Abstract:
A pulse detection system for expanded time radar, laser and TDR sensors detects specific cycles within bursts of cycles. A sensor transmits and receives short bursts of RF cycles. A transmit pulse detector triggers on a selected cycle of the detected transmit burst and starts a range counter. A receive detector triggers on a selected cycle within a received echo burst to stop the range counter, thereby indicating range. Cycle selection is enabled by an analysis window of time. The detection system can provide accuracies on the order of one picosecond and is well-suited to accurate ranging along an electromagnetic guide wire.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to radar detection circuits and more particularly to baseband carrier detection circuits for expanded time ranging systems. The invention can be used to accurately detect the time of occurrence of pulsed RF echoes for sampling radar, TDR (Time Domain Reflectometry) and laser sensors. 
   2. Description of Related Art 
   Short range, high resolution pulse-echo ranging systems, such as wideband and ultra-wideband pulsed radar and pulsed laser rangefinders often transmit a short sinusoidal burst on the order of 1-nanosecond in duration and consisting of about six cycles of RF. Radars having these parameters can be found in, for example, commercial pulse-echo rangefinders used to determine liquid levels in tanks. These radars operate in an expanded time mode, whereby the transmit pulse rate is slightly higher than the receiver gate, or sampling, rate, to produce a stroboscopic slow motion sampling effect, i.e., a down-sampling, time expansion effect. 
   The stroboscopic effect produces detected output pulses that resemble the received RF echo pulses, but occur on a vastly expanded time scale. Time expansion factors of 100,000 to 1-million are common. Accordingly, RF echo pulses having a 6 GHz carrier frequency produce sampled output echo pulses having a 6 kHz carrier frequency. These 6 kHz pulses are expanded time replicas of the RF echo pulses. At 6 kHz, pulse detection or other processing is vastly easier. Examples of expanded time radar architectures are disclosed in U.S. Pat. No. 6,191,724, “Short Pulse Microwave Transceiver,” by the present inventor, Thomas E. McEwan, and in U.S. Pat. No. 6,414,627, “Homodyne Swept Range Radar,” also by the present inventor. 
   A problem arises in precisely detecting pulsed RF produced by these systems. One sinewave cycle looks very much like the next within a sinewave burst, so a detector has difficulty detecting a particular sinewave cycle within the burst. For best ranging accuracy, the detector must consistently detect one specific cycle within the echo burst. Preferably, one particular point on a selected sinewave cycle must be detected. 
   One approach to the detection problem is a fixed threshold detector that triggers on the first sinewave cycle to cross the threshold. Unfortunately, variations in received signal amplitude make this approach unattractive since cycle jumps are inevitable as amplitude varies with target range, aspect angle and size. 
   Another approach is to detect the envelope of the sinusoidal burst and then threshold detect the envelope, with the detection time occurring at a threshold crossing. Alternatively, the envelope&#39;s time-of-peak (TOP) can be detected. Yet another technique is constant fractional maximum detection (CFD), wherein a peak detector detects peak amplitude, which is coupled through a voltage divider to set a tracking detection threshold at a constant fraction of the pulse maximum. In all these cases, the envelope is slower and of lower bandwidth than an individual cycle within the burst, and so detection accuracy suffers accordingly. A ten fold reduction in accuracy is not uncommon with these envelope detection techniques. 
   An automatic sinusoidal burst detection technique is disclosed in U.S. Pat. No. 6,137,438 “Precision Short Range Pulse-Echo Systems with Automatic Pulse Detectors,” by the present inventor. A peak detector detects peak envelope amplitude and sets a fraction of this peak—as a form of a CFD—as the threshold for the next repetition of the expanded time sinusoidal burst. Thus, a consistent detection point can be set on a selected cycle in the burst. This approach, while effective, has two limitations. First, rapid pulse-to-pulse variations are not tracked since the peak of one pulse is used to set a threshold on the next pulse. Second, detection does not occur at the zero axis crossings of the sinewaves where the voltage rate of change is fastest and detection can be accomplished with a minimum of noise and error. Thus, a better approach is needed for varying targets and for higher accuracy. 
   U.S. Pat. No. 5,457,990, “Method and Apparatus for Determining a Fluid Level in the Vicinity of a Transmission Line,” by Oswald et al, discloses a detection technique employing a threshold detector to define an analysis window of time. Whenever pulse amplitude exceeds the threshold, a TOP detector is enabled and detection occurs. The analysis window gates out noise outside the window. However, the &#39;990 patent fails to teach detection of multiple sinusoids in a burst—it is limited to single transients. Multiple cycles within a burst present an ambiguity as to which cycle to detect, and this problem is not addressed in the &#39;990 patent. Furthermore, the &#39;990 patent is limited to an analysis window derived from a single transient above threshold. A sinusoidal burst is not a transient. Thus, an entirely new technique is needed. 
   SUMMARY OF THE INVENTION 
   The present invention provides a detection system for expanded time radar, laser, or TDR sensors, which can include, but is not limited to, (1) a transmitter for transmitting a transmit burst, wherein the burst comprises two or more of RF sinusoidal cycles, (2) a receiver for receiving the transmit burst and echoes of the transmit burst and for producing an expanded time receiver output, wherein the receiver output comprises a detected transmit burst and a detected echo burst, (3) a transmit pulse detector for producing a start pulse when the detected transmit burst exceeds an amplitude threshold, (4) an envelope detector for producing an envelope pulse of the detected echo burst, wherein the envelope pulse includes a voltage peak, (5) a threshold detector for producing an analysis window if the envelope pulse exceeds a threshold value, (6) a time-of-peak detector for detecting the voltage peak and for producing a TOP pulse, (7) a comparator for threshold detecting sinusoidal cycles within the detected echo burst and for producing a carrier signal, and (8) a receive echo detector for producing a stop pulse during the analysis window in response to the TOP pulse and the carrier signal. 
   The system can also include a processor for measuring the start to stop pulse interval to determine echo range. Additionally, the system can include a gate to form a PWM pulse having a pulse width proportional the interval between the start and stop pulses, and a processor for measuring the PWM pulse width to determine echo range. Furthermore, the system can include a transmit pulse detector for producing a start pulse after the detected transmit burst exceeds an amplitude threshold N successive times, where N is an integer representing each sinusoidal cycle within the burst. 
   The invention includes a method for detecting expanded time radar, laser or TDR signals, comprising: (1) transmitting an RF burst of sinusoidal cycles, (2) sampling transmitted RF burst and a receive echo burst to produce a detected transmit burst and a detected echo burst, (3) threshold detecting the detected transmit burst to produce a start pulse, (4) threshold detecting the detected echo burst to produce a carrier signal, (5) envelope detecting the detected echo burst to produce an envelope pulse, (6) time-of-peak detecting the envelope pulse to produce a TOP pulse, (7) threshold detecting the envelope pulse to produce an analysis window of time, and (8) producing a stop pulse during the analysis window in response to the TOP pulse and the carrier signal. The method can further include processing the start pulse and the stop pulse to produce a range measurement. 
   The invention also provides a carrier phase detector for expanded time radar, laser, or TDR sensors, and includes (1) a transceiver (i.e., a transmitter-receiver including a common RF port) for producing a detected echo burst, wherein the detected echo burst comprises a limited number of sinusoidal cycles, (2) an envelope detector for producing an envelope pulse of the detected echo burst, wherein the envelope pulse includes a voltage peak, (3) a threshold detector for producing an analysis window if the envelope pulse exceeds a threshold value, (4) a time-ofeak detector for detecting the voltage peak and for producing a TOP pulse, (5) a comparator for threshold detecting sinusoidal cycles within the detected echo burst and for producing a carrier signal, and (6) a receive echo detector for producing an output pulse during the analysis window in response to the TOP pulse and the carrier signal. 
   Further, the invention provides a TDR sensor, including: (1) a transmitter for producing a transmit burst, wherein the burst consists of two or more of RF cycles, (2) a receiver for detecting echoes of the transmit burst and for producing an expanded time detected echo burst, (3) a transmission line probe coupled to the transmitter and to the receiver for conducting transmit and echo bursts, wherein the echo bursts are reflected transmit bursts from a liquid or other material in contact with, or close proximity to, the transmission line, (4) a threshold detector for producing an analysis window of time that is at least two cycles of the detected echo burst in duration if the detected echo burst exceeds a threshold value, (5) an analyzer responsive to a characteristic of the detected echo burst during the analysis window for producing an range pulse. The TDR sensor can also include a receiver for receiving the transmit burst and echoes of the transmit burst, and for producing an expanded time receiver output, wherein the receiver output consists of a detected transmit burst and a detected echo burst. Additionally, the TDR sensor can further include a processor responsive to the range pulse for measuring echo range. Also, the TDR sensor can further include a processor responsive to the detected transmit burst and the range pulse for measuring echo range. The TDR sensor can operate with a transmission line probe that is an electromagnetic guide wire or a Goubau line. 
   The present invention can be used in expanded time radar, laser, and TDR ranging systems as a high accuracy detection system that exhibits high accuracy, high dynamic range and excellent immunity to noise. Applications include pulse echo rangefinders for tank level measurement, environmental monitoring, industrial and robotic controls, digital handwriting capture, imaging radars, vehicle backup and collision warning radars, and universal object/obstacle detection and ranging. 
   One object of the present invention is to provide a precision pulsed RF radar pulse detection system. A further object is to provide a precision pulsed RF radar pulse detection system with high immunity to noise, interference and baseline clutter. Another object of the present invention is to provide a precision pulse detection system for TDR systems employing pulsed RF bursts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a radar sensor block diagram including the detection system of the present invention. 
       FIG. 2   a  depicts a first pulse start detector. 
       FIG. 2   b  depicts an Nth pulse start detector. 
       FIG. 3  is a diagram of a carrier phase detector. 
       FIG. 4  is a timing diagram for the system of  FIGS. 1 ,  2  and  3 . 
       FIG. 5  is a block diagram of a TDR system including a carrier phase detector of the present invention. 
       FIG. 6  is another detector logic configuration. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A detailed description of the present invention is provided below with reference to the figures. While illustrative component values and circuit parameters are given, other embodiments can be constructed with other component values and circuit parameters. All U.S. patents and copending U.S. applications cited herein are herein incorporated by reference. 
   General Description 
   The present invention overcomes the limitations of the various prior detection techniques by detecting the time-of-peak (TOP) of the expanded time RF burst envelope within an analysis window of time and then using that detection event to gate a carrier phase detector. The carrier phase detector detects the zero axis crossings of each sinewave cycle within a burst. The zero axis crossing of a selected cycle is gated by the TOP detection. Therefore, the accuracy of the detection is directly tied to the selected sinewave zero axis crossing, which is highly accuracy, and not to the TOP accuracy. Furthermore, the zero axis crossing occurrence time is amplitude independent and has the greatest immunity to noise. Noise can include random thermal noise, RF interference, and baseline clutter from undesired echoes. The use of the term sinewave can also include other repetitive waveforms, such as clipped sinewaves, triangle waves, etc, although the waveshape is generally sinusoidal due to the beneficial use of bandpass filters in the receive path. 
   Specific Description 
   Turning now to the drawings,  FIG. 1  is a block diagram showing a general configuration of a carrier phase detection system  10  of the present invention. A transmit clock signal on line  13 , labeled TXCLK, triggers a pulse generator  12 , which produces pulses at the TXCLK rate. The pulses gate-on an RF oscillator  16 , which is coupled to a radiating element  18 , which can be an antenna or an optical device such as a laser or LED. The radiated pulse waveform  48  includes a predetermined number of RF cycles. If the radiated signal is a microwave signal, waveform  48  represents the radiated electric field. If the radiated signal is optical, waveform  48  indicates amplitude modulation of light intensity. The TXCLK, pulse generator  12 , RF oscillator  16  and radiator  18  form a transmitter. 
   A receive dock signal on line  25 , labeled RXCLK, triggers a pulse generator  24 , which produces pulses at the RXCLK rate. The pulses gate a sampling receiver  22 , which samples signals from receiving element  20 . Receiving element  20  often can be configured as an antenna for microwave radiation. However when operating at predetermined optical frequencies, element  20  can be arranged as a photodetector such as, but not limited to, an avalanche CCD photodetector, a photomultiplier a photodiode, or any photodetector known by those skilled in the art that can receive desired frequencies within the spirit and scope of the present invention. The RXCLK, pulse generator  24 , sampling receiver  22  and receiving element  20  form a receiver. 
   The radiating and receiving elements can be combined into a single transmit-receive antenna  44  or a single lens, again indicated at  44 , for bidirectional operation as indicated by line  42  and element  44 . Element  44  can also be a launcher for a time domain reflectometer employing an electromagnetic guide wire or a Goubau line for use as an “electronic dipstick” or tank level sensor. 
   Sampling receiver  22  samples echoes at the RXCLK rate and produces expanded time sampled echo signals on line  26 . Several dozen samples can be continuously integrated together before being output on line  26 . The time expansion effect is caused by sampling at an offset frequency from the transmit pulses, in a similar fashion to observing a rapidly rotating fan blade that appears to rotate slowly under a strobe light set to a strobe frequency that differs slightly from the blades&#39; rotational rate. Accordingly, radars of this type are termed stroboscopic radars since they make realtime pulses propagating at the speed of light appear to propagate far slower, e.g., at the speed of sound. Expanded time signals are far easier to process accurately since the processing bandwidth is reduced in proportion to the time expansion factor. Time expansion is set by the TXCLK to RXCLK frequency difference A relative to the TXCLK frequency. That is, the time expansion factor=(TXCLK frequency)/Δ. Exemplary parameters are TXCLK frequency=2 MHz, Δ=10 Hz and the expansion factor=200,000. 
   Expanded time sampled transmit and echo signals are output from receiver  22  on line  26  and coupled to a transmit pulse detector  28 . Sampled transmit signals are present due to unavoidable proximity coupling between antennas  18 ,  20 , or optical elements  18 ,  20 , or via the dashed line between elements  18 ,  20  when a single element  44  is used. Transmit pulse detector  28  outputs a start signal on line  30 . 
   Sampling receiver  22  also outputs a sampled echo signal on line  50  to an optional variable gain amplifier  52 , or VGA, which is responsive to a range ramp input on line  54 . The range ramp increase VGA gain as the sampling receiver samples signals at greater ranges. Thus, echo amplitude versus range is held constant. Such a feature, while beneficial, is not essential to the invention. 
   The VGA output is coupled to bandpass filter (BPF)  56 , which rejects noise while passing expanded time replica echoes of radiated signal  48  along line  58 . For illustrative purposes, exemplary frequencies can be arranged with 6 GHz for the sinusoids in waveform  48 , and 6 kHz for the expanded time sinusoids output from receiver  22 . In such an example embodiment, BPF  56  has a passband centered at 6 kHz. BPF  56  is coupled to a receive pulse detector  32 . The output of receive pulse detector  32  is a stop signal on line  34 . The time interval between the start signal on line  30  and the stop signal on line  34  defines echo range. Optionally, the start and stop signals can be coupled to gate  36  to form a pulse width modulation (PWM) signal on line  37 . The pulse width of the PWM signal is proportional the time interval between the start and stop pulses and thereby indicates echo range. 
   An optional processor  38  can be used to perform various processing functions known in the art, such as averaging, range calibration and scaling, range error correction, etc. Processor  38  outputs on line  40  for display, memory or control functions. Processor  38  can determine echo range from the time interval between the start and stop signals or from the PWM signal. 
     FIG. 2   a  depicts an embodiment of transmit detector  28 . It includes a comparator  60  for detecting the detected transmit burst signals on line  26  when the burst amplitude exceeds a threshold Vth. Comparator  60  triggers latch  62 , which outputs a start signal on line  30 . For clarity, a reset line coupled to latch  62  is not shown. 
     FIG. 2   b  depicts another embodiment of transmit detector  28  that further includes a divide-by-N counter  66  in addition to comparator  60  for detecting the detected transmit burst signals on line  26  when the burst amplitude exceeds a threshold Vth. Counter  66  thus counts successive sinewave cycles detected by comparator  60  and outputs a trigger signal to latch  62  after N cycles have occurred. Accordingly, the start signal on line  30  corresponds to the Nth cycle of the detected transmit burst signal. Thus, transmit detector  28  of  FIG. 2   a  detects the first cycle above threshold Vth, while transmit detector  28  of  FIG. 2   b  detects the Nth successive cycle above threshold Vth. N can be beneficially set to match the selected cycle detected by receive detector  32 , thereby canceling errors resulting from frequency drift in the RF burst. In practice, the expanded time detector of  FIG. 2   a , when used in the system of  FIG. 1 , can be stable to a realtime equivalent of several picoseconds over a wide temperature range when RF oscillator  16  employs SiGe transistors at 6 GHz. 
     FIG. 3  is an block diagram of an embodiment of a receive detector  32 , i.e., a carrier phase detector. Carrier phase refers to the phase, or temporal location of the sinusoids in the detected echo burst. This phase varies with range, since the entire burst, and the phase of the sinusoids within the burst, occurs at a temporal location corresponding to physical range. Thus, detecting carrier phase is inherently a high accuracy range detection modality. 
   Walking though the functional blocks in  FIG. 3 , the detected echo burst is input on line  26  to an envelope detector  70 , which outputs an envelope signal on line  71 , whereby the envelope corresponds to a curve fitted to the sinusoidal peaks of the detected echo burst. The envelope signal on line  71  is differentiated by a differentiator  72  and output on line  74 . A comparator  76  detects a zero crossing of the voltage on line  74 , which corresponds to the time-of-peak (TOP) of the envelope signal. The TOP is approximately the center of the envelope signal. 
   A comparator  82  outputs a threshold signal on line  83  whenever the envelope signal received along line  80  is above a predetermined threshold Vref. The threshold signal defines an analysis window of time. No output from the receive detector can occur outside the analysis window. Accordingly, noise and spurious response are automatically eliminated outside the analysis window. Within the analysis window, gate  78  sets the D input of a latch  84  high via line  79 , and this high level is clocked through to the Q-bar output on line  34  at the next carrier phase signal transition on line  88 . The Q-bar output is the measurement stop signal. A comparator  86  triggers on zero crossings of the detected receive bursts and outputs a carrier phase signal on line  88 . The first carrier phase transition that occurs after the D-input is set high on line  79  toggles latch  84  and effects a receive detection on a selected sinusoid within the detected receive burst. Connections to latch  84  to hold it high until reset are not shown for clarity. 
     FIG. 4  is an exemplary timing diagram for the detection system of  FIGS. 1 ,  2 , and  3 . Referring to trace numbers at the left of each trace, trace  1  indicates the detection signals on line  26  from receiver  22 . Burst  100  is the detected transmit burst and burst  102  is the detected receive burst. Trace  2  is the transmit detector  28  output on line  30  of  FIG. 2   a , and its edge  104  is aligned with the first sinusoid in burst  100  to exceed threshold Vth. Threshold Vth is not shown for clarity. Alternatively, the transmit detector output  28  on line  30  of  FIG. 2   b , which includes a count-to-N counter, so detection edge  106  is aligned with the Nth sinusoid in burst  100  to exceed threshold Vth. N can be set to correspond to the detected cycle within the echo burst for optimal tracking of errors. 
   Trace  4  in  FIG. 4  is the detected echo burst after passing through VGA  52  and BPF  56 . The VGA attenuates detected transmit burst  100  into a small burst  108 . In practice, burst  108  is almost totally attenuated by the VGA. Burst  102  is amplified by the VGA into burst  110 , which can be substantially constant over a wide span of echo ranges. However, target cross-section can cause the amplitude of echo burst  110  to vary widely. Thus, for precision detection of the temporal location of burst  110 , it is beneficial to detect a zero axis crossing of one of the stronger sinusoids in burst  110 . 
   Trace  5  is the output  112  of envelope detector  70  on line  71  of  FIG. 3 . It is effectively a curve fitted to full-wave rectified sinusoid peaks of burst  110 . Threshold Vref is indicated by a dashed line  113 . Whenever output  112  is above threshold  113  (shown as a dashed line) comparator  82 , as shown in  FIG. 3 , outputs a signal  120  as seen in trace  8 . Signal  120  defines the analysis window of time. 
   Envelope signal  112  is differentiated by differentiator  72 , as shown in  FIG. 3 , and output on line  74  as a derivative signal  114  as shown in trace  6 . Comparator  76  outputs pulse  118  of trace  7  in response to derivative signal  114 . The dashed “X” markings of trace  7  indicate nearly continuous, but somewhat random, pulses similar to pulse  118  due to comparator  76  triggering on baseline noise. Once a strong signal is present, e.g., pulse  114 , then a relatively noise-free pulse  118  is produced. Gate  78  outputs pulse  122  of trace  9  on line  79 , as shown in  FIG. 3 , whenever pulses  118  and  120  are high. Pulse  122  is applied to the D input of latch  84 , and the stage is set for latch  84  to toggle on the next carrier phase signal edge, as shown by the dashed line, on line  88 . Trace  10  indicates the carrier phase signal (i.e., waveform  124 ) on line  88 , as shown in  FIG. 3 , which is provided by comparator  86 . Comparator  86  is referenced to ground, i.e., to zero, so it toggles on the slightest noise as well as the intended burst signal  110 . Thus, its output  124  is seen as a continuous waveform. Optionally, comparator  86  can be referenced to a non-zero voltage, to implement hysteresis or to eliminate noise outside the analysis window. The cycles in waveform  124  outside analysis window  120  are noisy and generally of a period set by BPF  56  of  FIG. 1 . Consequently, waveform  124  resembles a somewhat continuous waveform, but only the cycles within the analysis window  120  (e.g., specifically pulse  126 ) are able to trigger latch  84 , as shown in  FIG. 3 ) and produce a stop signal. Trace  11  shows the stop signal on line  34  with an edge (shown along the dashed line) corresponding to the first clock edge occurring after both analysis window pulse  120  and TOP pulse  122  go high. 
   Trace  12  is a PWM pulse  130  output from gate  36  of  FIG. 1 , with a leading edge aligned (shown aligned with the first dashed line) with start signal edge  104  (or  106 ) and a trailing edge aligned (shown aligned with the second dashed line) with the stop signal edge  128 . The trailing edge of PWM pulse  130  increases in proportion to echo range. The bottom line in  FIG. 4  indicates the time span for all the traces. The time span covers one sweep of the radar, e.g., as defined by the offset frequency Δ. One sweep can be on the order of 100 ms in time. 
   Derivative waveform  114 , as shown in trace  6 , is often produced by a differentiator configuration implemented with a simple resistor-capacitor, or RC, differentiator. An RC dfferentiator, as known in the art, can introduce some lag in the zero axis crossing of its output, indicated by the dashed “O” in waveform  114 . However, the RC parameters of the present invention can be set to provide an optimal detection margin between the “O” and a transition in waveform  124 , so the two do not occur at the same time. This assures the stop output is time aligned with a carrier phase transition in waveform  124  and not the TOP zero crossing “O”. 
   The success of this TOP method of  FIG. 3  depends on the stability and consistency of envelope  112 . Thus, the emitted waveform  48  must be stable. Laboratory tests indicate that waveform  48  is essentially invariant with temperature variation when SiGe transistors are employed in RF oscillator  16  at 6 GHz, so the invention can be practically realized with commonly available components. 
     FIG. 5  is a block diagram showing a general configuration of a carrier phase detection system  200  in a TDR configuration. Transmitter and receiver elements, as indicated by dashed block  210  are as described with respect to  FIG. 1 . A conductor  42  connects transmitter-receiver  210  to a pulse launcher  44 , which launches RF bursts  48  onto transmission line probe  46 . Object  47  reflects echo pulses back to launcher  44 , which also acts as a receive element, and coupled to a receiver  22  via conductor  42 . Object  47  can be a liquid surface in a tank, a material, e.g., corn in a silo, a sliding element, such as a piston in a cylinder, or countless other reflecting materials. Launcher  44  is beneficial when transmission line  46  is an electromagnetic guide wire or a Goubau line. It is not required with some transmission line geometries, such as coaxial, microstrip or balanced twin line, and so launcher  44  can be optional depending on transmission line probe  46 . 
   A receive clock signal, labeled RXCLK, triggers a pulse generator  24  so as to gate a receiver  22 . Receiver  22  after receiving such a gate signal, outputs a detected echo burst on line  26  to optional envelope detector  70 , which performs as described with respect to  FIGS. 3 and 4 . Comparator  82  outputs an analysis window pulse on line  83  as described with respect to  FIGS. 3 and 4 . Line  83  is coupled to an analyzer  85  which may include TOP detection as described with respect to  FIG. 3 , or it may contain other signal processing circuits, such as a CFD. Analyzer  83  is enabled by the analysis window pulse  120  on line  83 . Analyzer  85  outputs a stop signal to processor  38 . Processor  38  can be responsive to start pulse  104  (or  106 ), as shown by traces  2  and  3  of  FIG. 4 , or other range measurement signals, e.g., a signal derived from a range ramp, to produce a range output on line  40 . Processor  38  can average, scale, offset, error correct, and perform other such functions. 
   Envelope detector  70  can be omitted and a pulse stretcher  89  can be used as another example arrangement. In this configuration, comparator  82  triggers directly on a detected echo burst sinusoid received along line  80  exceeding threshold Vref, and it triggers pulse stretcher  89 , e.g., a monostable multivibrator, to produce an analysis window pulse  120 , as shown in trace  8  of  FIG. 4 . 
   TDR system  200  is responsive to sinusoidal echo burst signals within an analysis window of time, and can produce accurate range measurements by detecting a selected detected echo burst characteristic within the analysis window. An echo burst characteristic can include its amplitude, the amplitude of its detected envelope, its zero crossings or its differentiated envelope zero crossing, its envelope time-of-peak, its geometric mean, etc. 
     FIG. 6  is a block diagram of another detector logic configuration beneficial to the system of  FIG. 1  or  FIG. 5 . Line  79 , also shown in  FIG. 3 , couples pulse  122  and pulse  122   a  of trace  9  in  FIG. 4  to a START LOGIC circuit  90  and a STOP LOGIC circuit  96 . Pulse  122  is produced as described in reference to  FIGS. 3 and 4 . Pulse  122   a  is produced from a detected transmit burst pulse  108  rather than a detected echo burst pulse  110 ; otherwise the detection process is similar for both pulses. Comparator  82  produces an transmit analysis window of time, as indicated by pulse  120   a  of  FIG. 4 , when detected transmit pulse envelope  112   a  is above a threshold  113 . Comparator  82  also produces a receive analysis window of time, as indicated by pulse  120  of  FIG. 4 , when detected echo pulse envelope  112  is above a threshold  113 . Exemplary threshold  113  is the same for both transmit and echo pulses; however, it can differ as a design choice. 
   Line  88  couples detected carrier phase pulses  124  of trace  10  in  FIG. 4  to circuits  90  and  96 . START LOGIC circuit  90  outputs a START pulse on a first trigger edge to occur after pulse  122   a  goes high, effecting carrier phase detection of the detected transmit pulse. This is similar to edge detection  126  of trace  10  as shown for stop detection during pulse  122  but occurs during pulse  122   a  for start detection. This process is repeated by STOP LOGIC circuit  96  for pulse  122  and carrier phase pulses  124 , effecting carrier phase detection of the detected echo pulse as indicated by dashed line  126  and pulse edge  128  of  FIG. 4 . Optional delay element  92  provides a delayed enable to STOP LOGIC circuit  96  to set a minimum transmit-to-receive range in order to blank out false receive triggers on the main bang and close range clutter. A benefit to the configuration of  FIG. 6  is timing variations in BPF  56  and detector elements  70 ,  72 ,  76 ,  78  and  86  affect both the transmit and echo detection times equally and thus cancel out of the range measurement. Timing variations in receiver  22  also cancel, while timing variations in VGA  52 , gate  36  and processor  38  can be designed using common components to be negligible, since these elements operate in expanded time. The circuit benefits from detecting zero axis crossings of detected carrier pulses  108  and  110 , rendering timing accuracy independent of signal amplitude. A further benefit occurs when START and STOP detections occur on the same cycle within bursts  108  and  110 , rendering timing accuracy independent of cycle period within bursts  108  and  110 , i.e., independent of RF frequency. Accordingly, realtime offset errors of less than 1-picosecond, or 0.15 mm in range, can be realized without recourse to calibration or trimming. 
   Time-of-peak detection is an exemplary detection mode. Other detection modes can be implemented during the transmit and receive analysis windows of time that trigger on a characteristic of the detected burst signals, e.g., zero axis crossings, fractional maximum detection, pulse centroid detection, offset time-of-peak detection, etc. 
   The use of the word “radar” herein refers to traditional electromagnetic radar that employs microwaves or millimeter waves, and it also refers to optical radar, i.e., laser rangefinders, as well as guided wave radar, wherein radar pulses are guided along a electromagnetic guide wire or other conductor, as in TDR. “Radar” includes monostatic and bistatic systems, as well as radars having a single antenna/transducer. Frequency offset generally refers to an offset frequency between 1 and 1000 Hz between transmit and receive clock signals. However, the scope of the invention also encompasses larger offsets as may be required in various applications. 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.