PATENT ABSTRACT
A dual channel microwave sensor employs single sideband Doppler techniques in innumerable vibration, motion, and displacement applications. When combined with an active reflector, the sensor provides accurate range and material thickness measurements even in cluttered environments. The active reflector can also be used to transmit multi-channel data to the sensor. The sensor is a homodyne pulse Doppler radar with phasing-type Doppler sideband demodulation having a 4-decade baseband frequency range. Ranging is accomplished by comparing the phase of the Doppler sidebands when phase modulated by an active reflector. The active reflector employs a switch or modulator connected to an antenna or other reflector. In one mode, the active reflector is quadrature modulated to provide SSB reflections. Applications for the low-cost system include a mechanical motion/rotation sensor, a robust security alarm, a throat microphone, a stereo guitar pickup, a direction sensitive cardiac monitor, an electronic dipstick, a material thickness/dielectric sensor, a metal smoothness meter, a non-contact electronic readout, an RFID tag, silent “talking” toys, a passive-emitter data link, a beam interrupter, and a gold nugget finder.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to microwave motion sensing and ranging, and more particularly to single-sideband pulse-Doppler radar including an active reflector system. 
     2. Description of Related Art 
     The Doppler effect is used to detect or measure motion in police radars, security radars and automatic door radars. Microwave vibration sensing, while similar to Doppler sensing, relies on target-induced phase modulation of a radar return, and has generally been limited to specialized industrial/scientific use. One well-publicized exception was the microwave illumination of the U.S. embassy in Moscow during the Cold War, where voices may have caused a subtle vibration in reflecting objects that could be intercepted by a vibration-sensing microwave receiver outside the embassy. 
     Vibration sensing radars show great potential in medical diagnostics/monitoring and speech recognition, such as cardiac monitoring (e.g., “Body Monitoring and Imaging Apparatus and Method,” U.S. Pat. No. 5,573,012 to McEwan, 1996), respiration monitoring (e.g., “Respiration Monitor,” U.S. Pat. No. 3,993,995 to Kaplan et al, 1976), and vocal cord sensing (e.g., “Speech Coding, Reconstruction and Recognition Using Acoustics and Electromagnetic Waves,” U.S. Pat. No. 5,729,694 to Holzrichter et al, 1998). 
     One problem in sensing vibrations with radar is the occurrence of periodic nulls in the sensing field located every λ/2 of the RF wavelength. Thus, it is possible to fail to detect a vibrating object depending on its range from the radar. This problem is present in both single phase and quadrature phase radars. It can only be eliminated with SSB radar. 
     The direction of motion can be sensed by Doppler radars outfitted with quadrature detectors. A further adjunct to direction sensing is displacement sensing, where a direction-signed motion signature is integrated over a number of Doppler cycles such that a definite target displacement in one direction must occur before a detection threshold is reached (e.g., “Intrusion Detection System,” U.S. Pat. No. 3,942,178 to Hackett, 1976). 
     Displacement sensing overcomes the hair-trigger nature of a motion sensor by requiring a substantial target movement before triggering. Displacement sensing also reduces another false alarm nuisance: trigging on RF interference. It would be very difficult for RF interference to appear like it is moving in one direction only, i.e., appear as a single sideband to a microwave carrier that is consistently within perhaps 100 Hz of a 10.5 GHz carrier-generally, neither the radar nor the interference would have such coherence or stability. Unfortunately, prior art displacement sensors do not implement interference rejection in a robust fashion, nor do they fully demodulate the Doppler sidebands into separate analog channels. 
     Prior art Doppler radars are lacking in key features needed to enable innumerable applications, e.g., they lack dual channel analog single sideband demodulation combined with short-pulse range-gating while implemented in a low cost architecture. Also, the prior art does not teach the use of an active reflector to measure range or material thickness from the phase relation between the range-gated Doppler sidebands. 
     SUMMARY OF THE INVENTION 
     The present invention is a system comprised of a homodyne pulse Doppler radar employing SSB demodulation which can be used either separately or in combination with an active reflector, which may also have SSB capability. 
     The radar transmits RF pulses, or sinewave packets or bursts, at a pulse repetition frequency PRF. As used herein, the entire burst or packet is referred to as an RF pulse, with each pulse containing one or more cycles of a sinusoidal waveform at a carrier frequency. The transmitted pulses and receive echoes are applied in quadrature to a pair of peak-hold detectors that have a large hold time spanning multiple pulse repetitions. The RF pulse width may be 30 ns wide at a 915 MHz carrier frequency with a corresponding RF bandwidth of about 30 MHz. By integrating pulse repetitions over a long time, e.g., 1-second, Doppler sidebands only 0.1 Hz removed from the RF pulse carrier can be detected. 
     The maximum range, or range gate, is defined by the RF pulse width, since the RF peak detectors operate only while the transmitted pulse is present, such that echoes that arrive after the transmitter RF pulse ends do not get peak detected. 
     The RF peak-hold detector outputs are phase shifted and algebraically summed to provide upper sideband (USB) and lower sideband (LSB) outputs. USB outputs occur with objects moving toward the radar and LSB outputs occur with objects moving away from the radar. 
     One feature of the invention is that the USB and LSB outputs (or channels) can be rectified and filtered to provide a directional displacement indication. These indications can be combined in a weighted fashion such that a small amount of displacement in the wrong direction, for a given LSB or USB channel, will effectively reset that channel—an especially useful feature for eliminating false alarms from loitering or RF interference. 
     When sensing vibrations, either the USB or LSB channel provides an output free of nulls that occur every λ/2 with non-quadrature radar or in the I or Q channel of a quadrature radar, thus removing a barrier to practicality in many applications. 
     Stereo operation as provided by the LSB and USB channels is non-obvious and quite unexpected in many applications such as picking up guitar string vibrations, where each stereo output has the same general frequency of the vibrating string but a different tonal quality, providing a rich, airy sound on stereo headphones. Other vibration sensing applications include (1) sensing machine vibration or motion, including blade rotation with rotational direction indication, (2) direction sensitive cardiac motion sensing, (3) respiratory motion sensing, and (4) a vocal cord microphone. 
     When detecting a vibrating object, the phase between signals at the LSB and USB outputs varies 360 degrees with every λ/4 of distance from the vibrating object. For an RF center frequency of 915 MHz, 360 degrees represents a distance of 8.2 cm. Since the phase signals are very clean, it is an easy matter to measure sub-millimeter variations in distance, or equivalently dielectric constant or material thickness of objects inserted between the radar antenna and the source of vibration. One application of this effect is water or oil level measurement in a tank, by measuring the transit time through the height of the liquid. The advantage to this technique is that low frequency RF can be used without the need for a large focusing antenna, since the measurement is confined to reflections from the vibrating object, which is usually the only object in the field of view that is vibrating. 
     Changes in radar cross section (Δ-RCS) can be detected with the present invention, such as might occur when two metallic objects scrape against each other. If two smooth tweezers are slid across each other near the radar antenna, a large noise-like response is produced by the radar indicating the degree of surface smoothness. The Δ-RCS technique can also be used in combination with mechanical or acoustic pressure to locate gold nuggets in quartz, such as might be found in a gold mine. 
     Electronic objects that contain switching circuits or variable conduction devices, such as audio transistors, also provide a ΔRCS signature that is readily detected by the present invention. Applications include non-contact electronic circuit readout, toys that “talk” upon illumination by a radar, and RF identification (RFID) tags. 
     The Δ-RCS effect can be deliberately enhanced by designing a reflector that undergoes a large Δ-RCS under electronic control. Accordingly, another aspect of the present invention is a modulated dipole or other reflector/antenna, wherein the modulation is voice, music, continuous tones or data. The modulation can be provided by a very low power CMOS circuit, such as the type used in wrist watches, and the modulating device may be a FET, which requires essentially no current to drive its gate. The micropower active reflector thus formed emits no RF energy and is thus not subject to emission regulations, and yet it can transmit information when illuminated by the radar of the present invention. 
     The active reflector can be quadrature modulated to provide a SSB reflection that appears in either the LSB or USB channel of the illuminating radar. Thus, multi-channel operation is possible, beyond the usual code or frequency division techniques. 
     An RF beam interruption system can be configured with a radar and active reflector by forming a beam path through which passing objects will create an interruption. Advantages include (1) the ability to operate through panels and environmental overcoatings such as snow, and (2) the ability to freely locate micropower active reflectors as stick-on appliques. When solar powered, the active reflectors can be permanently attached to trees, lampposts, swimming pool gates, garage door frames, etc. for simplified security, safety, or home automation control. 
     A system of active reflectors can be combined with the radar to form a radiolocation system wherein the range to each reflector is measured by the radar and used to calculate the location of the radar and/or one or more reflectors. 
     The entire radar system of the present invention can be implemented with low cost off-the-shelf components. The applications cited herein are but a few of the many novel uses for this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a single sideband pulse Doppler radar of the present invention. 
     FIG. 2A is a sketch of the RF pulses and detector signal of the radar of FIG.  1 . 
     FIG. 2B is a sketch of the vibration-induced phase modulation in a vibration sensing mode of the radar of FIG.  1 . 
     FIG. 3 is a schematic diagram of the RF oscillator, bi-directional quad net and peak-to-peak detectors of FIG.  1 . 
     FIG. 4 is a schematic diagram of the highpass filter, amplifier and limiter of FIG.  1 . 
     FIG. 5A is a schematic of the baseband quadnet and SSB matrix of FIG.  1 . 
     FIG. 5B is a mathematical plot of the phase characteristics of the baseband quadnet of FIG.  1 . 
     FIG. 5C is hardware data showing the sideband separation within +/−20 Hz of the short-pulse RF carrier for the system of FIG.  1 . 
     FIG. 5D is hardware data showing the sideband separation within +/−2 kHz of the short-pulse RF carrier for the system of FIG.  1 . 
     FIG. 6A shows a general arrangement of a system for detecting vibrating or moving objects. 
     FIG. 6B illustrates a radar throat microphone arrangement of the system of FIG.  6 A. 
     FIG. 6C illustrates an alternative radar throat microphone arrangement of FIG.  6 A. 
     FIG. 6D plots the response to a pair of vibrating tweezers versus range for the setup of FIG.  6 A. 
     FIG. 6E plots the response to a person walking outbound and inbound for the setup of FIG.  6 A. 
     FIG. 6F plots the response to vibrating human vocal cords for the sound “hmmm” for the setup of FIG.  6 A. 
     FIG. 6G plots the response to human cardiac motion for the setup of FIG.  6 A. 
     FIG. 6H plots the response to human respiratory motion for the setup of FIG.  6 A. 
     FIG. 6I plots the response to the motion of a hammer drill for the setup of FIG.  6 A. 
     FIG. 6J plots the stereo response to a single plucked guitar string for the setup of FIG.  6 A. 
     FIG. 6K plots the response to a CW and CCW rotating blade for the setup of FIG.  6 A. 
     FIG. 7 is a diagram of a weighted object displacement sensor for the general setup of FIG.  6 A. 
     FIG. 8A is a diagram of a level or thickness gage using an active reflector with the system of FIG.  6 A. 
     FIG. 8B illustrates the sideband phase relation versus range for the setup of FIG. 8A 
     FIG. 8C indicates the shift in sideband phase with 3 cm of paper inserted in the target path. 
     FIG. 8D indicates the shift in sideband phase with a 1 cm increase in water within the target path. 
     FIG. 9A shows a general arrangement for detecting target Δ-RCS. 
     FIG. 9B plots the response to scraping smooth metal tweezers for the setup of FIG.  9 A. 
     FIG. 9C plots the response to gold nuggets embedded in quartz for the setup of FIG.  9 A. 
     FIG. 9D plots the response to an electronic calculator for the setup of FIG.  9 A. 
     FIG. 10A shows a general arrangement for using an active reflector in a system with the sensor of FIG.  1 . 
     FIG. 10B plots the response of the system of FIG. 10A when the active reflector is electronically modulated with a voice. 
     FIG. 10C plots the response when a human arm blocks the path between the active reflector and the sensor of the system of FIG.  10 A. 
     FIG. 11 is a diagram of a multi-reflector radiolocation system based on the system of FIG.  10 A. 
     FIG. 12A is a block diagram of a single sideband active reflector in a system with the sensor of FIG.  1 . 
     FIG. 12B is a schematic of a SSB reflector of FIG.  12 A. 
     FIG. 12C plots the USB and LSB response from the sensor of FIG. 12A for phase lead modulation of the active reflector. 
     FIG. 12D plots the USB and LSB response from the sensor of FIG. 12A for phase lag modulation of the active reflector. 
    
    
     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. 
     FIG. 1 is a block diagram of a homodyne SSB pulse Doppler sensor  10  (or transceiver apparatus) of the present invention. An RF oscillator  12  receives clock pulses from TX clock (PRF generator)  14  via pulse driver  16  and produces RF burst pulses (transmit pulses) at the transmit/receive antenna  20  after passing through bi-directional RF quadrature network  18 . Alternatively, RF oscillator  12  may drive a separate transmit antenna  22  for bistatic applications. TX Clock  14  typically produces a 2 MHz squarewave that is passed through a pulse driver  16  to form ˜30 ns wide pulses. Thus, the drive pulses are clock pulses of relatively short duration. Therefore, the TX clock  14  and pulse driver  16  may together be viewed as a clock signal generator  15 . Alternatively, if TX clock  14  produces sufficiently short pulses, a separate driver  16  is not required. These short pulses bias-on the RF oscillator  12 , which is designed to start and stop oscillating very rapidly as a function of applied bias. The oscillations are phase coherent with the drive pulses, i.e., the phase of the RF sinusoids relative to the drive pulse remains constant each time the oscillator is started-there is no significant clock-to-RF jitter. 
     The TX clock  14  can be modulated in frequency or phase, or with ON-OFF gating by optional modulator  13  for various purposes known in the art, such as spreading the spectral lines generated by RF oscillator  12  to reduce interference to other spectrum users. The modulation may be coded to distinguish the radar signal from that of others. Pulse driver  16  (or more generally clock signal generator  15 ) can be adjusted by range adjust  24  to control the length of the transmit pulses, and thereby the range. The frequency of the PRF generator  14  can be changed by ΔF as further described below. 
     Bi-directional RF quadrature network (or RF quadnet)  18  couples the RF oscillator to antenna  20 , and receives echo pulses picked up by an antenna  20  and provides two outputs that are in phase quadrature, e.g., at +45 and −45 degrees. These outputs are applied to peak-to-peak (P-P) detectors  26 ,  28  which detect and peak hold the P-P value of the RF pulses coupled from RF quadnet  18 . Alternatively, these detectors can be peak, rather than P-P detectors, or they can be multipliers (i.e., mixers). The preferred mode is the P-P detector since the P-P detector presents minimal RF loading to the RF quadnet while producing maximum output. The peak hold time of the P-P detectors is quite long relative to an RF cycle—typically many microseconds to several seconds. In practice it may be fixed at about 10-microseconds and further integration of the detected signals can be effected by optional lowpass filters (LPF)  44 ,  46  at the output of sensor  10 , as described further below. 
     An alternative embodiment of the RF quadnet is based on a waveguide topology rather than discrete components, wherein a transmit feed element and I, Q elements are physically arranged in the waveguide (or antenna feedhorn) to provide quadrature outputs to the P-P detectors. One implementation is given in FIG. 5 of co-pending application Ser. No. 09/073,159 “Pulsed RF Oscillator and Radar Motion Sensor,” by McEwan. 
     The P-P detector outputs are in phase quadrature as I (in-phase) and Q (quadrature phase) signals. The I,Q signals are applied to highpass filters (HPF)  30 ,  31  to strip off rectified DC components and undesired low frequencies. After amplification by amplifiers  32 ,  33 , the signals are coupled back to the HPF via limiters  34 ,  35 . If the signals from the amplifiers exceed a certain value, limiters  34 ,  35  conduct and reduce the gain of the amplifier/HPF path. This feature greatly reduces long power-up transients and large near-field transients when the HPF&#39;s are set to low values such as 0.1 Hz cutoff frequency (e.g., for respiration monitoring). 
     Amplifiers  32 ,  33  are coupled to baseband quadnet  36  which provides a constant 90-degree phase shift between its two outputs  38 ,  40  over a wide frequency range, such as 0.1 Hz to 2 kHz. Thus, the I, Q signals from amplifiers  32 ,  33  are shifted 90-degrees. SSB matrix  42  adds the two outputs from the lines  38 ,  40  to form the LSB signal and subtracts the signals from lines  38 ,  40  to form the USB signal. Both the LSB and USB signals are analog Doppler signals corresponding to outbound (LSB) and inbound (USB) motion relative to the radar. Optional lowpass filters (LPF)  44 ,  46  may be used to further integrate the signals and limit noise or unwanted frequencies. 
     Weighted direction detector  48  squares or rectifies and filters the LSB and USB signals to provide unipolar representations of the net inbound or outbound motion through distance, i.e., displacement of a moving object. These representations can then be algebraically added and threshold detected to provide net inbound or outbound digital displacement indications  50 ,  52  that are qualified by the lack of a weighted amount of opposite displacement, as will be more fully explained with reference to FIG.  7 . Alternatively, weighted direction detector  48  can be connected to the I and Q signals for the detectors  26 , 28 . 
     FIG. 2A depicts the pulsed RF sinusoidal bursts or packets  100  produced by RF oscillator  12 , which repeat every 1/PRF of clock  14 . The width of the sinusoidal packet, which may contain anywhere from 1 to an infinite number of cycles of the sinusoidal waveform, defines the maximum sensing range of the radar, since echoes must be received before the RF packet  100  ends. This gating mechanism occurs because the P-P detector output  102  has a value related to the peak amplitude of the transmit packet  100 . Echo signals are always of much lower amplitude and can only affect the P-P amplitude if they are received in time to add with the transmit packet. When the receive packets, i.e., echoes add to the transmit packet, even by partial overlap, they amplitude modulate (AM) the P-P detector output as an AM Doppler variation  104 . The beating of a Doppler return with the transmit packet is thereby detected by the P-P detector. This arrangement is effectively a sampled data system with a long-hold peak detector connecting the samples. 
     FIG. 2B depicts the radar and antenna combination, or sensor  110 , as shown in detail in FIG.  1 . Quadrature standing waves  112 ,  114  are effectively projected from the sensor. In other words, if a large target is moved toward sensor  110 , its I and Q signals will reproduce the waves  112 ,  114  as though they are fixed in space. When a vibrating object  116  is located at points  118 ,  120 , I VIB  and Q VIB  signals  122 ,  124  are produced. Also, it can be understood that an object vibrating at a peak or valley of wave  112  or  114  will not produce a vibration signal since the slope of the standing wave is zero. 
     Note that the vibration is substantially less than one cycle, as indicated by the intensified lines at points  118 ,  120 . However, the I VIB  and Q VIB  signals are approximately sinusoidal and in phase for a sinusoidal vibration. If a vibrating object  126  is located at points  128 ,  130 , out of phase I VIB  and QV IB  signals  132 ,  134  are produced. If I VIB  and Q VIB  are added, there will be either a doubling in amplitude or a null, depending on the location of objects  116 ,  126 . Thus, simple addition of I, Q will not result in reliable or consistent vibration sensing. 
     If I, Q are quadrature phase shifted and then added they will never add to zero since they are always orthogonal. In fact, they become constant amplitude signals, free of the usual nulls, and with a relative phase that varies 360° with every λ/4 change in range to sensor  110 . The phase shifted and algebraically added I, Q signals represent the USB and LSB outputs of sensor  110 . 
     FIG. 3 is a detailed schematic of the RF oscillator  12 , RF quadnet  18 , and P-P detectors  26 ,  28  of FIG. 1. A +5 to 0 volt drive pulse  148  is coupled to RF oscillator  12  via line  150  and capacitor-diode AC level shift network  152  to provide an approximate 0 to −5-volt bias pulse to Colpitts RF oscillator  12 . The RF oscillator output is coupled to class B biased buffer/amplifier  154 , which draws no current except when driven by the RF oscillator. The buffer/amplifier is coupled via harmonic-blocking filter  156  to form an RF pulse  158  (actual 915 MHz hardware data) to the RF quadnet  18 . Optionally, transmit antenna  22  may be coupled to the quadnet to provide a separate transmission location. This mode of transmission is generally less preferred, but has some application in bistatic radar configurations. The preferred antenna configuration is with the RF quadnet coupled to a single transmit/receive antenna  20 . 
     Quadrature network  18  is a bridge circuit having two branches, one comprised of an inductor in series with a resistor to provide a lagging phase shift, and the other arm comprised of a capacitor in series with a resistor to provide a leading phase shift. Over a band of about +/−10% of the RF carrier frequency, the phase difference appearing at nodes  160 ,  162  remains within 2° of exact quadrature, i.e., 90°. Since the carrier frequency is maintained within 1% of its design value, an accurate 90° phase shift is maintained. 
     RF pulse  158  passes through the RF quadnet  18  in transit to antenna  20 , and undergoes a +22.5-degree phase shift at node  160  and a −22.5-degree phase shift at node  162 . Received echo pulses undergo similar phase shifts to produce a net Doppler beat frequency phase shift of 90 degrees between nodes  160 , 162 . Thus, part of the 90-degree phase shift is incurred during transmission, and part during reception, making the network a bidirectional phase shift network. The relative amount of phase shift may not be equal in each direction; it is simply necessary that the total bidirectional phase shift is 90 degrees. One advantage to this type of network over a transmission line is that it acts at a single point and thus is less sensitive to voltage standing wave ratio (VSWR) effects that arise between harmonic filter  156  and antenna  20 . 
     Peak-to-peak detector  26  ( 28 ) is comprised of two diodes, one that conducts on the positive half of the RF cycle appearing at node  160  ( 162 ) and the other conducting during the negative half cycle. The voltage appearing at I det  (Q det ) equals the peak-to-peak RF voltage minus two diode drops. 
     P-P detectors  26 ,  28  are coupled to nodes  160 , 162  and provide I DET  and Q DET  detection voltages to the baseband quadnet of FIG.  5 A. The resistors and capacitors associated with these detectors are scaled to provide peak-hold times that span more than one RF pulse repetition, and thus perform coherent integration of the individual RF pulses. 
     FIG. 4 details the HPF  30 , amplifier  32 , and limiter  34  as block diagrammed in FIG.  1 . HPF  30  is an RC network which couples the I DET  signal to amplifier  32 , whose output is coupled to limiter scaling amplifier  170 . The operation of the other channel (Q DET ) is similar. The gain of the scaling amplifier sets the voltage required at the output of amplifier  32  to drive limiter diodes  34   a, b  of limiter  34  into condition to shunt away excess input signal to HPF  30 . An advantage to limiter  34  is that the HPF input capacitor  172  can be rapidly brought into a near zero voltage condition at node  174 . Input capacitor  172  is often very large and would take an enormous time to charge were it not for diodes  34   a, b.  Under normal operation, the Doppler voltages at node  174  are too small to cause diodes  34   a, b  to conduct. Diodes  34   a, b  are formed of back-to-back transistor diode junctions rather than discrete diodes due to the ready availability of very low leakage junctions in low cost transistors, e.g., 2N3904, as opposed to the scant availability of low cost, low leakage discrete diodes. 
     FIG. 5A illustrates the baseband quadnet  36  and SSB matrix  42  as block diagrammed in FIG.  1 . The baseband quadnet is comprised of a dual ladder of active allpass networks, which are well-known in the art. The SSB matrix adds and inverts the two quadnet output channels  38 ,  40  to form the LSB output, and subtracts and inverts the two quadnet channels to form the USB output. For best performance, all resistors and capacitors in FIG. 5A must be of a precision value (e.g., 1% accuracy or better). Some applications of the present invention operate at a single baseband frequency, so only one stage of the allpass quadnet is needed, rather than the 8 stages shown in FIG. 5A, which yields greater than 4 decades of frequency span. In general, N stages can be used, where N is an integer. 
     FIG. 5B plots the phase difference at the outputs  38 ,  40  of quadnet  36  across a broad frequency range, assuming the two inputs to the quadnet have zero phase difference. Most allpass phase splitters of this type operate over an octave or perhaps a decade of frequency range. FIG. 5B shows over four decades of range with only an 8-stage network. This mathematical simulation was verified in hardware. 
     FIGS. 5C, D plot the USB and LSB channel separation versus frequency, for a +/−20 Hz span and +/−2 KHz span about an RF center frequency of 916 MHz. The USB, LSB crossovers in the middle of the plots are surprising sharp. These plots were made by synchronizing PRF clock  14  to an HP8660C frequency synthesizer set to sweep +/−20 Hz in FIG.  5 C and +/−2 KHz in FIG. 5D about 916 MHz, in 1 Hz steps. Since RF oscillator  12  is phase locked to the PRF clock  14 , the RF oscillator has precise spectral lines every multiple of the PRF. The PRF was set to 2 MHz and the RF pulse width was 30 ns. Some of the coarseness seen in FIGS. 5C and 5D is due to the discrete steps, as well as aliasing artifacts in the plots. The baseband quadnet was fabricated with 5% components, which limited the separation levels, as seen by the slight signal at 180. 
     FIG. 6A depicts sensor  110 , which is based on FIG. 1, in a general vibration sensing setup. RF pulses  200  are transmitted to and reflected from an object  204 . In some arrangements a non-opaque material  202  is in the propagation path. Any or all of the sensor  110 , material  202 , or reflecting objects  204  may be spatially translated or rotated as indicated by the arrows in FIG. 6A, generally  210 , for imaging or application specific purposes. In some cases object  204  is simply translated or rotated through large distances, e.g., a moving person, car, or fan blade. In other cases, object  204  may vibrate, possibly as the result of exciter  208 , which may be a mechanical, acoustic (e.g., a microphone application), magnetic (e.g., a loudspeaker application), optical, or electromagnetic stimulus. Processor  212  can perform various operations on the USB, LSB Doppler sidebands, such as squaring, integrating and summing to provide a signed magnitude signal. Naturally, many other processing functions are possible, including taking the FFT of the Doppler sideband signatures. 
     One application for the setup of FIG. 6A relies on the detection of a shift in vibration characteristics due to overcoatings  206  on vibratory object  204 . The overcoating may be plating, polymer coatings, contaminants, etc., the thickness or other characteristics of which may be assessed on a non-contact basis with sensor  110 . 
     FIGS. 6B, C illustrate a vocal cord microphone application of FIG. 6A, wherein a headset combines a conventional boom microphone  220  with an antenna  222  or  224  of the sensor of FIG.  1 . Boom  226  houses a cable that connects the antenna  222  or  224  to bidirectional quadnet  18 . Antenna  222  is a dipole comprised of a radiator  227  and ground element  228 , which are generally connected to the shield of the interconnect cable. Antenna  224  is a dipole comprised of radiator element  230  and ground element  232 , which also serves as a mechanical support for acoustic microphone  220 . Obviously, other variations on the dipole antenna are possible. Using antenna  222 , the response to the sound “hmmm” is plotted in FIG. 6F for the I and USB signals of FIG.  1 . 
     FIG. 6D plots the I and USB responses to vibrating metal tweezers versus range. The problematic response nulls discussed with respect to FIG. 2B are clearly seen in the I channel but not in the USB channel, thereby demonstrating the value of SSB demodulation. 
     FIG. 6E plots the response of a person walking away or OUT from the sensor of FIG. 6A, pausing, and then walking IN to illustrate the separation of the Doppler sidebands. A slight channel bleed-through  240  results from non-precision components in the baseband quadnet  36 . 
     FIG. 6G reveals simultaneous inbound and outbound components of cardiac activity using a simple dipole antenna pressed to the chest, using a system depicted schematically in FIG.  6 A. This SSB signature is more consistent versus antenna location on the chest than prior art radars due to the absense of sensitivity nulls. The USB and LSB waveforms may be further processed into a signed magnitude indication for a more general and consistent signature. When combined with an EKG waveform, cardiac stimulus-response data becomes available with promising diagnostic/monitoring applications, including non-contact monitoring during surgery or long-term care. 
     FIG. 6H plots respiratory motion for the setup of FIG. 6A at about 1-meter range, indicating the potential for non-contact patient monitoring or SIDS detection. 
     FIG. 6I shows two distinct signatures from a cordless hammer drill at about {fraction (1/2 )} meter range, indicating the potential for the setup of FIG. 6A to monitor or diagnose machine motion. 
     FIG. 6J debuts a stereo guitar pickup. With a 915 MHz dipole positioned in parallel to the guitar strings and about 10 cm away, a large variety of distinctly different waveforms can be observed in the USB and LSB “stereo” channels even for a single plucking of the string. Listening tests with stereo headphones provide an interpretation of FIG.  6 J: both channels are of the same amplitude and have the same frequency, but the tonal quality is very different in the right and left headphones. Indian sitar players claim all of music is contained in a single note—it&#39;s all in the harmonics, as can be heard in the open and airy dimensionality of this stereo SSB Doppler radar pickup. 
     FIG. 6K was produced by rotating a metal blade 5 cm away from a 915 MHz dipole for the setup of FIG. 6A, using the I channel as a sync pickoff for the scope. As can be seen, the phase of the Q channel depends on the direction of rotation. Thus, a single dipole can sense the direction of blade rotation. 
     FIG. 7 is a block diagram of a weighted direction detector  48  of FIG. 1, which is generally used in the setup of FIG. 6A for moving objects. The USB, LSB outputs from the sensor of FIG. 1 are applied to absolute value circuits  250 ,  252 , or alternatively squaring circuits, to provide a unipolar fluctuating signal that is filtered in LPFs  254 ,  256  to produce an integrated motion-through-distance, or displacement, signature on line  258  for inbound movement, and on line  260  for outbound movement. Summation element  262  subtracts outbound signatures from inbound signatures with a weight W L &gt;1 (typically W L ˜10) assigned to the outbound channel (LSB), and similarly for summation element  264 , which subtracts inbound signatures from outbound signatures with a weight W U &gt;1 assigned to the inbound channel. Thus summation element  262  outputs a positive displacement signature from line  258  for inbound displacement, unless there is simultaneously an outbound displacement signature (normally an impossibility for a moving car or person, etc.). If there is a concurrent outbound displacement, the output of summation element  262  will swing sharply negative due to the application of the signature on line  260  to summation input —W L . Accordingly, arbitrarily small amounts of a concurrent opposite displacement will prevent positive outputs from occurring at the summation outputs. Bona fide displacements will cause a positive summation that can exceed thresholds V R1 , V R2  of threshold comparators  266 ,  268  and produce a digital IN or OUT indication (which may be further processed by processor  270  to control a process, etc). 
     Obviously, RF interference will produce roughly equal signatures on lines  258 ,  260 . By virtue of weights W L , W U , the probability of a positive summation and potential false triggering approaches zero as W L , W U  are increased in magnitude. Consequently, a sensor based on this scheme will become increasingly blinded with increasing RF interference, but may never trigger a false alarm due to RF interference. 
     FIG. 8A illustrates the general setup for measuring a material thickness, and in particular, the height of a liquid  301  in a tank  310 . Sensor  110  transmits RF pulses  200  through a material or liquid  301  which then reflect off a vibrating or active reflector  300 . Active reflectors will be discussed in further detail with reference to FIG.  10 A. Reflector  300  is an actively modulated antenna such as a dipole connected to a switch that is rapidly opened and closed under control of exciter  302 . Alternatively, another active reflector  304  may be a florescent lamp, a mechanically vibrating device, or any reflector capable of phase modulating the transmitted RF pulses  200 . 
     The USB, LSB outputs from sensor  110  have a phase relationship that varies with distance to the active reflector  300 , or alternatively with the combined height and dielectric constant of the intervening material. The USB, LSB phase is a linear function of apparent reflector range and is measured by phasemeter  306  to provide a depth or thickness indication, which may be processed by processor  308  to operate a level controller or other responsive apparatus. 
     Active reflector  300 ,  304  is shown outside the tank  310  for convenience, but may be located inside the tank, e.g., mounted on a rod  312  attached to the sensor  110  to form a unitized dipstick. 
     The center frequency of RF oscillator  12  can be changed by a small amount ΔF, by changing the PRF of clock  14  by ΔF as shown in FIG. 1, to offset the USB, LSB phase readings and produce a vernier effect with a range ambiguity related to 1/ΔF, which is much longer than the USB, LSB phase ambiguity which repeats every λ/4 in range. Accordingly, by comparing the USB, LSB phase at two frequencies separated by ΔF, coarse range measurements can be made across a range span related to 1/ΔF, while fine range measurements can be made by looking at the USB, LSB phase at just one frequency. The phase vernier effect based on ΔF is well-known in other applications. 
     FIG. 8B plots the USB response, which is used to trigger an oscilloscope as a timing reference, and the LSB response which is seen to vary in phase at ranges R 1 , R 2 , representing R 2 —R 2 ˜2 cm change in range. A pair of 250 Hz vibrating tweezers was used as the active reflector. The phase shift versus range cycles through 360 degrees for every ¼ wavelength of RF. This phase shift is independent of the modulation or vibration frequency of the active reflector. 
     FIG. 8C plots the change in phase when 3 cm of paper is inserted between the sensor  110  and the reflector  304 , indicating the invention&#39;s use as a material thickness gage, or a dielectric constant sensor—as may be useful with crop harvesters or dryers. 
     FIG. 8D plots the change in phase when 1 cm of water is inserted between the sensor  110  and the reflector  304  (i.e., 1 cm of water added to a tank containing 10 cm of water (REF waveform)). 
     FIG. 9A depicts an unusual active reflector modality based on reflector Δ-RCS or change in radar cross section, usually as an incidental or unanticipated effect. This effect can be ascribed to the fact that two objects have a different RCS when separated than when in electrical contact. For example, two quarter wave metal strips reflect differently when isolated than when in contact to form a single half wavelength reflector. If the two strips intermittently contact, they will produce a readily detectable noise-like signature. 
     FIG. 9B illustrates the Δ-RCS effect when two smooth metal rods (actually very smooth stainless steel tweezers) are gently slid or scraped across each other. 
     The system of FIG. 9A is similar to that of FIG.  6 A. Object  360  comprises two rods or other objects  359 ,  361 . In some arrangements a non-opaque material  202  is in the propagation path. Any or all of the sensor  110 , material  202 , or reflecting objects  359 ,  361  may be spatially translated or rotated as indicated by the arrows in FIG. 6A, for imaging or application specific purposes. Object  356  may vibrate, possibly as the result of exciter  208 , which may be a mechanical, acoustic, magnetic, optical, or electromagnetic stimulus. Processor  212  can perform various operations on the USB, LSB Doppler sidebands, such as squaring, integrating and summing to provide a signed magnitude signal. Naturally, many other processing functions are possible, including taking the FFT of the Doppler sideband signatures. 
     FIG. 9C provides a glimpse of the potential for the Δ-RCS detection method to find gold. Often, gold nuggets are suspended in quartz or other rocky materials that have fissures. When the fissures are flexed or moved slightly by a pressure P applied to the general area, nuggets lying on the fissure will scrape and produce a Δ-RCS signature as seen in FIG.  9 C. Nuggets embedded in quartz from a California gold mine were detectable at 2-meters range using a makeshift radar of the present invention. Pressure P may be propagated to the nuggets by a blow to a mine wall by a sledgehammer or small explosive charge, and sensor  110  may be outfitted with headphones or a recording means to detect Δ-RCS signatures, which will mainly occur from scraping metals embedded in the rock. The detection of gold nuggets  309  in a material  350  is illustrated in FIG. 9A by replacing object  360  with material  350  and applying pressure P to material  350 . 
     FIG. 9D indicates yet another use for the present invention—non-contact readout of electronic devices based on the Δ-RCS produced by switching transistors and their interconnects, or by variable conductivity devices such as audio transistors and their interconnects. FIG. 9D is a signature from a solar powered electronic calculator. The signature can be played into a loudspeaker, where the various processing functions can be heard as an amusing novelty. Obviously, toys can be based on this invention, such as a doll or Easter egg containing a reflector that voice modulates the radar return when illuminated by a toy radar “flashlight.” 
     FIG. 10A depicts sensor  110 , which is based on FIG. 1, in a general active reflector setup. RF pulses  200  are transmitted to and reflected from active reflector  400 . In some arrangements a non-opaque material  202  is in the propagation path. Any or all of the sensor  110 , material  202 , or reflecting objects  400  may be spatially translated or rotated as indicated by the arrows in FIG. 10A, for imaging or application specific purposes. In some cases active reflector  400  is simply translated or rotated through large distances, e.g., attached to a moving person, car, or fan blade. In other cases, active reflector  400  may be modulated in reflectance, possibly as the result of exciter or driver  406 , which may be a mechanical, acoustic, magnetic, optical, or electromagnetic stimulus. The driver can be actuated by a voice, music, CW or data signal  408 . Exciter  406  may be a transistor switch or other electronic driver. Processor  212  can perform various operations on the USB, LSB Doppler sidebands, such as squaring, integrating and summing to provide a signed magnitude signal. Naturally, many other processing functions are possible, including taking the FFT of the Doppler sideband signatures. A unique capability of this setup is the ability to measure the range between sensor  110  and reflector  400  from the USB/LSB phase relation, similar to that shown in FIGS. 8B, C, D. 
     Active reflector  400  is comprised of elements  402 , switch  404  connecting the elements  402 , and switch driver  406  connected to switch  404 . Element  402  may be a monopole or each of two dipole antenna elements, ideally λ/4 long. Alternatively, reflector  400  may be any reflecting arrangement, including a waveguide-mounted switch or reflecting plasma such as a florescent lamp. 
     FIGS. 10B illustrates the ability of the active reflector system of FIG. 10A to transmit voice information. This data was obtained by coupling the output of an AM radio as a driver  406  to an active reflector  400  wherein the switch  404  was formed of a silicon bipolar transistor. Uses for this configuration include RFID tags that “speak” their data when illuminated by a beam  200  from sensor  110 , with a loudspeaker connected to the USB or LSB output of sensor  110 . 
     FIG. 10C plots data for an RF beam interruption application using the setup of FIG.  10 A. Here, material  202  is opaque so it blocks beam  200  whenever inserted between sensor  110  and reflector  400 . The blockage  410  is seen in the data of FIG. 10C for the passage of a human arm for a sensor-reflector spacing of 0.5-meters. Applications for the RF beam interruption mode of FIG. 10A include machine safety sensors, pedestrian and automotive traffic control systems, home security and automation, etc. Obviously, more than one active reflector  400  can be used with a single sensor  110 , each having a different frequency or code provided by driver  406 . Driver  406  can be designed for micropower operation to allow solar power or multi-year battery operation of the active reflector. 
     FIG. 11 depicts a radiolocation application for the system of FIG.  10 A. Sensor  110  transmits beams  412 ,  414 , and  416  to active reflectors  424 ,  426 , and  428 . There may be intervening non-opaque materials  418 ,  420 , and  422  in the beam paths. These materials may be unintentional or unavoidable, such as building walls, or they may be placed in the beam path to obtain information about the material such as thickness or dielectric constant, or for imaging. Each active reflector may be time sequenced, multi-tone, or code modulated to allow all active reflectors to operate with sensor  110  on a non-interfering basis. The range R 1 , R 2 , R 3  to each reflector is deduced from the USB, LSB phase, as seen in FIGS. 8B, C, and D. Given the knowledge of R 1 , R 2  and R 3 , and the locations L 1 , L 2 , and L 3 , the location Lx of sensor  110  can be calculated. Depending on the number and location of the reflectors, not all ranges and locations need to be known to fix the location of other elements in the system. Potential uses for this system include machine control, inventory tracking and digital surgery. A feature of this system is its ability to obtain high resolution with relatively low RF frequencies, since the phase of the RF carrier is measured with very high resolution. 
     As in FIG. 8A, the center frequency of RF oscillator  12  can be changed by a small amount ΔF to offset the USB, LSB phase readings and produce a vernier effect with a range ambiguity related to 1/ΔF, which is much longer than the USB, LSB phase ambiguity which repeats every λ/4 in range. 
     FIG. 12A depicts a sensor  110  in an active quadrature reflector setup. RF pulses  200  are transmitted to and reflected from active quadrature reflector  500  formed of elements  502  which are connected to quadrature network  504 . In some arrangements a non-opaque material  202  is in the propagation path. Any or all of the sensor  110 , material  202 , or reflecting objects  500  may be spatially translated or rotated as indicated by the arrows in FIG. 10A, for imaging or application specific purposes. In some cases active reflector  500  is simply translated or rotated through large distances, e.g., attached to a moving person, car, or fan blade. In other cases, active reflector  500  may be modulated in reflectance, possibly as the result of quadrature driver  506 , which may be a mechanical, acoustic, magnetic, optical, or electromagnetic stimulus, generally providing quadrature phase baseband data (Φ1, Φ2) in response to inputs (CTRL and MOD) to RF quadrature network  504 . Processor  212  can perform various operations on the USB, LSB Doppler sidebands, such as squaring, integrating and summing to provide a signed magnitude signal. Naturally, many other processing functions are possible, including taking the FFT of the Doppler sideband signatures. A unique capability of this setup is the ability to measure the range between sensor  110  and reflector  500  from the USB/LSB phase relation, similar to that shown in FIGS. 8B, C, and D. 
     FIG. 12B is a schematic of one embodiment of quadrature network  504 . Quadrature baseband data (Φ1, Φ2), which may be analog or digital, is provided by a quadrature driver  506  to network  504 . Network  504  contains two switches comprised of bipolar transistors  507 ,  508  that switch phase shifting elements  510 ,  512  to ground, thereby modifying the phase of the reflections from antenna/reflector elements  502  of reflector  500 . Clearly, other embodiments of switch elements  507 ,  508 , whether analog or digital, and phase shift elements  510 ,  512  are possible within the scope of the invention. 
     FIGS. 12C and D provide an indication of the USB, LSB channel separation possible with the setup of FIG.  12 A. FIG. 12C shows the responses at the USB, LSB outputs of sensor  110  with Φ 1  leading Φ 2 , thereby producing a response in the USB channel only. FIG. 12D shows the LSB response when Φ 1  lags Φ 2 . The quad driver  506  input labeled CTRL controls the phase lead/lag condition, or sideband selection, and the quad driver input labeled MOD is the voice, data or tone modulation input, which is split into two baseband phases Φ 1 , Φ 2  within the quad driver  506  function. 
     When the active reflections are set for LSB, reflector  500  appears to be moving away from sensor  110  of FIG. 12A, and when the reflections are set for USB, reflector  500  appears to be moving towards sensor  110 . If the I, Q outputs of sensor  110  (as indicated in FIG. 1) are observed, reflector  500  appears to be rotating in a direction defined by the phase relation between Φ 1 , Φ 2 . 
     Although the invention has been described with reference to a 915 MHz system because of the enhanced materials penetration at this frequency, the principles of the invention can be applied to other frequencies, e.g., 2.4 GHz, 5.8 GHz or 24 GHz. Changes and modifications in the specifically described embodiments, including the use of multiple sensors or reflectors, 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.