Pulse discriminator

Details Pulse discriminator Pulse discriminator

2002-11-04

2003-11-11

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S068000

Reexamination Certificate

active

06646479

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The invention relates to detecting pulses, such as with laser rangefinder receivers and, more particularly, to measuring thresholds of return pulses with a pulse discriminator (PD).
BACKGROUND OF THE INVENTION
In laser rangefinder systems, the distance to a target is determined by measuring the time interval between when a transmitted pulse is produced by a laser pulse source and when its reflection (return pulse) from a target is detected. Usually, a digital range counter is started at the time t
0
when the transmitted pulse is detected and stopped at the time t
r
when the returned pulse is detected. The pulses are normally detected by means of a photodetector, producing corresponding analog signals representative thereof. These analog signals are then processed electronically to generate command signals to start and stop the digital range counter. The resultant time interval measurement (t
r
-t
0
) is indicative of the distance between the laser pulse source and the target.
Due to the shape of the analog signal produced by a laser pulse impinging upon a photodetector (often Gaussian) and finite receiver bandwidth, the pulse's leading edge exhibits a risetime. If a fixed-threshold comparator were used to detect and separate the pulses from (lower amplitude) noise signals, the actual time at which the return pulse crossed the comparator threshold would be a function of the amplitude of the pulse signal. This would introduce considerable error and uncertainty into laser rangefinder distance measurement.
The effect of a fixed-threshold comparator on the accuracy of range measurement can be illustrated by considering its response to two different return pulses of differing amplitude: a strong return pulse and a weak return pulse. The strong return pulse signal would result in a threshold crossing at a relatively low point (early) along its leading edge, resulting in an relatively early counter stop command, thereby producing a time interval measurement indicative of a relatively shorter distance between the pulse source and the target (range to target). Conversely, a relatively low-amplitude return pulse signal that just barely crosses the comparator threshold would result in a threshold crossing very high (late) on its rising edge, producing a relatively later counter stop command, thereby producing a time interval measurement indicative of a relatively longer range to target than would be produced by a stronger return pulse occurring at the same time.
To overcome the amplitude sensitivity of a fixed-threshold comparator, a type of pulse discriminator (PD) known as a constant fraction discriminator (CFD), has been developed to help ensure that the time at which a pulse's threshold crossing is detected is substantially independent of pulse amplitude. By using the same CFD circuit to detect both the transmitted pulse and the return pulse (thereby producing both the start and stop commands to the digital range counter), the time interval measurement (t
r
-t
0
) is substantially unaffected by any delay in the CFD or amplitude variations in the start or stop pulses.
A CFD operates by monitoring the amplitude of the incoming signal (pulse) and continually adjusting its detection threshold to a fixed (i.e., constant) fraction thereof. This threshold level may be produced by attenuating the incoming signal by a fixed attenuation factor and “stretching” the peak of the attenuated signal (e.g., via a peak-hold or “pulse-stretching” circuit). This threshold level is then compared to a delayed version of the incoming signal (e.g., by subtracting it threshold level and detecting zero crossings). The amount by which the incoming signal is delayed is selected to allow sufficient time for the attenuation and peak “stretching” circuit to “set up” a valid and stable threshold level. This approach is substantially,independent of pulse shape.
Simpler CFDs do not employ the “pulse-stretching” aspect of the CFD described above. An example of such a simpler CFD is shown in FIG.
1
.
FIG. 1
shows a simple prior-art CFD
100
wherein an input signal
102
is applied to a fixed attenuator
104
and a delay line
108
. The attenuator
104
scales down the input signal
102
by a fixed attenuation constant “K” to produce an attenuated input signal
106
. The delay line
108
delays the input signal
102
by a fixed amount, producing a delay signal
110
. The delay signal
110
is inverted (multiplied by −1) by an inverter
112
, to produce an inverted, delayed signal
114
. A summing block
116
adds the attenuated input signal
106
to the inverted, delayed signal
114
to produce a summation signal
118
. A comparator
120
compares the summation signal
118
to a “zero” level to produce a positive output
122
(“OUT”) whenever the summation signal is greater than the zero level.
In effect, the CFD of
FIG. 1
uses the attenuated input signal
106
as a comparison threshold against which the delayed input signal
110
is compared. The delay is selected to produce the desired threshold crossing point. (In using this technique, it is desired that the shape of the input pulse is constant.)
Another commonly used prior-art pulse detection technique is to differentiate input pulses from a baseline noise level by considering only input signals above a predetermined minimum noise-rejection threshold (effectively a “squelch” level). The “squelched” input signal is then differentiated. Due to the natural properties of differentiation, with the correct differentiation time-constant, the differentiated input signal will cross zero at a point corresponding to the peak of the input pulse. Pulse symmetry between rising and falling edges is desirable for an accurate zero crossing time.
Some of the disadvantages of these prior art techniques are:
a) They are complex, especially when multiple channels are used to extend the dynamic range of the CFD or to allow the use of detector arrays.
b) When multiple return pulses are close together, (e.g., as a result of a target behind a tree, with a signal from both the tree and the target) the second return may interfere with the delayed first return, causing a range error or lack of target discrimination, (i.e., the inability to separate and distinguish between the two return pulses). This is especially problematic when the first return pulse is stronger, or when the trailing edge of the first return pulse is elongated (e.g., due to a sloping first target), or distributed in range (e.g., due to multiple closely-spaced echoes from the leaves of a tree).
c) The simpler techniques are sensitive to the pulse shape.
BRIEF DESCRIPTION (SUMMARY) OF THE INVENTION
It is a general object of the present invention to provide an improved technique for discriminating between return pulses and improving range accuracy.
It is a further object of the invention to provide a simpler, less-expensive, lower-power pulse discriminator (PD), suitable for use in arrays and expanded dynamic range requirements.
It is a further object of the invention to provide a PD that will allow the resolution of closely spaced pulses, even when a subsequent pulse is small enough that it only appears as a modulation on the trailing edge of a first pulse.
It is a further object of the invention to provide a PD that is easily expandable in dynamic range.
It is a further object of the invention to provide a technique for ensuring an accurate measurement of pulse timing over a wide dynamic range, or in the presence of multiple pulses.
According to the invention, a non-delayed input signal is provided to a first comparator input and a delayed input signal (the delay is applied by a delay line or equivalent delay circuit) is applied to a second comparator input. An offset voltage is applied between the delayed and non-delayed signals at the comparator inputs to provide a “bias” so that the output of the comparator is “normally” (when no signal is present) at an “inactive” state. Typically, the comparator will be a “fast” comparator, suited to comparing high-speed analog sig

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