antenna and RF detector. The processing procedure would be very close to that shown herein.
The proportional steering command which is produced by my sig-nal processing technique is shown in Figure 2. This steering command signal which is linear over a region of - 2 about the boresight axis was pro-duced by a system using a 3 radius defocused spot. The defocused sFot size may be varied to obtain the desired linear region. The solid curve is the steering ccmmand produced at the output of the difference amplifier 40 as the target is positioned so as to move the defocused spot on the detector over a region of - 3 about the boresight axis. Figure 2 shows that the steering command voltage is linear with angle over a region which is approximately
Laser Seeker signal processors conventionally employed for proportional tracking utilize linear signal amplification of a type which limits the total instantaneous dynamic range to approximately 20 db, or -10 db about the average pulse amplitude. However, the scintillation in the reflected laser energy from a target caused by missile and illumlnator aiming motion can cause pulse to pulse amplitude variations exceeding 20 db. The resulting saturation or dropping of pulses will reduce the data rate and degrade guidance accuracy. In addition, terrain masking can occur, which is responsible for creating false pulses and preventing a large percentage of the energy from reaching the target.
In several instances, during field tests of laser illumLnated tactical tArgets, a 25 db variation from pulse to pulse was observed due to scintillation and terrain masking. Under these conditions the 20 db instantaneous dynamic range of a conventional proportional processor will cause pulses to be lost with the resultant degradation in accuracy.
Further, with conventional processing equipment, it is possible to get a series of returns, such as from foreground bushes or other objects, which will have the effect in signal processors of limited dynamic range of causing the signal processor to track the false return. It will be seen that l~q~75~
if a false pulse arrives earlier than the true pulse and is of a higher amplitude, if the false pulse is greater than 1/2 of the instantaneous dynamic range than the true pulse, then the system may lock upon the false target. If this type of situation is to be avoided, the signal processor must have a wide instantaneous dynamic range.
Summary of the Invention According to the invention there is provided a method of processing two detector output signals related to a common sensing plane so as to produce a normalized output signal related to the two detector output signals, comprising applying the detector output signals to separate signal channels each containing a logarithmic amplifier in order to produce signals representing the logarithms of the detector output signals;
converting signal outputs of nanosecond duration from each logarithmic amplifier into signals of at least sufficient duration to provide sufficient time for comparison of the channel output signals thereby produced; comparing the channel output signals to produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the input signal level thus inherently accomplishing normalization; and limiting the steering command output to a preselected voltage level below a non-linear region of that output.
The invention also provides a proportional signal processor for processing two detector output signals related to a common sensing plane by the method of the invention, comprising ., 10"7754 detector means for sensing the bearing of a target; at least one pair of channels connected to the detector means to receive detector output signals related to a common sensing plane which output signals represent target position; a logarithmic amplifier in each signal channel each connected to receive one of the detector output signals and to produce a signal related to the logarithm of a wide range of levels of the received detector output signal; sample and hold circuit means in each signal channel for converting pulse type signal outputs of nanoseconds duration into signals of at least sufficient duration to provide sufficient time for comparison of the signals in the two channels; difference amplifier means connected to receive the outputs of said channels and operable to compare the outputs and produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the detector output signal level thus inherently accomplishing normalization;
and limiter means for limiting the output of said difference amplifier means to a preselected voltage level below a non-linear region of that output.
The invention thus provides a signal processor having an instantaneous dynamic range of - 30 db, which enables my system to distinguish true target returns from false ones.
Whereas previous proportional type signal processors were of limited capability, the present invention makes possible an instantaneous dynamic range of 60 db or greater in a proportional system, and accurate tracking information will be provided when - 3a -10~7754 pulse to pulse variations are as great as + 30 db from the average signal level. The instantaneous dynamic range of my invention is to a large extent determined by the dynamic range capability of a logarithmic amplifier utilized in the amplification arrangement of each channel of my device. However, even a logarithmic amplifier may not by itself have sufficient dynamic range to cope with signal variation due to scintillation when this is superimposed with variation due to range closure changes.
Accordingly, I use an AGC arrangement that enables the amplification arrangement to operate about the middle of its linear range, which makes it possible with a 60 db logarithmic amplifier to handle a pulse to pulse variation equivalent to the square root of 1,000, which is about 31.6 to 1.
- 3b -It is to be noted that normalization previously obtained only by the use of additional circuitry is inherently accomplished in accordance with the present invention by taking the quotient of the logarithm of the up and down channels, which produces a steering command voltage whose slope is independent of signal level.
As should now be apparent, the distinct advantage of extremely wide dynamic range is thus obtained by the utilization of the logarithmic amplifies, while at the same time, circuit complexity and cost are reduced inasmuch as normalization of the guidance signal is inherent in the signal processing utilized in accordance with this invention, thereby eliminating the discrete normalization circuits previously needed. A further cost reduction results from the fact that matched gains or tracking in the video amplifiers is required between only two channels rather than necessitating a matching between four channels as required in present proportional processors.
Thus, wide instantaneous dynamic range in accordance with this invention, in conjunction with last pulse logic, uill allow my signal processor to track the true target even though the true target is 30 db lower than an earlier false pulse.
It is therefore a primary object of the present invention to provide a signal processor having extremely wide dynamic range.
It is another object of this invention to provide a signal processor for use in conjunction with laser illuminators and the like for proportional tracking, which provides an instantaneous dynamic range of 60 db or greater.
It is yet another object of this invention to provide a signal processor having increased dynamic range obtained with reduced circuit complexity.
It is still another object of this invention to provide a signal processor of reduced complexity and cost, made possible because the normalization of the guidance signal is inherent in the signal processing technique utilized.
10~7 7~4 These and other objects, features and advantages will be more apparent from a study of the drawings in which:
Figure 1 is a somewhat simplified block diagram illustrating my novel signal processor in conjunction with a more or less conventional quad-rant detector;
Figure 2 is a graph which shows the steering command voltage produced by the present invention as a function of target motion about the optical system boresight axis; and Figure 3 is a graph which shows the relation between the received target energy level and the sample and hDld output level as controlled by the AGC system.
Detailed Descrip ion T~rning now to Figure 1, I have there revealed a typical e~bodi-ment of my signal processor 10, which is shown operatively associated with a detector 12. The detector is disposed in such a position that light from, for example, a laser illuminated target 14 is imaged thereon as a defocused spot by means of a suitable optical system, represented here by a lens 16.
The detector 12 may be a four quadrant PIN diode, utilizing quadrants iden-tified as A, B, C, & D, reading in a clockwise direction. As will be under-stood, my signal processor may be used as an intrinsic part of a cccker head of a missile, for example, but is obviously not to be so limited. As an example, the detector may relate to use with a non-optical arrangement, such as an RF direction finder in which the incoming signal is detected by four directional antennas. Thus, an emkodiment of my invention can use either a quadrant type detector as illustrated, or can use four related but separate detectors.
My signal processor will be explained in conjuction with a pair of channels related to the same sensing plane, such as with the channels con-cerned with the derivation of up-down commands. The channels shown in Figure 1 provide a pitch guidance signal proportional to the vertical displace-ment of the defocused spot frcm the center of the detector, and in accordance with the teachings of the present invention, pitch error is equal to Log (A+B) m mus Log (C+D). However, it is to be understood that the signal processor for the orthogonally related channel is essentially identical, except that of course the yaw error is equal to Log (A+D) minus Log (B+C). The processor for the left-right channels therefore does not need to be separately treated here.
It will De seen in Figure 1 that the output signals from quadrants A, B, C and D are delivered to respective preamplifiers 18, 20, 22 and 24, each of which has a bandwidth of 25 megacycles. Incorporated in each pre-amplifier is a diode attenuator network that makes gain control possible. The signal handling range of each preamplifier is 60 db, with the AGCing of the diode attenuators in a manner described hereinafter affording an additional 90 db of gain control.
Figure 1 further reveals that the outputs of preamplifiers 18 and 20 are summed in a linear summing amplifier 26, the bandwidth of which is 35 megacycles and the gain of which is unity. Simiarly, the outputs of preamplifiers 22 and 24 are summed in a linear summing amplifier 28, the characteristics of which are identical to those of summing amplifier 26.
The outputs from summing amplifiers 26 and 28 are respectively applied to logarithmic amplifiers 30 and 32, the gain charac eristics of which may for example be logarithmic over a 60 db dynamic range. The utilization of logarithmic amplifiers is of key importance to my signal processor in that the function they provide to the circuit to a large extent makes possible the wide dynamic range capability of my device, but the log amps per se are not a part of my invention, and may for example be of integrated circuit construc-tion, such as are obtainable from Texas Instruments and others. Thus, the use herein of amplifier means including log amps makes possible the ampli-fication of wide range of signal levels.
The output from log amps 30 and 32 is resepctively connected to sample and hold circuits 34 and 36, where the short duration pulses, such as 15 nanosecond pulses from a laser, may be stretched to hold a constant value between consecutives pulses. The sample and hold circuits are prefer-ably known devices of a two stretch type, that serve to stretch the pulses from the nanosecond to the millisecond region in accordance with conventional practice.
Steering ccmmands are developed by now taking the difference of the tWD sample and hold outputs, this being accomplished by connecting the sample and hold devices 34 and 36 to a difference amplifier 40, the output of which is linear over an angular region of the detector 12 corresponding to 2/3 of the radius of the defocused spot. The output ~rom the difference amplifier is delivered to a limiter 42 in order to provide a steering ccmman~
at output 44 of constant amplitude heyond the 2/3 radius point. Figure 2 reveals the clamping of the signal at an appropriate location to provide a constant amplitude steering ccmmand beyond the linear region.
The outputs of the sample and hold circuits 34 and 36 are also applied to an OR gate 46 and thence to AGC 48 in order to develop an ACG
voltage. A hold off bias is applied to the OR gate so that the average signal level rises 30 db above threshold (1/2 of the logarithmic range of the ampli-fiers prior to the development of an ACG voltage). After the AGC threshold is reached, the AGC output voltage is fed from 48 kack to the four preampli-fiers 18, 20, 22 and 24 as shown in Figure 1 in order to maintain the output of the sample and holds at a constant value. This AGC is effective over an additional 90 db of dynamic range.
The AGC arrangement serves to hold the average signal strength in the middle of the log amp dynamic range by suitably changing the gain of the preamps. This gain change can be accomplished for example by the use of a diode attenuator netw~rk, as previously mentioned. Typically, if a 60 db instantaneous dynamic range is required, a 30 db threshold w~uld be employed in the AGC. No AGC would be developed through the OR gate 46 until the stronger of the tWD channels exceeds 30 db above threshold~ The AGC w~uld then be applied to the linear amplifiers to maintain the average pulse amplitude at the midpoint of the 60 db log amp dynamic range. Pulse to pulse variations of - 30 db cound thus occur without affecting the accuracy of the proportional tracking signal.
The instantaneous dynamic range of my design is dictated by the ratio of the main loke to side loke energy of the target illuminator. With present day state of the art laser illuminators, an instantaneous dynamic 1~97~S4 range of greater than - 30 db might well result in the processor tracking fal æ target created by side lobe energy. An A~C system is therefore highly desirable in conjunction with the log amp dynamic range, to cover the 120 db total dynamic range required by most laser _eekers. However, a total log amplifier range of 120 db is possible with the present state of the art, and if u æd, would elimLnate the need for the AGC arragnement.
In operation, energy reflected from the target 14 is received through the optical system and imaged onto the four quadrant detector 12.
The signal from each quadrant is amplified in a linear manner by the respec-tive preamplifiers 18 through 24. The signals from the A and B quadrants are summd in the summing amplifier 26, applied to the log amplifier 30, and the pulse output from the log amplifier representing the logarithm of the (A + B) sum signal is then stretched for one int~r pulse period in the sample and hold circuit 34. Similarly, the signals from the C and D quadrants are amplified in the preamps 22 and 24, c~mbined in the summing amplifier 28, and applied to the log amplifier 32. The output of the log amplifier repre ænting the logarithm of the (C + D) sum signal is then stretched in the sample and hold circuit 36 for one inter pulse period.
The difference in the sample and hold outputs is then taken in the difference amplifier 40, coupled through the limiter 42, which then pro-duces at 44 the steering command for the up/down channel. As the missile closes on the target, the average signal level at the output of the sample and h~lds 34 and 36 will increase and when it reaches a point 30 db akove thres-hold, the biased diodes in the O~ gate 46 will couple a signal to the AGC 48, which will ~e fed back to the preamps 18 thorugh 24 in such a manner as to ~. ~
10~7754 maintain the larger of the sample and hold outputs at a constant amplitude.
Most importantly, therefore, the output of the sample and holds is held con-stant at the midpoint of the logarithmic range of the log amps, despite range closure.
The signal may therefore vary on a pulse-to-pulse basis by a factor of + 30 db from the average value once the AGC threshold has keen reached, without loss of signals. This enables, through utilization of last pulse logic, my seeker to develop accurate guidance information, even in the presence of scintillation that produces large pulse-to-pulse signal variations, or even in the presence of terrain masking, which produces false signal returns which may be as much as 30 db greater than the true target return.
Last pulse logic develops the steering information from the last signal energy which exceeds the threshold sensitivity of the system. ~ach pulse which exceeds the threshold is processed and stored in the sample and hold circuits. A succeeding pulse discharges the steering informations~ored in the sample and hold from the previous pulse, and therefore produces a steering ccmmand from the last pulse only.
Inherent also in the processing technique in accordance with this invention is the implementation of the steering commands so that the steering ocmmand voltage which is proportional to the angle between the optical axis and t~rget bearing remains constant over a range of signals of - 30 db about the average value.
The normalization technique inherent in this implementation is accomplished by taking the quotient of the logarithm of the up and down chan-:lOg7754 nels, which produces a steering command voltage slope is independentof signal level. Significantly, as taught herein, the numker of parts required in order to obtain normalization is appreciably reduced from the number re-quired in the usual normalization procedures, which necessitated taking A + B, subtracting C + D, and then dividing the sum of A + B + C + D.
My signal processing technique is capable of use in other applica-tions, such as in a monopulse R. F. direction finder. In an RF direction finder each of the quadrants of the detector would be replaced by an antenna and RF detector. The processing procedure would be very close to that shown herein.
The proportional steering command which is produced by my sig-nal processing technique is shown in Figure 2. This steering command signal which is linear over a region of - 2 about the boresight axis was pro-duced by a system using a 3 radius defocused spot. The defocused sFot size may be varied to obtain the desired linear region. The solid curve is the steering ccmmand produced at the output of the difference amplifier 40 as the target is positioned so as to move the defocused spot on the detector over a region of - 3 about the boresight axis. Figure 2 shows that the steering command voltage is linear with angle over a region which is approximately
2/3 of the defocused sFot size, or - 2. The steering ccmmand is therefore restricted to the linear region by means of limiter 42 which limits the differ-ence amplifier output 40 to + 5 volts beyond the +2 angle and to -5 volts bey~n~ the -2 angle, as shown by the dashed curve in Figure 2.
1~)977~;4 A further advantage resulting from the inherent configuration of ny signal processing technique is that the steering command signal is normal-ized, that is the slope of the steering command voltage vs. angle off axis is independent of the target signal strength over the full dynamic range of the logarithmic amplifier. If the target is moved off the boresight axis by a given amount, for example 1, the ratio of the target signal powers produced in the A + B and C + D channels is constant and is independent of target signal level. The difference in the logarithms of the two constant ratio pulses (the difference of log amp 30 and 32 outputs as measured by difference ampli-fier 40) is a constant voltage which is independent of the aboslute value of the signal pulses. The slope of the resulting steering command 44 produced at the output of limiter 42 is therefore independent of target signal level and a normalized steering conmand signal is obtained with a significant simplifica-tion in circuit complexity over the convention normalization technique.
The primary advantage of my signal processing technique is the improved quidance ac acy resulting from a wide instantaneous dynamic range in a proportional tracking system. Large pulse to pulse signal varia-tions oc in both RF and optical seeker systems because of scintillation and/or terrain masking as in the case ofa moving target illuminated by a ground or airborne mounted laser. Variations also oc in the pulse to pulse output from present state-of-the-art lasers. Target signature measure-ments of tactical laser illuminated targets revealed variations approaching - 30 db. The limited dynamic range of present generation proportional laser ~ccks, usually -+ 10 db, will result in reduced guidance accuracy through reduced data rate (individual pulses falling below the threshold level) or saturated pulses (individual pulses exceeding the linear range).
In order to utilize the 1 30 db dynamic range offered by my signal processing method, an AGC system must be employed which maintains the average signal amplitude at the midpoint of the instantaneous dynamic range.
The AGC characteristic for my slgnal processing method is shown in Figure 3.
The output of the sample and hold outputs 34 and 36 rises from the threshold value of 1 volt to 5.5 volts (+ 30 db ab~ve threshold) as the seeker closes on the target. The OR circuit 46 bias is exceeded at a sample and hold output voltage of 5.5 volts and the AGC 48 develops a gain control voltage which is fed back to the diode attenuator netwDrks in the preamps ]8 through 24. The output from the larger of the sample and hold outputs, represented by the horizontal line in Figure 3, is held at 5.5 volts for an additional increase of 90 db. The instantaneous pulse amplitude, represented by the sloped line in Figure 3, may therefore vary by - 30 db from the average value with~t loss of accuracy once the average signal strength has risen 30 db above threshold. For illustration the instantaneous load line of - 30 db is drawn at the + 90 db signal level above threshold point in Figure 3. This instan-taneous operating line actually progresses from the + 30 db ab~ve threshold point toward the higher signal levels as the seeker closes on the target.
An instantaneous dynamic range of - 30 db and a total dynamic range of 120 db are used for illustration only. The instantaneous dynamic range and total dynamic range may be varied to meet application requirements.
As should now be apparent, I have provided a highly advantageous logarithmic proportional signal processor admira~ly suited for use with laser see~ers, in that it provides a significant increase in dynanic range, accom-plished ~y circuitry whose cost and complexity are considerably reduced from the ordinary. Because the signal outputs are the logarithm of the signal inputs, the slope of the steering ccmmand is independent of input signal amplitude, and normalization is inherent. It should further be noted that for each factor of 10 increase in power, I obtain the same ~ in the output voltage.
Significantly, my device thus produces, independent of the abso-lute signal level represented by the def sed spot, a steering command whose slope (volts vs. degrees off axis) is constant for a given angular displacement of the defocused spot away from the midpoint of the detector, with this being true irrespective of whether the input is n OE the threshold, or at the upper end of the dynamic range, which of course may be a value one million times greater.
A preferred emkcdiment of my invention may involve a signal processor having a wide instantaneous dynamic range and usable in conjunc-tion with a pair of channels relatable to the same sensing plane, comprising detector means, and at least one pair of channels arranged to receive outputs from said detector means. Amplifier means including a logarithmic ampli-fier æ e operatively disposed in each of the channels, which are æ ranged to receive the respective outputs of said detector means and functions to amplify a wide range of signal levels. Difference ~l~lifier means æ e provided for producing a signal whose polarity is indicative of the channel having the higher output, such that appropriate commands can be generated.
Either one or two pairs of channels can be utilized, and if tw~
pairs are employed, the plane of one pair of channels is orthogonal to the plane of the other pair of channels, thus ma~ing it p~ssible to generate l~g7754 steering command usable for controlling the movement of a vehicle, such as a missile or the like.
The signal processor may utilize automatic gain control means, the latter means being operative for selectively changing the gain of said amplifier means such that the logarithmic amplifier of each channel can function substantially at the midpoint of its operating characteristic. Sample and hold means may also be utilized in each channel, for converting pulse type signal outputs into signals of longer duration, thus to provide sufficient time for the comparison by said difference amplifier means of the outputs of said channels. Limiter means may be provided for limiting the output of the difference amplifier means to a preselected voltage level.
It should by now be apparent that normalization is inherent in my invention, in that the difference amplifier provides a steering command having a slope representing voltage versus target bearing off boresight, that is independent of input signal level.
The principles, preferred embodiments and mode of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention.
1~)977~;4 A further advantage resulting from the inherent configuration of ny signal processing technique is that the steering command signal is normal-ized, that is the slope of the steering command voltage vs. angle off axis is independent of the target signal strength over the full dynamic range of the logarithmic amplifier. If the target is moved off the boresight axis by a given amount, for example 1, the ratio of the target signal powers produced in the A + B and C + D channels is constant and is independent of target signal level. The difference in the logarithms of the two constant ratio pulses (the difference of log amp 30 and 32 outputs as measured by difference ampli-fier 40) is a constant voltage which is independent of the aboslute value of the signal pulses. The slope of the resulting steering command 44 produced at the output of limiter 42 is therefore independent of target signal level and a normalized steering conmand signal is obtained with a significant simplifica-tion in circuit complexity over the convention normalization technique.
The primary advantage of my signal processing technique is the improved quidance ac acy resulting from a wide instantaneous dynamic range in a proportional tracking system. Large pulse to pulse signal varia-tions oc in both RF and optical seeker systems because of scintillation and/or terrain masking as in the case ofa moving target illuminated by a ground or airborne mounted laser. Variations also oc in the pulse to pulse output from present state-of-the-art lasers. Target signature measure-ments of tactical laser illuminated targets revealed variations approaching - 30 db. The limited dynamic range of present generation proportional laser ~ccks, usually -+ 10 db, will result in reduced guidance accuracy through reduced data rate (individual pulses falling below the threshold level) or saturated pulses (individual pulses exceeding the linear range).
In order to utilize the 1 30 db dynamic range offered by my signal processing method, an AGC system must be employed which maintains the average signal amplitude at the midpoint of the instantaneous dynamic range.
The AGC characteristic for my slgnal processing method is shown in Figure 3.
The output of the sample and hold outputs 34 and 36 rises from the threshold value of 1 volt to 5.5 volts (+ 30 db ab~ve threshold) as the seeker closes on the target. The OR circuit 46 bias is exceeded at a sample and hold output voltage of 5.5 volts and the AGC 48 develops a gain control voltage which is fed back to the diode attenuator netwDrks in the preamps ]8 through 24. The output from the larger of the sample and hold outputs, represented by the horizontal line in Figure 3, is held at 5.5 volts for an additional increase of 90 db. The instantaneous pulse amplitude, represented by the sloped line in Figure 3, may therefore vary by - 30 db from the average value with~t loss of accuracy once the average signal strength has risen 30 db above threshold. For illustration the instantaneous load line of - 30 db is drawn at the + 90 db signal level above threshold point in Figure 3. This instan-taneous operating line actually progresses from the + 30 db ab~ve threshold point toward the higher signal levels as the seeker closes on the target.
An instantaneous dynamic range of - 30 db and a total dynamic range of 120 db are used for illustration only. The instantaneous dynamic range and total dynamic range may be varied to meet application requirements.
As should now be apparent, I have provided a highly advantageous logarithmic proportional signal processor admira~ly suited for use with laser see~ers, in that it provides a significant increase in dynanic range, accom-plished ~y circuitry whose cost and complexity are considerably reduced from the ordinary. Because the signal outputs are the logarithm of the signal inputs, the slope of the steering ccmmand is independent of input signal amplitude, and normalization is inherent. It should further be noted that for each factor of 10 increase in power, I obtain the same ~ in the output voltage.
Significantly, my device thus produces, independent of the abso-lute signal level represented by the def sed spot, a steering command whose slope (volts vs. degrees off axis) is constant for a given angular displacement of the defocused spot away from the midpoint of the detector, with this being true irrespective of whether the input is n OE the threshold, or at the upper end of the dynamic range, which of course may be a value one million times greater.
A preferred emkcdiment of my invention may involve a signal processor having a wide instantaneous dynamic range and usable in conjunc-tion with a pair of channels relatable to the same sensing plane, comprising detector means, and at least one pair of channels arranged to receive outputs from said detector means. Amplifier means including a logarithmic ampli-fier æ e operatively disposed in each of the channels, which are æ ranged to receive the respective outputs of said detector means and functions to amplify a wide range of signal levels. Difference ~l~lifier means æ e provided for producing a signal whose polarity is indicative of the channel having the higher output, such that appropriate commands can be generated.
Either one or two pairs of channels can be utilized, and if tw~
pairs are employed, the plane of one pair of channels is orthogonal to the plane of the other pair of channels, thus ma~ing it p~ssible to generate l~g7754 steering command usable for controlling the movement of a vehicle, such as a missile or the like.
The signal processor may utilize automatic gain control means, the latter means being operative for selectively changing the gain of said amplifier means such that the logarithmic amplifier of each channel can function substantially at the midpoint of its operating characteristic. Sample and hold means may also be utilized in each channel, for converting pulse type signal outputs into signals of longer duration, thus to provide sufficient time for the comparison by said difference amplifier means of the outputs of said channels. Limiter means may be provided for limiting the output of the difference amplifier means to a preselected voltage level.
It should by now be apparent that normalization is inherent in my invention, in that the difference amplifier provides a steering command having a slope representing voltage versus target bearing off boresight, that is independent of input signal level.
The principles, preferred embodiments and mode of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention.
Claims (15)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of processing two detector output signals related to a common sensing plane so as to produce a normalized output signal related to the two detector output signals, comprising applying the detector output signals to separate signal channels each containing a logarithmic amplifier in order to produce signals representing the logarithms of the detector output signals; converting signal outputs of nano-second duration from each logarithmic amplifier into signals of at least sufficient duration to provide sufficient time for comparison of the channel output signals thereby produced;
comparing the channel output signals to produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the input signal level thus inherently accomplishing normalization; and limiting the steering command output to a preselected voltage level below a non-linear region of that output.
comparing the channel output signals to produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the input signal level thus inherently accomplishing normalization; and limiting the steering command output to a preselected voltage level below a non-linear region of that output.
2. The method according to Claim 1 wherein the detector output signals have a wide instantaneous dynamic range and each of the logarithmic amplifiers has a predetermined dynamic range and the levels of the detector output signals applied to the logarithmic amplifiers are controlled in response to the channel output signals in order to maintain the detector output signals applied to the logarithmic amplifiers substantially within the dynamic range of the logarithmic amplifiers.
3. The method according to Claim 1 or Claim 2, comprising sampling and storing the channel output signals and subtracting one of the stored output signals from the other to produce the steering command output.
4. The method according to Claim 1 or Claim 2, wherein the two detector output signals are pulse type signals which are converted into signals of longer duration, one of which signals of longer duration is subtracted from the other signal to produce the steering command output which is related in signal level and polarity to the difference in signal level between the two detector output signals.
5. The method according to Claim 1 wherein, in each channel the logarithmic amplifier has a predetermined instantaneous dynamic range extending upwardly from a lower threshold limit, and the channel output signal from the logarithmic amplifier is sampled and stored in a sample and hold circuit, and the sample and hold circuit is discharged of previous channel output signals and the output signal from a logarithmic amplifier is sampled and stored upon the occurrence of a succeeding logarithmic amplifier input signal that exceeds the lower threshold limit of the logarithmic amplifier so that detector output signals below the lower threshold limit are rejected and detector output signals above the lower threshold limit are retained until a yet later detector output signal exceeding the lower threshold limit is applied to the logarithmic amplifier.
6. The method according to Claim 5, comprising controlling the levels of the detector output signals applied to the logarithmic amplifiers in accordance with the level of the logarithmic amplifier output signals to maintain the levels of the detector output signals at a desired operating point in the instantaneous dynamic range of the logarithmic amplifiers upon the occurrence of logarithmic amplifier output signals exceeding a predetermined value.
7. A proportional signal processor for processing two detector output signals related to a common sensing plane, by the method according to Claim 1, comprising detector means for sensing the bearing of a target; at lease one pair of channels connected to the detector means to receive detector output signals related to a common sensing plane which output signals represent target position; a logarithmic amplifier in each signal channel each connected to receive one of the detector output signals and to produce a signal related to the logarithm of a wide range of levels of the received detector output signal; sample and hold circuit means in each signal channel for converting pulse type signal outputs of nanoseconds duration into signals of at least sufficient duration to provide sufficient time for comparison of the signals in the two channels; difference amplifier means connected to receive the outputs of said channels and operable to compare the outputs and produce a steering command output whose polarity is indicative of the channel having the higher output signal level and which has a slope representing volts versus target bearing which is independent of the detector output signal level thus inherently accomplishing normalization;
and limiter means for limiting the output of said difference amplifier means to a preselected voltage level below a non-linear region of that output.
and limiter means for limiting the output of said difference amplifier means to a preselected voltage level below a non-linear region of that output.
8. The signal processor according to Claim 7 wherein said detector means comprises a quadrant type optical target detector.
9. The signal processor according to Claim 7 wherein said detector means comprises a radio frequency antenna array target detector.
10. The signal processor according to Claim 7 wherein the sample and hold means in each signal channel is operable to sample and hold the output signal from the logarithmic amplifier in that channel.
11. The signal processor according to Claim 10 wherein the logarithmic amplifiers each have a predetermined instantaneous dynamic range extending upwardly from a lower threshold limit, and the sample and hold means is arranged to be discharged of previous logarithmic amplifier output signals upon the occurrence of a succeeding signal being applied to the logarithmic amplifier, which signal exceeds the lower threshold limit of the logarithmic amplifier.
12. The signal processor according to Claims 7, 8 or 9 wherein each of the logarithmic amplifiers has a predetermined instantaneous dynamic range, and means are provided for controlling the levels of the detector output signals applied to the logarithmic amplifiers in response to a logarithmic amplifier output signal exceeding a predetermined value in order to maintain the level of the detector output signals applied to the logarithmic amplifiers substantially within said dynamic range.
13. The signal processor according to Claims 7, 8 or 9 wherein each of the logarithmic amplifiers has a predetermined instantaneous dynamic range, and amplifying means is included in each signal channel for controlling the level of the detector output signals applied to the logarithmic amplifiers in response to the output signals form the logarithmic amplifiers.
14. The signal processor according to Claims 7, 8 or 9 wherein the signal processor is used for target tracking utilizing a laser illuminator which has main lobe energy and side lobe energy, said signal processor being intended to track a target illuminated by the main lobe energy and to reject apparent targets created by the side lobe energy, and automatic gain control means utilized to provide last significant pulse logic by maintaining the average target signal amplitude at a desired operating point of the instantaneous dynamic range of said logarithmic amplifier so that insignificant level signals created by side lobe energy of the target illuminator which may occur later in time than the true target return are rejected because they fall below the lower limit of the instantaneous dynamic range of said logarithmic amplifier.
15. The signal processor according to Claims 7, 8 or 9 wherein the signal processor is used for target tracking utilizing a laser illuminator which has main lobe energy and side lobe energy, said signal processor being intended to track a target illuminated by the main lobe energy and to reject apparent targets created by the side lobe energy, and automatic gain control means utilized to provide last significant pulse logic by maintaining the average target signal amplitude at a desired operating point of the instantaneous dynamic range of said logarithmic amplifier so that insignificant level signals created by side lobe energy of the target illuminator which may occur later in time than the true target return are rejected because they fall below the lower limit of the instantaneous dynamic range of said logarithmic amplifier, said desired operating point being substantially at the midpoint of the instantaneous dynamic range of said logarithmic amplifier.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA268,113A CA1097754A (en) | 1976-12-17 | 1976-12-17 | Wide instantaneous dynamic range proportional signal processor |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA268,113A CA1097754A (en) | 1976-12-17 | 1976-12-17 | Wide instantaneous dynamic range proportional signal processor |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1097754A true CA1097754A (en) | 1981-03-17 |
Family
ID=4107527
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA268,113A Expired CA1097754A (en) | 1976-12-17 | 1976-12-17 | Wide instantaneous dynamic range proportional signal processor |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1097754A (en) |
-
1976
- 1976-12-17 CA CA268,113A patent/CA1097754A/en not_active Expired
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