Adaptive pulse detector utilizing delayed input comparison

An adaptive signal level detector. The inventive detector includes a first circuit for receiving an input signal and providing a second signal identical to said input signal and delayed relative thereto. The amplitude of the delayed signal is compared to the maximum amplitude of the input signal. When the second signal is at a predetermined level relative to the input signal an output signal is provided. In a specific implementation, the invention (10) includes a first circuit (18) for detecting the signal level of the transmitted signal and providing a second signal in response thereto. The second signal is a logarithmic representation of the input signal The second signal is delayed (20) to provide a third signal. The third signal is subtracted (24) from the second signal to provide a fourth signal. When the fourth signal exceeds a predetermined threshold, an output signal is provided. The invention provides a threshold detection circuit that will detect the instant when the amplitude of the leading or trailing edge of a received pulse crosses a threshold which is a predetermined fraction of the peak pulse amplitude of the received signal independent of the amplitude thereof. The invention enables several different receivers at separate locations to detect the same point on a received pulse signal independently of variations in antenna gain, receiver gain or received signal strength. Because the invention provides measurement independent of pulse amplitude, highly accurate matching of receivers is no longer required. The inventive circuit will greatly enhance the accuracy of comparison of the time of arrival of an transmitted signal at separate receivers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to communication systems. More specifically, 
the present invention relates to receivers with signal processors for 
determining the location of a transmitter and the time of arrival of a 
pulse therefrom. 
While the present invention is described herein with reference to 
illustrative embodiments for particular applications, it should be 
understood that the invention is not limited thereto Those having ordinary 
skill in the art and access to the teachings provided herein will 
recognize additional modifications, applications, and embodiments within 
the scope thereof and additional fields in which the present invention 
would be of significant utility 
2. Description of the Related Art 
In many applications, there is a need to determine the time and angle of 
arrival of a transmitted pulse. For example, in military applications, the 
conventional technique for determining the ability of a gunner to hit a 
target with a munition is visual. That is, the target is simply visually 
examined to determine the number of hits or score. 
Unfortunately, the visual scoring method is costly due to the requirement 
that the target be retrieved and examined to provide a score. Visual 
scoring from a remote location is often complicated by numerous range 
and/or battlefield conditions including darkness, haze, smoke, dust, and 
etc. In addition, there is no real time feedback of a gunners score during 
the firing operation. Accordingly, the opportunity for real time 
correction is not provided with conventional scoring techniques 
U.S. patent application entitled RADIO FREQUENCY DEVICE FOR MARKING 
MUNITION IMT POINT, U.S. Pat. No. 5,280,751, by J. O. Muirhead and G. 
E. Held discloses a unique and advantageous automatic system for gunnery 
scoring The system includes a plurality of miniature radio transmitters 
which are mounted on one or more of the munitions. The transmitters are 
energized when the projectile impacts the target. The transmitted signal 
is detected by one or more receivers. Detection occurs when the received 
signal exceeds a fixed threshold. By accurately monitoring the time of 
arrival at a number of different locations, the angle of arrival may be 
determined at each receiver from which the impact point of the round can 
be determined by triangulation. 
Unfortunately, due to attenuation, the amplitude of the signal transmitted 
from the impact point varies inversely with the square of the distance 
from the impact point to the receiver. Thus, each receiver may receive the 
transmitted signal at a different level. With each receiver having a 
detector with the same fixed threshold, differences in signal levels 
translate to differences in the time of detection of the transmitted 
signal. Differences in the time of detection, contribute to error in the 
location computations Other sources of error include differences in 
antenna and receiver gain patterns. 
Thus, there is a need in the art for further improvements in gunnery 
scoring systems utilizing munitions equipped with radio transmitters. More 
specifically, there is a need in the art for a technique for minimizing 
computation error due to signal level variations at the receivers of 
gunnery scoring systems using munitions equipped with radio transmitters. 
SUMMARY OF THE INVENTION 
The need in the art is addressed by the present invention which provides an 
adaptive signal level detector. The inventive detector includes a first 
circuit for receiving an input signal and providing a second signal 
identical to said input signal and delayed relative thereto. The amplitude 
of the delayed signal is compared to the maximum amplitude of the input 
signal. When the second signal is at a predetermined level relative to the 
input signal an output signal is provided. 
In a specific implementation, the invention includes a first circuit for 
detecting the signal level of the transmitted signal and providing a 
second signal in response thereto. The second signal is a logarithmic 
representation of the input signal. The second signal is delayed to 
provide a third signal. The third signal is subtracted from the second 
signal to provide a fourth signal. When the fourth signal exceeds a 
predetermined threshold, an output signal is provided. 
The invention provides a threshold detection circuit that will detect the 
instant when the amplitude of the leading or trailing edge of a received 
pulse crosses a threshold which is a predetermined fraction of the peak 
pulse amplitude of the received signal independent of the amplitude 
thereof. The system is adaptive to pulses of different amplitudes by the 
use of logarithmic pulse amplification prior to subtraction of a delayed 
version of the pulse from the undelayed pulse. When the delayed pulse is 
delayed by an amount equal to or greater than the expected rise time of 
the pulses, the output is the instantaneous difference between the 
logarithms which is the logarithm of the instantaneous amplitude ratios of 
the delayed to the undelayed pulse. This may be achieved with the leading 
or the trailing edges or both. The log ratio video voltage is compared to 
a preset value (e.g., decibels). 
The invention enables several different receivers at separate locations to 
detect the same point on a received pulse signal independently of 
variations in antenna gain, receiver gain or received signal strength. 
Because the invention provides measurement independent of pulse amplitude, 
highly accurate matching of receivers is no longer required. The inventive 
circuit will greatly enhance the accuracy of comparison of the time of 
arrival of a transmitted signal at separate receivers.

DESCRIPTION OF THE INVENTION 
Illustrative embodiments and exemplary applications will now be described 
with reference to the accompanying drawings to disclose the advantageous 
teachings of the present invention. 
FIG. 1 is a schematic diagram of an illustrative implementation of a system 
incorporating the adaptive signal level detector of the present invention. 
The system 1 includes a first receiver 10 and an identical second receiver 
12. The first receiver 10 is shown in detail and includes an antenna 14 
and a conventional radio frequency (RF) detector 16. In the alternative, 
the RF detector 16 may be replaced with an intermediate frequency (IF) 
detector. The RF detector 16 receives a signal in the form of a pulse from 
a source such as an RF munition such as that disclosed in the 
above-mentioned U.S. patent application entitled RADIO FREQUENCY DEVICE 
FOR MARKING MUNITION IMT POINT, U.S. Pat. No. 5,280,751, by J. O. 
Muirhead and G. E. Held. 
The received signal is input to a logarithmic amplifier (log amp) 18. The 
log amp 18 converts the input RF signal to a voltage proportional to the 
logarithm of the instantaneous amplitude of the input signal. A typical 
transfer function for a log amp is 25 millivolts per decibel of input 
power over the dynamic range of the log amp. Dynamic ranges of log amps 
vary from a low end of -80 to -60 dBm (decibels relative to 1 milliwatt) 
to a high end of 0 dBm. The selection of log amp characteristics will 
depend on the application and need only be known to permit proper set up 
of the threshold voltage and S/N threshold reference voltages. 
The output of the logarithmic amplifier 18 is split in two paths, one with 
a delay element 20. In the illustrative embodiment, the delay element is 
an inductor. However, depending on the frequency of the received signal, a 
transmission line of a predetermined length may be used to delay the 
signal in the first path in a manner well known to those skilled in the 
art. 
The delay line must provide sufficient delay to ensure that the pulse under 
measurement (log video) has time to rise to peak amplitude before the 
delayed video has risen to within the desired detection threshold. The 
amount of delay required is dependent upon the rise time of the pulses 
expected. 
As the delay element will attenuate the signal in the first path, an 
attenuator 22 is included in the second path to ensure that the insertion 
losses in the two paths are equal. This assures that both the delayed and 
undelayed inputs to the subtractor 24 have the same transfer function in 
terms of volts per decibel. The attenuator may be a resistor or other 
suitable element. 
A subtractor 24 subtracts the delayed signal from the undelayed signal 
output by the logarithmic amplifier 18. The subtractor output is the 
voltage difference between the undelayed video signal and the delayed 
video signal and is referred to herein as the "difference video". The 
difference video is the logarithm of the instantaneous ratio of the 
delayed video to the undelayed video. It may be considered to be scaled in 
terms of volts per decibel. 
The-output of the subtractor 24 is thresholded by a first comparator; a 
threshold comparator 26. The threshold comparator compares the 
instantaneous log ratio to the input threshold setting (threshold 
voltage). When the difference video falls below the threshold voltage, the 
threshold comparator outputs a threshold detection mark if the output is 
enabled. The threshold voltage may be preset or adjustable. The value of 
the threshold voltage is dependent on the desired pulse power detection 
level relative to the peak pulse power and the volts per decibel of the 
difference video. 
The threshold comparator 26 is enabled by the output of a second 
comparator, a signal-to-noise (S/N) comparator 28. The S/N comparator 
prevents the circuit from triggering on noise in the absence of input 
pulses. The S/N comparator 28 compares the undelayed output of the 
logarithmic amplifier to a noise threshold to provide enable signals for 
the first comparator 26 and a conventional time measurement circuit 30. 
When an undelayed video pulse rises above the S/N threshold voltages the 
S/N comparator 28 will change output levels thereby enabling the threshold 
comparator. The output enable from the S/N comparator 28 may also be 
provided to external logic. The setting of the S/N threshold will 
determine the false alarm rate caused by noise. 
A time measurement circuit 30 computes the time of arrival of the pulse 
based on an internal reference clock. FIG. 2 is an illustrative 
implementation of the time measurement circuit 30. The illustrative time 
measurement circuit 30 includes an AND gate 34 which receives the output 
from the threshold comparator 26 and the enable output signal from the S/N 
comparator 28. The AND gate output enables a counter 36 which counts 
pulses from a clock 38 to provide the indication of the time of arrival of 
the pulse from a transmitter. 
A similar circuit (not shown) would compute the time of arrival of the 
signal as received by the second receiver 12 from a second antenna element 
11. 
The output of the time measurement circuit is provided to a processor 32 
which computes angle of arrival information based on the time measurement 
signal and the distance between the receivers. 
FIG. 3 is a graph of the equivalent signal-to-noise ratio of the undelayed 
and delayed video signal in the adaptive signal level detector of the 
present invention with an exemplary delay of 100 nanoseconds. The output 
enable and threshold detection mark signals are shown in relation to the 
time line of the graph. Note that as the energy density of the received RF 
signal rises, the log of the undelayed signal rises to a peak S/N ratio of 
18 dB. With a delay of 100 nanoseconds set by the delay element 20, the 
log of the delayed video signal begins to rise at the 100 nanosecond mark. 
Note that with a S/N threshold of 12 dB set by the S/N comparator 28, an 
output enable signal is provided at 100 nanoseconds. As the delayed video 
signal rises, the difference video signal begins to peak and then fall off 
as shown. When the difference video signal from the subtractor 24 falls 
below the threshold set by the threshold comparator 26, that is, 3 dB, the 
threshold detector outputs the threshold detector mark signal, shown as a 
high to low transition. It may now be appreciated that the threshold set 
by the threshold comparator 26 is the threshold for the log of the delayed 
video signal. That is, setting a 3 dB threshold, for example, at the 
threshold comparator 26 is tantamount to setting a point at which the 
delayed video signal is 3 dB down from its maximum value. More 
importantly, as the difference between the logarithm of two values is the 
log of the ratio of the two values, it can be seen that the present 
invention provides a system for determining when a delayed signal has 
reached a predetermined proportion of a maximum value. As the amount of 
the delay is known, the present invention allows for a determination of 
the time at which the undelayed signal reaches the predetermined 
proportion of its maximum value independent of the amplitude thereof. 
Thus, the present invention has been described herein with reference to a 
particular embodiment for a particular application. Those having ordinary 
skill in the art and access to the present teachings will recognize 
additional modifications applications and embodiments within the scope 
thereof. For example, the invention is not limited to the illustrative 
application. The invention may be used for numerous other applications as 
well. FIG. 4, for example, is a schematic diagram of a pulse width 
measuring system utilizing the teachings of the present invention. The 
circuit of FIG. 4 is identical to that of FIG. 2 with the exception that 
an additional threshold comparator 40' is added and the time measurement 
circuit is implemented differently as shown in FIG. 5. The second 
threshold comparator 40' detects the trailing edge and provides a reset 
signal to a set-reset flip-flop 44' of FIG. 5 through a second AND gate 
42'. The flip-flop 44' is set by the output of the first AND gate which is 
fed by the output of the first threshold comparator 26'. The counter 
number must be added to 2 times the delay line delay in the same units to 
obtain the total pulse width. 
It is therefore intended by the appended claims to cover any and all such 
applications, modifications and embodiments within the scope of the 
present invention. 
Accordingly,