Sensor front-end with phase coding capability

A sensor front end for an electronic radar sensor is disclosed that provides for a lower parts count while providing technical functionality by using multifunction parts, i.e., parts that are used both in transmitting and receiving. The sensor front end includes a continuous wave signal source that functions as a signal source when the front end is transmitting a signal and as a local oscillator when the front end is receiving a signal. The sensor front end also includes a tri-mode mixer that functions as a phase-modulator and transmit switch when the front end is transmitting a signal and as a mixer/down-converter when the front end is receiving a signal. The sensor front end further includes a common aperture antenna that acts as both a transmitting antenna for transmitting a sensor signal and for receiving a reflected signal from a object. A phase shifter can be added to provide a predetermined phase shift in the transmitted sensor signal, the received reflected signal, or both, such that in-phase and quadrature signal components are provided. In addition, phase coding may be added to the signal to reduce the degenerative impact of interfering signals. A receiver module is coupled to the tri-mode mixer such that, when receiving a reflected signal, the receiver provides a baseband sensor output signal that can be used to determined the position and velocity of the object. A sampling module can be added such that the sensor output signal is sampled and provided as an analog signal, or the sampled sensor output signal can be provided to an analog-to-digital converter to convert the sensor output signal into a digital format, or both.

CROSS REFERENCE TO RELATED APPLICATIONS

BACKGROUND OF THE INVENTION

Proximity sensors of various types are used in a variety of applications in which the distance to an object and, in some circumstances, the closing velocity of that object are to be determined. This data is often provided to a processing system for analysis. Typically, this analysis determines whether one or both of the distance and velocity exceed a predetermined safety threshold and whether an alarm is to be set or other action taken. Proximity sensors are used, for example, in a variety of applications that can include burglar alarms, obstacle detection, and automobiles. Proximity sensors in automobiles are used to determine the relative position and closing velocity of other automobiles or objects in the vicinity of the automobile. These sensors must be physically small, light weight, highly reliable, and low cost. The requirements of the systems that utilize these sensors are often quite stringent both in terms of performance and in the physical and economic factors as well. The more complex the sensor, the larger the parts count, and concomitantly, the higher the cost, the higher the mass, the larger the physical volume of the sensor and the lower the reliability of the sensor.

Therefore, it would be desirable to provide a sensor that utilizes fewer components to reduce the cost, size, and weight of the sensor and provide the necessary functionality and reliability.

BRIEF SUMMARY OF THE INVENTION

A sensor front end for an electronic sensor is disclosed that provides for a lower parts count while providing technical functionality by using multi-mode parts, i.e., parts that are used both in transmitting and receiving. The sensor front end includes a continuous wave signal source that functions as a signal source when the front end is transmitting a signal and as a local oscillator when the front end is receiving a signal. The sensor front end also includes a tri-mode mixer that functions as a phase-modulator when the front end is transmitting a signal and as a mixer/down-converter when the front end is receiving a signal. The sensor front end further includes an antenna that acts as both a transmitting antenna for transmitting a sensor signal and for receiving a reflected signal from a object. A phase shifter can be added to provide a predetermined phase shift in the transmitted sensor signal, the received reflected signal, or both, such that in-phase and quadrature signal components are provided for. A receiver module is coupled to the tri-mode mixer such that, when receiving a reflected signal, the receiver provides a baseband sensor output signal that can be used to determine the position and velocity of the object. A sampling module can be added such that the sensor output signal is sampled and provided as an analog signal, or the sampled sensor output signal can be provided to an analog-to-digital converter to convert the sensor output signal into a digital format, or both.

In particular, a sensor front end is disclosed that includes an antenna having an antenna port and a common aperture for transmitting a sensor signal and receiving a reflected signal. A continuous wave signal source is coupled to a first input of a tri-mode mixer that provides a predetermined amount of signal between the first input and a first input/output port. The signal provided from the first input port to the first input/output port is pulse and phase-modulated by a phase-modulation signal provided to a second input/output port of the tri-mode mixer by a phase-modulator. The phase-modulated signal exits the tri-mode mixer at the first input/output port and is provided to the antenna port for transmission therefrom as the sensor signal. If a object is within the beam width of the antenna, a portion of the sensor signal is reflected back to the antenna aperture as the reflected signal and is coupled to the first input/output port of the tri-mode mixer. The continuous wave signal source coupled to the first input acts as a local oscillator and the tri-mode mixer mixes the local oscillator and the received reflected signal and provides a baseband video output signal from the second input/output port. A transmit-receive switch is used to switch the second input/output port between the phase-modulator and a receiver-processor. The receiver processor includes a phase-demodulator that demodulates the baseband video-output signal and provides the demodulated baseband video-output signal as a sensor output signal.

Other forms, features and aspects of the above-described methods and systems are described in the detailed description that follows.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1depicts a block diagram of the architecture of a sensor front end consistent with the present invention.FIGS. 2 and 3depict the transmitter functions including a phase-modulator and the receiver functions including phase code demodulator respectively.FIGS. 4-9depict circuits suitable for use in the sensor front depicted inFIGS. 1-3, andFIG. 10depicts another embodiment of the sensor front end. Although the following embodiments are described with respect to microwave frequencies and components, the apparatus and methods described herein may be applied to other frequencies and systems.

As depicted inFIG. 1, a controller104provides a plurality of control signals to ensure the proper timing and operation of the various components in the sensor front end100. The sensor front end100includes a transceiver106that can simultaneously transmit a phase-modulated sensor signal121from an antenna102and coherently receive a reflected signal123and down convert this signal for further processing as a baseband video-signal.

In particular, the transceiver106receives two control signals from the controller104. A first control signal is a transmit-receive control signal101that determines the routing of signals within the transceiver106. A second control signal is provided to the transceiver106as an I/Q signal115which determines if the sensor signal is an “in-phase” or “quadrature phase” signal. The controller determines if the signal is to be a quadrature signal and if so, the controller will provide the I/Q signal115to a phase shifter(not shown inFIG. 1) to phase shift one or both of the transmitted and received signals. Transceiver106also receives a phase-modulation signal121from phase-modulator108. Phase-modulator108receives a phase-modulator control signal113from the controller104and is responsive to the phase-modulator control signal113by selecting one of a plurality of predetermined signal phase states. In the illustrative embodiment two signal states are employed. The phase-modulator108provides the phase-modulation signal103that corresponds to the phase-modulation control signal113to the transceiver106. Transceiver106phase-modulates the transmitter signal using the phase-modulation signal103.

The phase-modulated transmitter signal is provided to the phase shifter108and if the transmitter signal is a quadrature signal, the transmitter signal is phase shifted by the phase shifter108. This phase-modulated, and possibly phase shifted, transmitter signal is provided to the antenna port and radiated from the common aperture of the antenna102as sensor signal121. If an object122is present within the beam width of the antenna102, the object122reflects the sensor signal121and the antenna captures a portion of the reflected signal123in the common aperture. The captured portion of the reflected signal123is provided to the transceiver106, via line119, for down-conversion and further processing.

Transceiver106receives the captured portion of the reflected signal123via antenna102and line119and provides, as an output, a baseband video output signal117. The baseband video output signal117is the down-converted video signal and representative of the amplitude and phase of the captured portion of the reflected signal123. The down-converted baseband video signal117is provided to a pre-amp110coupled to the transceiver106. The preamplifier110provides, as an output, an amplified signal that is a function of the broadband video signal output117received from transceiver106. Preamplifier110, in one embodiment, is also coupled to controller104and receives a sensitivity time control (STC) signal therefrom. The STC signal is a gain control signal to reduce a receiver gain setting for nearby objects or objects to prevent the receiver from saturating from the reflected signals from a nearby object.

The output of pre-amp110is provided to the input of phase-demodulator112. The phase-demodulator112also receives a phase-demodulation signal107from the controller104and is responsive to the phase-demodulator signal by applying a phase-demodulator scheme to the preamplified baseband video-signal that is the converse of the phase-modulation scheme selected by phase-modulator108. The phase-demodulator112provides, as an output, a phase-demodulated signal.

A sample and hold module114is coupled to the phase-demodulator112and to the controller104. The sample and hold module receives a sample and hold signal109from the controller104and is responsive to the sample signal109by sampling the phase-demodulated output signal. The sample and hold module114provides this sampled signal as an analog sensor output signal118. The sampled signal may also be coupled to an analog-to-digital converter116that provides a digitized output120of the sensor outputs118for digital storage and analysis.

FIG. 2depicts the transmitter and phase-modulator portion of the sensor front end100depicted in FIG.1. In particular,FIG. 2depicts the microwave components201and the transmit and phase-modulator components203. The microwave wave components201include a continuous wave (CW) signal oscillator202that provides a transceiver signal having a first frequency, a first amplitude, and a first phase. As discussed above, the transceiver signal is used during transmission, when it is the signal that will be modulated and radiated from the antenna102as the sensor signal121. In addition, the transceiver signal is also used when receiving a captured portion of the reflected signal123as the local oscillator signal in the tri-mode mixer that is used to down-convert the captured portion of the reflected signal123. The (CW) signal oscillator202can be any active element consistent with the desired operating frequency. Typically for a desired operating frequency through X-Band, a bipolar junction transistor is appropriate, and for desired frequencies through W-band, field effect transistors or GUNN devices are appropriate. A high Q resonator (not shown) may be added to provide increased frequency stability.

Mixers in general are used in transmitters as up-converters and phase-modulators and in receivers as down-converters. Typically a mixer will have two inputs, one receiving the local oscillator signal and the other receiving the signal to be down-converted. In the sensor front end disclosed herein, the tri-mode mixer204functions not only as a mixer in the receiver mode, but will also function as a transmit pulse modulator and phase-modulator in the transmit mode. This tri-mode operation allows the sensor front end to reduce the parts count of the front end by using some of the components both while transmitting and while receiving.

The tri-mode mixer204receives the transceiver signal from the CW signal oscillator202at the first input port205. The first input/output port207of the tri-mode mixer204is used to provide the phase-modulated transceiver signal to a phase shifter206during transmitter operation, or to receive the captured portion of the reflected signal123from the phase shifter206during receiver operation.

The signal that is to be transmitted from the antenna102as the sensor signal121, is provided by the tri-mode mixer204. The tri-mode mixer204passes a signal pulse having sufficient amplitude between the first input port205and the first input/output port207such that the portion of the transceiver signal provided therethrough is provided as an output from the first input/output port207. The amount of the signal provided must be sufficient such that the portion of the transceiver signal switched from the tri-mode mixer204to the phase shifter206and transmitted from antenna102is sufficient to detect objects according to the desired system specifications. The amount of power required is typically a function of the specified detection range, the radar cross-section of the specified object, the gain of the antenna, and the sensitivity of the receiver. In addition, when transmitting, the tri-mode mixer operates as a phase-modulator and will adjust the phase state of the transceiver signal in response to the phase-modulation signals115received at a second input/output209from the phase-modulator203.

In the embodiment depicted inFIG. 2, the phase-modulator203employs a bi-phased shift keyed “BPSK” phase-modulation scheme, wherein the BPSK phase-modulation scheme includes two phase states representative of a zero “0” and a one “1”, which in the illustrated embodiment are either in phase, i.e. 0 degrees out of phase with one another, or out of phase, i.e. 180 degrees out of phase with one another respectively. The phase-modulator203provides one of these two phase states to the balanced mixer input/output204. The particular choice of the two bi-phase components is selected via switch212that is controlled by signal113from controller104. In the illustrated embodiment, the two phase states are created by the reverse polarity of the voltage sources214and216and applying one of the two opposing polarities to the second input/output port209of the tri-mode mixer204.

The switch210provides for the switching of the transceiver transit/receive function under control of transmit-receive signal101received from the controller104. Thus, when the switch210is in the transmit, “T”, position, phase-modulator203, responsive to the phase-modulation control signal113, provides the phase-modulation signal103to the second input/output209of the tri-mode mixer204. The phase modulation signal is applied to the transceiver signal and is reflected in the phase of the transceiver signal provided, as an output, from the first input/output207of the tri-mode mixer204.

As discussed above, in a preferred embodiment the transceiver can be operated in an in-phase/quadrature phase-mode (“I/Q mode”). In this embodiment, phase shifter206, in response to the I/Q signal115, shifts the phase of the phase-modulated transceiver signal by a predetermined number of degrees prior to transmission from the antenna102. In a preferred embodiment, the transceiver signal is shifted by 45 degrees prior to transmission by the antenna102. I/Q mode is particularly advantageous for detecting stationary objects, or objects that maintain a constant distance from the antenna, by using two signals that are 90 degrees out of phase with one another, i.e., the two signals are orthogonal to one another. Each of the two orthogonal signals represent position vectors in an orthogonal vector space and the vector that results from the addition of these two signal vectors represents a position vector to the object for the time period of the two measurements.

FIG. 3depicts a transceiver106when switch210is in the receiver, or “R” position. In particularFIG. 3illustrates the microwave devices201which include the CW signal source202, the tri-mode mixer204, the phase shifter206and antenna102receiving reflected energy123. A portion of the reflected signal energy123is captured by antenna102and is provided to phase shifter206. As discussed above, when the sensor front end100is to be operated in an I/Q mode, the phase shifter206provides a 45 degree phase shift prior to transmission from antenna102. When operating in the I/Q mode the phase controller206shifts the captured portion of the reflected signal123by a predetermined amount, which typically is 45 degrees. In this way, the received signals are 90 degrees out of phase, i.e., are in quadrature. In one embodiment in which multiple pulses are used for each signal, there is no overlap between the I & Q pulses such that a predetermined number of in-phase pulses are transmitted followed by a predetermined number of quadrature pulses. If the sensor front end is not operating in I/Q mode, the phase shifter206passes the received reflected signal to the first input/output port207of the tri-mode mixer204.

The captured portion of the reflected signal123provided at the first input/output port207is combined with the transceiver signal from the CW signal oscillator provided at the first input205, as the local oscillator signal, in the tri-mode mixer204and down-converted signal to a baseband signal. The output of the mixer204, i.e., the baseband video signal is provided at the second input/output port209. In the illustrated embodiment the baseband video signal is derived from a BPSK phase-modulated signal and the mixing in tri-mode mixer204is coherent in nature thus, the baseband video signal will also be bi-polar, i.e., the baseband video signal will have both positive and negative voltages. The baseband video signal is connected by the transmit-receive switch210, in the R position, to preamplifier110.

The preamplifier110receives the baseband video signal and may also receive the STC command signal105. As discussed above, the STC command signal is used to adjust the gain of the preamplifier110to avoid receiver saturation that may be caused by nearby objects. The preamplified baseband video signal is provided to the phase-demodulator112that includes capacitor304and306and phase-demodulator switch308. Phase-demodulator switch308is responsive to the phase-demodulation control signal on line107provided by controller104and is used to provide the necessary phase-demodulation to the baseband video signal. Switch308switches substantially synchronously with switch212(FIG. 2) in order to provide a demodulation scheme that is substantially synchronous with the modulating signal. In the illustrated embodiment in which BPSK phase-modulation is used one output of the preamplifier110is an inverting output and the other output is a non-inverting output. The proper phase-demodulation of the preamplified baseband video signal will therefore convert the bipolar baseband video signal into a unipolar signal. The output of the demodulator112is provided to the sample module114, which is responsive to the sample signal109provided by controller104by sampling the phase-demodulated signal. The sampled signal is provided as a sensor output118, or as an input to an analog to digital converter116, or both. The analog to digital converter116is responsive to the a/d convert signal111by providing as an output a digital representation120of the sampled phase-demodulated signal.

FIG. 4depicts a series of graphs that illustrate the operation of the sensor front end depicted inFIG. 1during two consecutive transmit and receive cycles401and403respectively. Graph402depicts the sensor transmit signal121, graph404depicts the received reflected signal123, graph406depicts the baseband video signal, graph408depicts the phase-demodulated video signal, and graph410depicts the sampled output signal. In particular, the first transmitted sensor signal pulse416is arbitrarily defined as a “1” state and the second sensor signal pulse418, which is 180 degrees out of phase with the first pulse416, is arbitrarily defined as a “0” as depicted in graph402. The first and second received reflected signal pulses420and422correspond to the first and second transmitted pulses416and418respectively, and are attenuated and time delayed versions thereof as depicted in graph404. The down-converted baseband video pulses424and426corresponding to the first and second receive pulses are bipolar in nature as discussed above and depicted in graph406. The down-converted baseband video signals are bi-phase due to the operation of the tri-mode mixer in mixing the received reflected signals with the CW signal source coherently. In the illustrated embodiment, the baseband video output signals are proportional to the relative phase of the received signals compared with the coherent signals provided by the CW signal source used in the tri-mode mixer, and to the strength of the captured portion of the reflected signals. The bi-phase baseband video signals are converted to uni-phase signals using the known phase code by the demodulator as depicted in graph408. Finally, the sampled output432is provided as depicted in graph410.

FIGS. 5A and 5Bdepict two embodiments of tri-mode mixers that are suitable for use in the sensor front end100depicted in FIG.1.FIG. 5Ais a double balanced mixer (“DBM”)500that includes first and second baluns504and508respectively and a quad diode ring506. In particular, the first balun504is coupled to the first input205, which is the local oscillator “L” input. The balun504splits the input signal and provides a symmetric signal to the quad diode ring506. In this embodiment of the tri-mode mixer, the diodes501,503,505, and507can be thought of as switches, and in particular pairs of switches that are used to reverse the polarity of the signal applied to the second balun508. Diodes507and503form a first diode switching pair and diodes501and505form a second diode switching pair.

When operating as a transmitter, the two pairs of diode switches are turned on and off by applying a positive or negative current to the second input/output port209. A positive current provided to the second input/output port209will turn on diodes507and503and turn off diodes501and505. Similarly, a negative current applied to the second input/output port will turn on diodes501and505and turn off diodes503and507. In this way, the phase of the signal transmitted from the L input, i.e., the first input205to the R input, or the first input/output port207can be affected.

When operating as a receiver, the pairs of diode switches are turned on and off by the signal provided at the first input205, i.e., the L input of the tri-mode mixer. For a positive going signal, diodes503and507are turned on and diodes501and505are turned off. For a negative going signal at the first input205, diodes501and505are turned on and diodes503and507are turned off. As can be seen this will have the effect of reversing the polarity of the output balun508, effectively multiplying the signal input from the first input/output port by a series of pulses at the first frequency, effectively mixing the two signals together.

FIG. 5Bdepicts another embodiment of a tri-mode mixer suitable for use in the sensor front end depicted in FIG.1. Mixer520is a single balanced mixer. The single balanced mixer520has intrinsic isolation between the first input port205and the first input/output port207by the null associated with the bipolar drive signal, from balun524, across diodes521and523. Inductor530is provided to prevent RF energy from the first input/output port207from entering the second input/output port209. Capacitor528is added to prevent the down-converted baseband video signal from leaking through the first input/output port209.

When operated as a phase-modulator when transmitting the sensor signal, positive and negative current input to the second input/output port209will switch diodes521and523on and off accordingly. This will have the effect of reversing the phase of the signal leaking through the mixer from the first input port205to the first input/output port207.

When operated as a mixer when down-converting the received reflected signal, as the signal from the first input port205changes, diodes521and523will be biased on and off accordingly. In this way, the signal from the first input port205is mixed with the received signal input from the first input/output port207in either diode521or523.

FIGS. 6A and 6Bdepict an embodiment of a single pole double throw (SPST) switch using FET switches and a suitable pulse generator to control the FET switches respectively. In particular, a SPDT switch602and an equivalent circuit using a pair of FET switches600is shown inFIG. 6A. Afirst FET604is normally connected to an input terminal605using a first voltage applied to the gate608. A complimentary voltage is applied to the gate610of the second FET606to turn off the second FET and provide a normally open contact. As the voltages applied to the gates612and614are switched, the center terminal605is disconnected from the normally closed terminal and connected to the normally open terminal. For modest switching speeds, e.g., 10 nsec or more, properly configured TTL logic circuits may be used to provide the pulses used to switch the FET switches.

If faster pulses are required, a pulse generator620suitable for use with the FET switches is depicted inFIG. 6B. Astep recovery diode (SRD)642is used that has a rapid transition time, i.e., it will rapidly switch from a conducting to a non-conducting state when a reverse bias is applied. As the drive signal from driver624falls, the SRD will switch off and the fast negative going pulse is provided to the capacitors644and652will differentiate the negative going pulse and provide an negative going impulse654at the first output647, and a complimentary pulse at a second output649.

FIG. 7depicts one embodiment of a phase shifter206suitable for use with the sensor front end described herein. Phase shifter206includes a quadrature hybrid706that has four terminals,702,704,705, and707. Terminal702is arbitrarily set as the input terminal. Quadrature hybrids are devices that divide an input signal at one terminal into two signals that are output on the terminals on the opposite side of the hybrid. The two output signals typically have one-half the power of the input signal and are ninety degrees out of phase with one another. In the illustrated embodiment, a signal input at terminal702will be divided and phase shifted between terminals705and707. If a signal is input at terminal702any reflections present at terminals705and707will be propagated through the hybrid and will be output at terminals702and704. As such, the impedance and reflectivity of any transmission line or circuit elements coupled to the terminals705and707can cause reflections back into the quadrature hybrid706and provide a phase shifted version of the signal input at terminal702as an output at terminal704. Transmission lines708and710are a quarter wavelength at the frequency of interest and will act as impedance transformers for the terminating impedances. The phase shift control signal is input to terminal722and will act to turn on or off the pin diodes712and714. When on, the PIN diodes will short the terminal end of the quarter wavelength transmission lines708and710to ground resulting in a reflectivity of 1 and a nearly infinite impedance at the input to the two transmission lines. In the event that the PIN diodes712and714are turned off, the impedance of the PIN diodes712and714will be transformed by the quarter wavelength transmission lines708and710respectively.

FIG. 8depicts a sample module800that is suitable for use in the sensor front end depicted in FIG.1. In particular the sample module800includes a front end801that provides short duration pulses. The front end801is described with respect to FIG.6B. The differentiated pulses produced by the SRD642are applied to the diodes802and804forward biasing them. Forward biasing the diode802and804allows at least a portion of the signal current present at the sample input806to be provided to the capacitors644and652to provide a sample output808.

Signal processing techniques known in the art may be added to increase the signal to noise ratio, to enhance object detection, or both. Non-coherent signal integration is utilized to reduce the noise fluctuations on the received signal. This is analogous to reducing the variance of a random variable around its mean. In one embodiment, a sample command can be issued at particular times during the reception of the reflected signal123so that particular range bins are sampled and observed. For any particular range bin, the time to the object can be determined by:τd=2*Rdc
where c is the speed of light, Rdis the range bin “d” and τdis the time delay. If an I/Q mode is being used, then multiple I channel and multiple Q channel samples are taken during alternating pulse repetition frequency cycles and processed according to:Iav=∑k=1n⁢IkQav=∑k=1n⁢Qk,andE=∑k=1n⁢(Iav2+Qav2)
where n is the total number of I and Q samples. If the resulting value E exceeds a predetermined threshold value, it may be determined that an object has been detected and is present in the range bin “d” Both in phase and quadrature phase signals should be used if possible, to ensure that stationary objects within the range bin are detected. It can be shown, that the signal to noise ratio can be improved as the square root of the number of samples taken.

Other signal processing techniques known to those of skill in the art can be used as well.FIG. 9is a block diagram of one embodiment of a signal processing technique that can be used on the sampled signals. In particular, the exponential averager900includes an input902coupled to a first multiplier904. The first multiplier904multiplies the signal present on the input line902and a first constant906. The resultant product is provided to the summing module908that adds the product from the first multiplier904with the product of a second multiplier914. The second multiplier914multiplies a second constant916, which preferably is the difference of one minus the first constant, and the resultant output916that has been stored in storage register912. It can be shown that the reduction in noise power variance resulting from the exponential averaging is:σoutσin=α2-α
where α is the value of the first coefficient and σ is the noise power variance of the demodulated signal.

Advantageously, only a single storage register is used in the above implementation. Two separate exponential averagers have to be used when sampling both I and Q channels in an I/Q system. In the exponential averaging system depicted inFIG. 9, the weights of the first and second coefficients can be changed according to the system requirements. A smaller first coefficient and concomitantly larger second coefficient will attenuate the input samples and the past averaged outputs are then the dominant terms. If a larger first coefficient is used and concomitantly smaller second coefficients are used, the input samples will be the dominant terms in the equation and thus the system is able to respond more quickly to changes in the input data. The selection of the appropriate values of the first and second coefficients is determined by the particular system requirements.

As discussed above, the sensor signal pulses may be phase coded. If a sufficient number of pulses are included in the phase code sequence, the receiver is able to de-correlate interfering signals and improve the signal detection and signal to noise ratio of the receiver. In addition, the signal may be phase coded to reduce the degenerative impact of interference from other signal sources. In particular, as discussed above, the received signal is mixed with the first signal such that an output is provided only when both the received signal and the first signal are present. Phase coding the first signal and correlating the phase code of the received signal with the first signal can allow the sensor front end described herein to reject interfering signals and increase the reliability and security of the system.

FIG. 10depicts another embodiment of a sensor front end1000. In particular, the front end1000includes a CW signal source1002coupled to a FET1006that, along with its associated components1004, operates as an amplifier while transmitting and as an unbiased mixer during receiving. This embodiment allows greater transmitter power to be used, however, as depicted inFIG. 10, phase coding is not possible without an additional component specifically added to implement a phase coding scheme. Except for the lack of phase-modulation and phase-demodulation, operation of the other components of the sensor front end1000are identical to the sensor front end100and the circuits described herein inFIGS. 4-9.

Those of ordinary skill in the art should further appreciate that variations to and modification of the above-described methods, apparatus for providing a phase coded sensor front end may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope spirit of the appended claims.