Abstract:
An automatic pulse detector compares a radar video pulse to a delayed and amplified version of itself. The radar video pulse serves as an amplitude reference for a comparator. A delayed and amplified version of the same pulse serves as the pulse to be detected. Time of detection is amplitude independent and is not degraded by flat-topped pulses. Pulse detection occurs at a fixed, fractional point on the leading edge of a pulse where noise has less temporal influence than at the top of a pulse. Unlike a time-of-peak detector, the self-referencing pulse detector is well-suited to detecting wide, flat-topped pulses produced by expanded-time, pulse-echo radars operating in relatively narrow ISM bands.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to radar detection circuits and more particularly to automatic pulse detection circuits. The invention can be used to detect pulses for wideband and UWB radar rangefinders, time domain reflectometers and radiolocation systems. 
         [0003]    2. Description of Related Art 
         [0004]    Radar echo pulses exhibit large amplitude variations, depending on target size, range and dielectric constant, and these variations produce range measurement errors when the pulses are detected with a fixed threshold detector. Echo amplitude variations also occur to a lesser extent with TDR-based tank level sensors, mainly being limited to dielectric constant variations of the liquid in the tank. However, accurate TDR-based tank level sensors require accurate, amplitude-independent pulse detectors. 
         [0005]    Detectors with amplitude-tracking thresholds or other means to achieve amplitude independence are generally termed automatic pulse detectors and several automatic pulse detectors have been in existence for more than 30 years. U.S. Pat. No. 5,610,611, High Accuracy Material Level Sensor, to the present inventor, Thomas E. McEwan, describes the well-known constant fraction discriminator, or CFD, for use in a TDR-based tank level sensor. The CFD in the &#39;611 patent uses a peak detector to determine the peak amplitude of repetitive equivalent-time pulses and sets a trigger point that is a fraction of the peak amplitude, such as the half-way point on the rise of the pulse (half-max detection). Unfortunately, the CFD exhibits latency errors caused by slow peak tracking when the pulses decrease in amplitude. Latency is a particular problem when the CFD is used in a TDR level sensor for sloshing liquids in a tank, such as an automotive gas tank. Another potential problem with the peak detector is it can erroneously lock-on to the strongest peak in a radar or TDR waveform, such as the main bang peak, unless the CFD is provided with analog gating. Another problem with the CFD is it can trigger on baseline noise when no echo pulses are present, so a threshold detector is needed to prevent false triggering. To overcome the limitations to a CFD, additional circuitry is often needed. 
         [0006]    Another well-known automatic pulse detector is the time-of-peak (TOP) detector. The TOP detector differentiates pulses and triggers on the resulting zero axis crossings. To prevent false triggering on baseline noise, the desired pulses must be above a threshold before zero-axis detection is enabled. This standard detector is utilized in an application to TDR in U.S. Pat. No. 5,457,990, Method and Apparatus for Determining a Fluid Level in the Vicinity of a Transmission Line, to Oswald, 1995. However, the TOP detector can be less accurate than the CFD for the simple reason that a pulse peak is somewhat flat and has a low rate of change, making temporally accurate detection difficult. Small baseline perturbations, such as baseline ringing or radar clutter, can sum with the pulse and substantially displace the exact time-of-peak. In contrast, a CFD can detect at a fast slewing point during the pulse risetime where detection time is much less sensitive to baseline perturbations. A major limitation to a TOP detector is its inability to operate properly with flat-topped pulses—the pulse should have a sharp peak. 
         [0007]    An amplitude-independent pulse detector is needed that (1) triggers on a high-slew point of a pulse like the CFD to avoid the inaccuracies of the TOP detector, (2) does not have the latency of the CFD, and (3) does not have the complexity of prior automatic detectors. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is a self-referencing pulse detector that includes a method of detecting a radar video pulse (RVP), comprising: coupling a RVP to a first input of a two-input comparator, delaying the RVP to produce a delayed RVP; amplifying the delayed RVP to produce a delayed and amplified RVP; coupling the amplified and delayed RVP to a second input of the comparator; and, providing a detected output pulse from the comparator. The method of detecting a RVP can further comprise threshold detecting the RVP to produce a threshold pulse and gating the threshold pulse with the detected output pulse from the comparator to produce a thresholded detected output pulse. 
         [0009]    The self-referencing radar pulse detector can also comprise: a detection comparator having a first and a second input for producing a detected radar pulse; an input line for coupling a radar video pulse (RVP) to the first input of the comparator; a delay circuit having an input coupled to the input line for producing a delayed RVP; and, an amplifier having an input coupled to the delay circuit and an output coupled to the second input of the comparator for producing a delayed and amplified RVP. The self-referencing radar pulse detector can further comprise: a threshold comparator having a first input coupled to the input line and a second input coupled to a voltage reference for producing a threshold pulse; and, a logic gate for gating the threshold pulse with the detection comparator output for producing a thresholded detected radar pulse. The self-referencing radar pulse detector can include a delay circuit is at is one of a transmission line, an allpass network, or a lowpass filter. 
         [0010]    Objects of the present invention are: (1) to provide an amplitude-independent automatic pulse detector that is accurate, simple, and inexpensive; (2) to provide an amplitude-independent automatic pulse detector with zero latency; and (3), to provide an amplitude independent automatic pulse detector that can operate equally well with impulses and with flat-topped pulses. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a diagram of a self-referencing pulse detector of the present invention. 
           [0012]      FIG. 2  is a timing diagram of the self-referencing pulse detector. 
           [0013]      FIG. 3  is a diagram of a radar system incorporating the self-referencing pulse detector. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    A detailed description of the present invention is provided below with reference to the figures. While illustrative parameters and embodiments are given, other embodiments can be constructed with other parameters. 
       General Description 
       [0015]    The present invention overcomes the limitations of prior radar video pulse (RVP) detectors by using the RVP itself as an automatic reference voltage. A delayed and amplified version of the RVP is then used as the pulse to be detected. Detection occurs whenever the delayed and amplified RVP exceeds the instantaneous amplitude of the RVP itself. Since the detection reference is set by the RVP itself, i.e., since it is self-referencing, the temporal location of the detection point is independent of the RVP amplitude. 
         [0016]    The temporal location of the detection point is effectively on the leading edge of the RVP, generally in the middle of the risetime where the slew rate is greatest. Accordingly, sensitivity to noise is minimized, particularly when compared to a TOP detector. Furthermore, the self-referencing arrangement is free of latency and can adapt to pulse amplitude changes on each individual RVP. 
       Specific Description 
       [0017]    Turning now to the drawings,  FIG. 1  is a block diagram of a self-referencing radar pulse detector, generally  10 . An exemplary radar video pulse (RVP)  28  is coupled to comparator  22  via input line  12 . The RVP is also coupled to delay circuit  18 , which is coupled to amplifier  20 . Amplifier  20  produces a delayed and amplified version of the RVP, shown as pulse  30 . When the instantaneous amplitude of pulse  30  exceeds the instantaneous amplitude of pulse  28 , a detected output pulse is produced by comparator  22  on line  17 . The exact detection time t is indicated by small circles on waveforms  28 ,  30  and  32 . Time t is on the rapid rise portion of waveform  30 , generally the lowest noise detection point. 
         [0018]    Delay element  18  can be comprised of: (1) a classic LC (inductor-capacitor) transmission line; (2) a classic RC (resistor-capacitor) transmission line; (3) a conventional cable or microstrip transmission line; (4) an allpass network; (5) a lowpass filter, or (6) any other device (e.g., SAW) that can delay RVP  28 . Delay  18  can be comprised of passive or active elements. In one embodiment, delay  18  is comprised of a third order active lowpass filter that also has gain. Thus delay element  18  and amplifier  20  can be unitized. The passband characteristics of an active lowpass can be optimized so delayed-and-amplified pulse  30  is not distorted by a recovery tail or by ringing; i.e., critical damping can be used. The more accurately pulse  30  resembles a delayed-and-amplified version of RVP pulse  28 , the less chance there is of detection error when RVP  28  has a complex waveshape, as may be the case when radar clutter is present. 
         [0019]    The amount of delay introduced by delay element  18  is somewhat of a design choice, with the delay shown in  FIG. 2  being optimal for most applications. This amount of delay can be seen from  FIG. 2  to be equal to about ½ of the risetime of pulse  28 . 
         [0020]    An optional threshold comparator  14  and logic gate  24  can be included to provide a threshold feature. Whenever the amplitude of RVP  28  exceeds a threshold level V th , indicated by the dashed line on pulse  28 , comparator  14  outputs a threshold pulse. The threshold pulse is gated with the detected pulse from comparator  22  to produce a gated detection pulse  32  on line  26 . One benefit of this optional threshold feature is to prevent false triggers on noise when no RVP is present. 
         [0021]    Amplifier  20  can be eliminated, with delay circuit  18  coupled directly to comparator  22 , and an attenuator  13  can be inserted in series with line  12 , before comparators  14  and  22  of  FIG. 1 , to produce a comparable result. The relative amplitudes of pulses  28  and  30  can be held in the same proportion by using either amplifier  20  or attenuator  13 . Detection comparator  22  operates on the relative amplitudes of its input pulses. Consequently, the self-referencing detection function of the present invention occurs with either configuration. Whether one uses an attenuator or an amplifier is a design choice. In either case, the benefit of the invention is fully realized. For the sake of brevity, the amplifier configuration is described and claimed, but functionally and structurally, the amplifier and attenuator configurations are considered to be the same. In other words, if the pulse from delay circuit  18 , or from amplifier  20 , is larger than the reference pulse applied to the other input of comparator  22 , it is considered to be amplified. 
         [0022]      FIG. 2  is a timing diagram of a self-referencing radar pulse detector. RVP  28  is the input pulse and pulse  30  is the delayed and amplified pulse, as described with reference to  FIG. 1 . When pulse  30  intersects pulse  28 , detection occurs at time t. One can easily visualize that time t does not vary with the amplitude of the RVP, since both pulses  28  and  30  vary in equal proportions. It is also evident by inspection that if the RVP became flat-topped and broadened in width, as indicated by dashed line  80 , and consequently by dashed line  82 , the intersection of the resulting waveforms would still occur at point t. Thus the detector is also independent of pulse width and can operate with wide pulses. Such wide pulses are common in radars that must operate within the FCC&#39;s designated ISM bands at, for example, 2.4 GHz, 5.8 GHz and 24 GHz. It is also evident from  FIG. 2  that RVP  28  may have ringing after detection point t with no effect on the temporal location of point t. Thus, RVP  28  need not necessarily be a unipolar pulse. RVP  28  can comprise ½ cycle or more—up to many cycles—of a somewhat sinusoidal shaped signal. In such cases, the self-referencing radar pulse detector can automatically trigger on the first ½ cycle. 
         [0023]    Comparator  22  outputs a digital output detection pulse  86  on line  17  whenever RVP  28  is more positive than delayed and amplified pulse  30 . The output detection time is indicated at point t. Optional threshold comparator  14  outputs a threshold pulse  84  on line  15  whenever RVP  28  exceeds a reference voltage V th . Threshold pulse  84  and comparator output pulse  86  are coupled to an optional AND gate, which produces a thresholded detection output pulse  32 . Pulse  32  carries the detection timing point at time t. Pulse  32  can be used to trigger a latch, it can start or stop a range counter or control logic, or it can be used for other functions in processor  60 . 
         [0024]      FIG. 3  is a block diagram of an exemplary radar transceiver including the present invention. Transmitter  50  transmits radar pulses into free space via antenna  52 . Echoes are received via antenna  54  and received by receiver  56 , which can produce raw video pulses on line  57  that often comprise one or more sinusoidal cycles. In such cases, the raw video pulses can be rectified and filtered by envelope detector  58  to produce a substantially unipolar radar video pulse (RVP)  28 . Some radars can produce an unipolar RVP without the envelope detector. For example, some impulse radars can produce such pulses, time domain reflectometers (TDR) can produce unipolar pulses, and radars having a power sensitive, as opposed to a voltage sensitive, detector, can produce unipolar pulses. Pulse detector  10  and lines  12  and  17  are as described with reference to  FIG. 1 . Optionally, threshold comparator  14  and gate  24  may be included, in which case the output from element  10  will be on line  26 . Processor  60  receives detected radar pulses and can produce a range output signal, or other processed signal, at port O, often in relation to timing signals to or from transmitter  50  on line  62 . 
         [0025]    An exemplary radar for use with the present invention can be a sampling type, expanded time radar such as that described in U.S. Pat. No. 6,137,438, “Precision Short Range Pulse-Echo Systems with Automatic Pulse Detectors,” by the present inventor Thomas E. McEwan. RVP  28  can have a duration of about 1-millisecond after time-expansion and an amplitude of about 1-volt. Amplifier  20  can be a TLV-272 by Texas Instruments, Inc., and comparators  14  and  22  can be type LM-339 by Fairchild, Inc. One embodiment of the self-referencing radar pulse detector using these components in an expanded time radar exhibits less than an equivalent of 5 ps in detection variation over a 10:1 amplitude variation and over −55 to +65° C. temperature range, when RVP  28  is equivalently 1.5 ns wide in realtime, after backing out a time expansion factor of about 650,000. 
         [0026]    Herein, the term “radar” can refer to a free-space radar that propagates pulses through air or a dielectric medium, e.g., a tank gauging radar. Radar can also mean a conducted or guided wave radar (GWR), such as a tank gauging radar or an “electronic dipstick.” Radar can also mean a bistatic radiolocation radar for a radar-tracked writing pen or other object locator. In a GWR, antennas  52  and  54  of the exemplary radar of  FIG. 3  can be replaced with a single GWR pulse launcher as is well-known in the art. The free-space radar of  FIG. 3  can also be implemented using a single antenna for transit and receive as is well-known in the art. 
         [0027]    The specific comparators and their input polarities, the specific logic gates and radar architectures given in this disclosure are examples. One skilled in the art can readily create other configurations as a design choice. Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.