Wide instantaneous dynamic range proportional signal processor

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.

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 variation 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 illuminator 
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 illuminated 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 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 
In accordance with the present invention, I have provided 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 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. 
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 amplifiers, 
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, will 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.

DETAILED DESCRIPTION 
Turning now to FIG. 1, I have there revealed a typical embodiment 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 
identified as A, B, C, & D, reading in a clockwise direction. As will be 
understood, my signal processor may be used as an intrinsic part of a 
seeker 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 embodiment 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 conjunction with a pair of 
channels related to the same sensing plane, such as with the channels 
concerned with the derivation of up-down commands. The channels shown in 
FIG. 1 provide a pitch guidance signal proportional to the vertical 
displacement of the defocused spot from the center of the detector, and in 
accordance with the teachings of the present invention, pitch error is 
equal to Log (A+B) minus 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 be seen in FIG. 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 preamplifier 
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 
attentuators in a manner described hereinafter affording an additional 90 
db of gain control. 
FIG. 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. Similarly, 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 characteristics 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 construction, such as are obtainable from Texas Instruments and 
others. Thus, the use herein of amplifier means including log amps makes 
possible the amplification of a wide range of signal levels. 
The output from log amps 30 and 32 is respectively 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 consecutive pulses. The sample and hold circuits are preferably 
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 commands are developed by now taking the difference of the two 
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 from the difference 
amplifier is delivered to a limiter 42 in order to provide a steering 
command at output 44 of constant amplitude beyond the 2/3 radius point. 
FIG. 2 reveals the clamping of the signal at an appropriate location to 
provide a constant amplitude steering command 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 AGC voltage. A 
hold off bias is applied in the OR gate so that the average signal level 
rises 30 db above threshold (1/2 of the logarithmic range of the 
amplifiers prior to the development of any AGC voltage). After the AGC 
threshold is reached, the AGC output voltage is fed from 48 back to the 
four preamplifiers 18, 20, 22 and 24 as shown in FIG. 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 network, as previously mentioned. Typically, if a 60 db 
instantaneous dynamic range is required, a 30 db threshold would be 
employed in the AGC. No AGC would be developed through the OR gate 46 
until the stronger of the two channels exceeds 30 db above threshold. The 
AGC would 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 could 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 lobe to side lobe energy of the target illuminator. With present 
day state of the art laser illuminators, an instantaneous dynamic range of 
greater than .+-.30 db might well result in the processor tracking false 
targets created by side lobe energy. An AGC 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 seekers. However, a total 
log amplifier range of 120 db is possible with the present state of the 
art, and if used, would eliminate the need for the AGC arrangement. 
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 respective 
preamplifiers 18 through 24. The signals from the A and B quadrants are 
summed 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 to one inter pulse period in the sample 
and hold circuit 34. Similarly, the signals from the C and D quadrants are 
amplified to the preamps 22 and 24, combined in the summing amplifier 28, 
and applied to the log amplifier 32. The output of the log amplifier 
representing 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 
produces 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 holds 34 and 36 will increase and when it reaches a point 
30 db above threshold, the biased diodes in the OR gate 46 will couple a 
signal to the AGC 48, which will be fed back to the preamps 18 through 24 
in such a manner as to 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 constant 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 been 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. Each pulse 
which exceeds the threshold is processed and stored in the sample and hold 
circuits. A succeeding pulse discharges the steering information stored in 
the sample and hold from the previous pulse, and therefore produces a 
steering command 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 
command voltage which is proportional to the angle between the optical 
axis and target 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 channels, which 
produces a steering command voltage whose slope is independent of signal 
level. Significantly, as taught herein, the number of parts required in 
order to obtain normalization is appreciably reduced from the number 
required in the usual normalization procedures, which necessitated taking 
A + B, subtracting C + D, and then dividing by the sum of A + B + C + D. 
My signal processing technique is capable of use in other applications, 
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 signal processing 
technique is shown in FIG. 2. This steering command signal which is linear 
over a region of .+-.2.degree. about the boresight axis was produced by a 
system using a 3.degree. radius defocused spot. The defocused spot size 
may be varied to obtain the desired linear region. The solid curve is the 
steering command 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.degree. about the boresight axis. FIG. 2 shows that 
the steering command voltage is linear with angle over a region which is 
approximately 2/3 of the defocused spot size, or .+-.2.degree.. The 
steering command is therefore restricted to the linear region by means of 
limiter 42 which limits the difference amplifier output 40 to +5 volts 
beyond the +2.degree. angle and to -5 volts beyond the -2.degree. angle, 
as shown by the dashed curve in FIG. 2. 
A further advantage resulting from the inherent configuration of my signal 
processing technique is that the steering command signal is normalized, 
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.degree., 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 amplifier 40) is a constant voltage which is 
independent of the absolute 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 
command signal is obtained with a significant simplification in circuit 
complexity over the conventional normalization technique. 
The primary advantage of my signal processing technique is the improved 
guidance accuracy resulting from a wide instantaneous dynamic range in a 
proportional tracking system. Large pulse to pulse signal variations occur 
in both RF and optical seeker systems because of scintillation and/or 
terrain masking as in the case of a moving target illuminated by a ground 
or airborne mounted laser. Variations also occur in the pulse to pulse 
output from present state-of-the-art lasers. Target signature measurements 
of tactical laser illuminated targets revealed variations approaching 
.+-.30 db. The limited dynamic range of present generation proportional 
laser seekers, 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 .+-.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 signal processing method is shown in 
FIG. 3. The output of the sample and hold outputs 34 and 36 arises from 
the threshold value of 1 volt to 5.5 volts (+30 db above 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 networks in the 
preamps 18 through 24. The output from the larger of the sample and hold 
outputs, represented by the horizontal line in FIG. 3, is held at 5.5 
volts for an additional increase of 90 db. The instantaneous pulse 
amplitude, represented by the sloped line in FIG. 3, may therefore vary by 
.+-. 30 db from the average value without 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 FIG. 3. This instantaneous operating line 
actually progresses from the +30 db above threshold point toward the 
higher signal levels as the seeker closes on the target. 
An instantaneous dynamic range of .+-.30 db and 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 admirably suited for use with 
laser seekers, in that it provides a significant increase in dynamic 
range, accomplished by 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 command 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 .DELTA. in the output voltage. 
Significantly, my device thus produces, independent of the absolute signal 
level represented by the defocused 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 near the threshold, or at 
the upper end of the dynamic range, which of course may be a value one 
million times greater. 
A preferred embodiment of my invention may involve a signal processor 
having a wide instantaneous dynamic range and usable in conjunction 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 amplifier are 
operatively disposed in each of the channels, which are arranged to 
receive the respective outputs of said detector means and function to 
amplify a wide range of signal levels. Difference amplifier means are 
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 two pairs are 
employed, the plane of one pair of channels is orthogonal to the plane of 
the other pair of channels, thus making it possible to generate steering 
commands usable for controlling the movement of a vehicle, such as a 
missile or the like. 
The signal processor may utilize automatic gain control means, 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.