A homodyne radar system includes an oscillator, an antenna, a low noise amplifier, a mixing subsystem and a directional coupler. The oscillator is configured to generate a transmit signal and a local oscillator signal. The antenna is configured to transmit the transmit signal and to receive a receive signal. The low noise amplifier is configured to amplify the receive signal. The mixing subsystem is configured to receive and mix the transmit signal and the receive signal to produce an output signal. The directional coupler is coupled to the antenna, the oscillator, the low noise amplifier and the mixing subsystem. The directional coupler is connected and configured to provide a low-loss transmission path from the antenna to the low noise amplifier and a high loss transmission path from the oscillator to the antenna.

BACKGROUND

Short range Doppler radars use a homodyne system, in which the received signal is mixed with the transmitted signal. Mixing the received and transmitted signals results in a low frequency output that is proportional to the Doppler shift between the received signal and the transmitted signal. However, since the mixing process uses the transmitted signal, isolating the received signal from the transmitted signal is difficult.

FIG. 1shows a conventional simple Doppler radar system100for measuring the velocity of a radar signal relative to the ground. The transmitted signals104are generated by an oscillator110and transmitted through an antenna112. The transmitted signals reflect of the target (e.g., the ground) and produce reflected signals102which are received at the antenna112. The transmitted signals104is split into a transmitted in-phase (“I”) signal104aand a transmitted quadrature (“Q”) signal104b, where the I signal104ais ninety degrees out of phase with the Q signal104b. Similarly, the received signals102include a received I signal102aand a received Q signal102b. The received I signals102aand the transmitted I signals104aare mixed at an I mixer106. The received Q signals102band the transmitted Q signals104bare mixed at a Q mixer108. Differences between the I signals102aand104aand between the Q signals102band104bare used to determine the direction of travel of the radar system100.

In the Doppler radar system100, the transmitted signals104are not separated from the received signals102. Thus, the received signal102cannot be independently amplified before it is sent to the mixers106and108. The power of the received signal102is low, and is then further divided between the I mixer106and the Q mixer108. Furthermore, the I mixer106and the Q mixer108are both very lossy. This results in a low Signal to Noise Ratio (“SNR”). In the radar system100, there are two ways to improve the SNR. One way to improve the SNR is to increase the transmitted signal power (thereby increasing the received signal power). However, increasing the transmitted signal power causes the radar system100to draw more DC power and may raise safety concerns. The other way to improve the SNR is to increase the antenna gain by using a larger antenna; however this adds size and cost to the system.

Accordingly, it is often desirable to isolate the received signals102from the transmitted signals104. To achieve isolation between the transmitted signal104and the received signal102in a homodyne radar, one existing solution is to use two separate antennas: a receive antenna and a transmit antenna.FIG. 2is a schematic diagram of a Doppler radar system120with a separate receive antenna122and transmit antenna124. As shown inFIG. 2, the transmitted signal104is generated at the oscillator110, and then divided at a first power divider132. Part of the transmitted signal is directed through the transmit antenna124, and the other part is further divided at a second power divider134. The second power divider134divides the transmitted signal104into a local oscillator I signal104aand a local oscillator Q signal104b. The local oscillator I signal104ais sent to a balanced I mixer136and the local oscillator Q signal104bis sent to a balanced Q mixer138. The received signal102is received through the receive antenna122and sent to a low noise amplifier126for amplification. The amplified received signal102is then divided into a received I signal102aand a received Q signal102bat the power divider128. The received I signal102is sent to the balanced I mixer136and the received Q signal is sent to the balanced Q mixer138.

Another way to isolate the received signal102from the transmitted signal104in a homodyne radar is to use a circulator.FIG. 3is a schematic diagram of a Doppler radar system150including a circulator152. The circulator152separates microwave signals based on their direction. The circulator152includes three ports: one “transmit in” port for the transmitted signal104from the oscillator110, one combined “transmit-out/receive-in” port for sending the transmitted signal104to the antenna112and for receiving the reflected signal102from the antenna, and one “receive out” port for sending the received signal102to the low noise amplifier126. The circulator152provides isolation between the transmitted signal104and the received signal102.

Another way to isolate the received signal102from the transmitted signal104in a homodyne radar is to use a Wilkinson power divider.FIG. 4is a schematic diagram of a Doppler radar system170including a Wilkinson power divider172. The Wilkinson power divider172separates the received signal102from the transmitted signal104. In a similar design aimed at isolating received signals from transmitted signals in RFID systems, a branch line coupler has been used instead of the Wilkinson power divider172. Both of these approaches attenuate the received signal significantly, reducing the SNR of the system.

SUMMARY

Conventional homodyne radar systems, and conventional methods for achieving isolation between the transmitted and received signals in such systems, have several limitations and disadvantages. For example, expensive, large or complicated technology is used to isolate the received signal from the transmitted signal.

Aspects and embodiments are directed to systems and methods for isolating the received signal from the transmitted signal in short-range homodyne radars using a directional coupler. According to various aspects, a directional coupler is smaller and less expensive than other technologies used to isolate the received signal from the transmitted signal. According to another aspect, a directional coupler may be integrated into a short-range homodyne radar, thereby avoiding complex installation procedures.

According to one aspect, a homodyne radar system is provided, the homodyne radar system configured to isolate the received signal from the transmitted signal. The homodyne radar system includes an oscillator, an antenna, a low noise amplifier, a mixing subsystem, and a direction coupler. The oscillator is configured to generate a transmit signal. The antenna is configured to transmit the transmit signal and to receive a receive signal. The low noise amplifier is configured to amplify the received signal to provide an amplified signal. The mixing subsystem is configured to receive and mix the transmit signal and the amplified signal to produce an output signal. The directional coupler is coupled to the antenna, the oscillator, the low noise amplifier and the mixing subsystem, and the directional coupler is connected and configured to provide a low-loss transmission path from the antenna to the low noise amplifier and a high-loss transmission path from the oscillator to the antenna.

In one embodiment, the directional coupler includes first a first transmission line that provides the low-loss transmission path, and an input port at a first end of the first transmission line, and the antenna is coupled to the input port. The directional coupler also includes a second transmission line. The directional coupler includes a through port at a second end of the first transmission line, and the low noise amplifier is coupled to the through port. In one embodiment, the directional coupler includes a coupled port at a first end of the second transmission line which is proximate to the first end of the first transmission line, and the oscillator is coupled to the coupled port. In one example, the directional coupler includes an isolated port at a second end of the second transmission line which is proximate the second end of the first transmission line, and the mixing subsystem is coupled to the isolated port. In one embodiment, the oscillator is configured to provide the transmit signal to the mixing subsystem from the coupled port to the isolated port.

According to another embodiment the mixing subsystem includes at least one power divider and first and second balanced mixers. In one example, the first balanced mixer is configured to mix an in-phase portion of the receive signal with an in-phase portion of the transmit signal, and the second balanced mixer is configured to mix a quadrature portion of the receive signal with a quadrature portion of the transmit signal.

According to one embodiment, the directional coupler is configured to impart less than approximately one decibel of loss to the receive signal along the low-loss transmission path. According to another embodiment, the directional coupler is configured to impart a loss of approximately ten decibels to the transmit signal along the high-loss transmission path.

In another embodiment, the directional coupler is a velocity-compensated directional coupler. In one example, the directional coupler includes a second transmission path and the low-loss transmission path and the second transmission path each include a plurality of notches. In another embodiment, the directional coupler is a microstrip directional coupler. In a further embodiment, the directional coupler is a forward wave directional coupler. In another embodiment, the directional coupler is fully monolithic and compatible with modern semiconductor manufacturing processes.

According to another aspect, a method is provided for isolating a received signal from a transmitted signal in a radar system. The method includes generating a transmitted signal at an oscillator, sending the transmitted signal through a high loss path of a directional coupler to an antenna for transmission, receiving a received signal at the antenna, sending the received signal through a low loss path of the directional coupler to an amplifier, amplifying the received signal to provide an amplified signal, and mixing the transmitted signal and the amplified signal.

In one embodiment, the method further includes dividing the transmitted signal into a transmitted I signal and a transmitted Q signal using a first power divider coupled to the oscillator. The method may further include sending the amplified signal from the amplifier to a second power divider, and dividing the amplified signal into a received I signal and a received Q signal. In one example, mixing the transmitted signal and the amplified signal includes mixing the received I signal with the transmitted I signal, and mixing the received Q signal with the transmitted Q signal.

DETAILED DESCRIPTION

Short range Doppler radars use a homodyne system, in which the received signal is mixed with the transmitted signal. According to one example, radar systems compare the received signal to the transmitted signal in order to measure velocity. However, the received signal is often attenuated or masked by noise, and it can be difficult and costly to distinguish the received signal from the transmitted signal.

As discussed above, although several different approaches have been used to achieve isolation between the received signal and the transmitted signal in conventional homodyne radar systems, each of these approaches suffers from disadvantages. For example, although using separate receive and transmit antennas (such as in the radar system120discussed above with reference toFIG. 2) generally achieves adequate isolation between the transmitted and received signals, the radar system120is large, heavy, and costly compared to other radar systems, such as the radar system100ofFIG. 1. Using a circulator for isolation, such as in the radar system150ofFIG. 3, also has several disadvantages, including that is it mechanically difficult to integrate a circulator into a radar system, which adds significant complexity in building the system. For example, both the two-antenna approach ofFIG. 2and circulator approach ofFIG. 3require the use of mechanical interconnects, and low loss, low RF-reflection interconnects at high frequencies (e.g. millimeter-wave frequencies) are difficult to manufacture in a production environment. This problem is particularly noticeable in systems using circulators because there are the three separate interconnections required, one for each of the three circulator ports. Additionally, circulators are expensive, significantly increasing the cost of the radar system150. In one example, adding a circulator152to a radar system150adds about fifty dollars to the production cost. In another example, adding a circulator152to a radar system150doubles the cost of the radar system150. Furthermore, because a typical navigation system provides orthogonal 3-axis velocity measurements, such systems use three radars, one for each axis. Thus, according to one example, using three radar systems150with circulators152adds about $150 to the cost of the navigation system. Furthermore, a circulator typically provides less than about 20 dB of isolation between the received signal102and the transmitted signal104.

The Wilkinson power divider172used in some systems, as discussed above with reference toFIG. 4, introduces large losses into the radar system170. Specifically, the Wilkinson power divider172of the radar system170adds at least about three decibels (dB) of loss to the received signal102before it reaches the LNA126. The 3 dB loss results in a significant decrease in the signal-to-noise ratio (SNR) and a significant increase in the overall receive noise figure. Furthermore, similar to the circulator discussed above, a Wilkinson power divider172may provide only about 20 dB of isolation between the received signal102and the transmitted signal104.

Thus, conventional homodyne radar systems have numerous limitations and disadvantages. According to one embodiment, in a short-range, low-power homodyne radar system, the received signal102may be isolated from the transmitted signal104by using a directional coupler as shown inFIG. 5. In one example, the directional coupler is a microwave directional coupler. The directional coupler may be included in the radar system's millimeter-wave integrated circuit, thus adding little cost to the radar and avoiding the need for additional millimeter-wave RF interconnects that are required for two-antenna or circulator designs. As discussed further below, the directional coupler provides a lower-loss, lower-noise solution than using a Wilkinson power divider or branch line coupler, and eliminates the size, weight and cost penalties associated with two-antenna systems.

Referring toFIG. 5, there is illustrated a schematic diagram of a homodyne Doppler radar system200including a directional coupler202according to one embodiment. The directional coupler202is connected to provide a low loss path between the antenna212and the receiver low noise amplifier (LNA)226. The directional coupler202includes a pair of closely spaced, electromagnetically coupled transmission lines: a first (receive path) transmission line204aand a second (transmit path) transmission line204b. In one example, the directional coupler202is implemented as a backward wave microstrip coupler and the transmission lines204aand204bare each approximately one quarter wavelength long. Energy applied at a first port244of the directional coupler202flows through the first transmission line204ato a second port254, and is also preferentially coupled out a third port246in the second line204bopposite the flow of power in the first line204a. According to one feature, a signal applied at an input port of the directional coupler202, for example, the received signal applied from the antenna212to the first port244, generates an electromagnetic wave in the first transmission line204a. Some of the electromagnetic field is coupled to the coupled second transmission line204b. Similarly, an input signal applied at the third port246is coupled to the first port244, as discussed further below.

As shown inFIG. 5, in one embodiment an antenna212is coupled to the first port244of the directional coupler202, and the receiver LNA226is coupled to a second port254. The second, transmit path transmission line204bis coupled to an oscillator210(which generates the transmit signal to be transmitted by the antenna212) at the third port246, and to a first power divider234at a fourth port256. The first power divider234splits the transmit signal from the oscillator210into an in-phase (“I”) local-oscillator signal and a quadrature (“Q”) local-oscillator signal. Similarly, the received signal is amplified by the LNA226and then split into a received I signal and a received Q signal by a second power divider228. As discussed above, the received I signals and the transmitted I signals are mixed at an I mixer236. The received Q signals and the transmitted Q signals are mixed at a Q mixer238. Differences between the two I signals and between the two Q signals are used to determine the direction of travel of the radar system200.

From the receiver point of view in the radar200, the first port244is the input port of the directional coupler202, with the signal received by the antenna212being applied at that port. The second port254is the through port of the directional coupler202, the third port246is the coupled port, and the fourth port256is the isolated port. Thus, the signal received by the antenna212flows from the first port244through the low loss receive path transmission line204ato the LNA226at the second port254. In one example, the loss in the path from the antenna212to the LNA226is approximately a fraction of a decibel (dB), for example, less than 0.5 dB.

From the transmitter point of view, the third port246becomes the input port of the directional coupler202, with the transmit signal generated by the oscillator210applied at that port. To the transmit signal from the oscillator210, the first port244is the coupled port, the second port254is the isolated port, and the fourth port256is the through port. Thus, the transmit signal from the oscillator210is transmitted from the third port246to the fourth port256to drive the I and Q mixers236,238, as discussed above. The transmission line204bis a low loss line, and transmission of the transmitted signal104from the third port246and the fourth port256results in only very small signal losses, comparable to the loss in the received signal102as it is transmitted from the antenna to the LNA. In one example, the loss in the transmitted signal104from the third port246to the fourth port256is less than about 0.5 dB.

Still referring toFIG. 5, a portion of the signal from the oscillator210is also coupled to the first port244and then transmitted by the antenna212. The transmission of the transmitted signal104from the third port246to the first port244results in high losses to the transmitted signal. Specifically, the power of the transmitter signal at the antenna212is reduced, relative to the power of the signal when generated by the oscillator210and provided at the third port246, by the coupling factor of the directional coupler202. The amount of energy coupled from one transmission line204aor204bto the other is controlled primarily by the spacing between the two lines. In one embodiment, the directional coupler is configured to couple between about 1/10thand 1/100thof the energy at the input port to the coupled port, corresponding to a coupling factor of about 10 dB to 20 dB. Thus, in one example, the power in the transmitted signal104coupled from the oscillator210at the third port246to the antenna212at the first port244is reduced by about 10 dB.

For a short range radar system, the transmit power required can be significantly lower than the signal power needed to drive the mixers236,238. As a result, the transmit signal104can be “tapped off” the mixer drive signal using the directional coupler202, as discussed above. In a short range radar, for example, having a range of a few meters and where the transmit power is generally only a few milliwatts, the loss introduced to the transmit signal104by the directional coupler202is not only acceptable, but beneficial in at least some embodiments due to the difference in the desired power level of the transmitted signal relative to the mixer drive signal. In some radar systems, low transmitted power is required to reduce interference problems with other equipment, and maintain Low-Probability-of-Intercept (LPI) for tactical operations. For example, a short-range radar system200may have a range of about 1 to 2 meters and use a transmit power of between about 1 mW and about 5 mW, whereas the oscillator210may provide a signal power of about 10 to 50 mW which can be used to drive the mixers236,238as discussed above.

In addition, the energy coupled from the oscillator at the third port246into the first transmission line204awill primarily be sent to the antenna212at the first port244, with very little energy leaking back to the second port254and the LNA226. In one example, the power travelling in the reverse direction from the oscillator210to the LNA226at the second port254may be at least 30 or 40 dB lower than the transmit signal104going to the antenna212. Thus, the directional coupler202provides very good isolation at the LNA226(e.g., greater than 30 dB) between the received signal102traveling through the receive path transmission line204afrom the antenna212to the LNA226and the transmitted signal104traveling from the oscillator210to the antenna212. This allows the use of the LNA226to amplify the received signal102prior to the received signal being provided to the mixers236,238, thereby improving the signal to noise ratio at the mixers. In addition, the radar200may use a high gain LNA226without the LNA being overridden by leakage from the oscillator210.

Thus, according to one feature, an inexpensive and small low-power radar200suitable for short-range radar systems can be provided by including and configuring the directional coupler202to provide a low loss path for the received signal102to the LNA226and to attenuate the signal from the oscillator210to provide a suitable low power transmit signal104. Furthermore, separating the received signal102from the transmitted signal104using the directional coupler202, as discussed above, allows for the use of double balanced mixers236and238. According to one example, double balanced mixers236and238may be used because the transmit signal104is blocked from the receive path by the directional coupler202, and the received signal102may be fed directly into the RF port of the double balanced mixers236and238. According to one feature, double balanced mixers236and238may be used when the received signal102is isolated, and not superimposed with the high level transmitted signal104. According to one feature, using double balanced mixers largely eliminates the DC offset created by unbalanced mixers which are used in the radar system100ofFIG. 1.

Directional couplers have been used in conventional RFID systems; however, these systems are based on minimizing transmit losses. For example,FIG. 6is a schematic diagram of a system180using a directional coupler182connected in the system in a manner that provides a low-loss path for the transmitted signal. This configuration, however, results in high losses to the received signal. In particular, the first transmission line184aof the directional coupler182is coupled to the first power divider132and the antenna112such that the transmitted signal104travels through a low loss path to the antenna112. However, a received signal102from the antenna112is coupled from the antenna port to the second transmission line184bof the directional coupler182, and therefore suffers high losses. In one example, the directional coupler182causes the amplitude of the received signal102to decrease by about 10 dB. Thus, if the configuration ofFIG. 6were applied to a Doppler radar system, the high losses in the receive path would significantly degrade the overall receive SNR, making such a configuration highly undesirable.

According to various examples, the radar system200may provide many advantages over conventional homodyne radar systems. For example, as discussed above with reference toFIG. 2, a radar system120using separate transmit and receive antennas122and124is large, heavy, expensive, and requires two separate millimeter-wave connections to the integrated circuit140in the radar system120. Low loss, low RF-reflection interconnects at millimeter wave frequencies are difficult to produce, and are therefore expensive. The radar system150using the circulator152is expensive, and requires three separate millimeter wave connections to the circulator152. The radar system170with the Wilkinson divider172may result in a higher loss to the received signal102and increased noise as compared with the radar system200. Additionally, since the radar system200may have a higher gain at the LNA226than the system150or the system170(because the directional coupler202may provide better isolation than either the circulator or Wilkinson power divider approaches), the gain at the baseband signal electronics of the radar system200may be reduced, reducing the overall power consumption of the radar system200compared to the systems150and170, and also reducing the number of parts used in the radar system200.

According to various embodiments, the radar system200may be used as a ground velocity sensor. In one embodiment, the radar system200may be a ground velocity sensor used in a navigation system, for example, to supplement a GPS (global positioning system) navigation unit when the GPS signal is not available (referred to as a “GPS-denied” environment). The radar system200may be used in a vehicle-based navigation system or in a handheld navigation system. In various examples, a handheld navigation system including the radar system200may be used by civilian first-responders, such as firefighters and other rescue workers, and it may be used by soldiers. According to one feature, the low power usage of the radar system200allows for safe usage in handheld devices. In another example, the radar system200may be included in robots or robotic vehicles. In one embodiment, the radar system200may be used in automotive radars. In one example, the radar system200may be used in automotive applications to measure the distance to nearby objects by introducing a frequency sweep on the transmitted signal.

Where the radar system200is used, it may be desirable to include velocity compensation techniques in the directional coupler.FIG. 7is a schematic diagram of one example of a microstrip directional coupler302having velocity compensation features. In the illustrated example, the directional coupler302includes two strips of metal (a first transmission line304aand a second transmission line304b) positioned over a ground plate (not shown), and has first310, second312, third314and fourth316ports, as discussed above. According to one feature, the first port310is the input port, the second port312is the through port, the third port314is the coupled port, and the fourth port316is the isolated port.

According to one embodiment, notches306a-306gand308a-308gin the velocity-compensated directional coupler302provide velocity compensation. At very high frequencies (e.g., millimeter-wave frequencies), velocity compensation corrects for differences in even-mode velocities and odd-mode velocities in the coupled transmission lines304aand304b. According to one feature, velocity compensation assists in achieving a directional coupler with very high isolation, for example, approximately 35 dB or greater, which is considerably higher than may be achieved with millimeter wave circulators or Wilkinson dividers.

Referring toFIG. 8, there is illustrated a graph350showing an example of a response of a directional coupler (including velocity compensation), according to one embodiment. The example directional coupler has a length320of about 255 μm, and a width322(measured across both transmission lines304aand304b) of about 100 μm. As shown in the graph350, the response of the example coupler was measured over the frequency range of about 79 GHz to about 81 GHz. In this range, the coupling factor (represented by trace354) is approximately 10.5 dB, the through-loss (i.e., the loss through the first transmission line304afrom the first port to the second port; represented by trace352) is about 0.5 dB, and the isolation is approximately 33 dB to 34 dB.

In the embodiments discussed above, the directional coupler202has been illustrated as a backward wave coupler. As discussed above, for a backward wave coupler implemented using microstrip transmission lines, the lines may be approximately ¼ wavelength long. According to another embodiment, the directional coupler may instead be implemented as a forward wave directional coupler, as illustrated for example inFIG. 9.FIG. 9is a schematic diagram of one example of a homodyne Doppler radar system270including a forward wave directional coupler272according to one embodiment. Similar to the backward wave coupler, the forward wave directional coupler272includes two electromagnetically coupled transmission lines: a first (receive path) transmission line274aand a second (transmit path) transmission line274b. The transmission lines274aand274bmay each be several wavelengths long. In various examples, the transmission lines274aand274bare each about one, about two, about three, about four or about five wavelengths long. However, although the forward wave coupler272at several wavelengths in length is larger than the backward wave coupler202, which may be only a quarter wavelength long, at millimeter wave frequencies, the forward wave coupler can still be less than several millimeters in length.

As shown inFIG. 9, the receive path transmission line274aof the forward wave directional coupler272has a first port276coupled to the antenna212and a second port286coupled to the receiver LNA226. The transmit path transmission line274bhas a fourth port288coupled to the oscillator210, and a third port278coupled to the first power divider234. From the receiver point of view in the radar270, the first port276is the input port, the second port286is the through port, the third port278is the isolated port, and the fourth port288is the coupled port. From the transmitter point of view, the fourth port288becomes the input port, the first port276is the coupled port, the second port286is the isolated port, and the third port278is the through port. The transmitted signal104applied at the fourth port288from the oscillator210is coupled to the first port276to be transmitted by the antenna212, and is attenuated by the coupling factor of the coupler. Thus, transmission of the transmitted signal104from the fourth port288to the first port276results in high losses to the transmitted signal104, as discussed above. In one example, the power in the transmitted signal104coupled from the oscillator210at the fourth port288to the antenna212at the first port276is reduced by about 10 dB. In another example, the power in the transmitted signal104coupled from the oscillator210at the fourth port288to the antenna212at the first port276is reduced by between about 6 dB and about 15 dB. The received signal102is transmitted via the low-loss transmission line274to the LNA226, as discussed above.

Thus, similar to the radar system200discussed above, another example of an inexpensive and small low-power radar270suitable for short-range radar systems can be provided by including and configuring the forward wave directional coupler272to provide a low loss path for the received signal102to the LNA226and to attenuate the signal from the oscillator210to provide a suitable low power transmit signal104. According to one feature, forward wave couplers such as the forward wave coupler272have looser fabrication tolerances than backward wave couplers, and therefore may be easier and/or less expensive to manufacture. Furthermore, forward wave couplers are easily made with high directivity, and accordingly may be desirable for certain applications.

FIG. 10is a flow chart showing a method400of isolating the received signal from the transmitted signal in a short-range homodyne radar, according to one embodiment. At step402, an oscillator in the radar generates a transmitted signal. At step404, the transmitted signal is sent through a high loss path of a directional coupler to an antenna for transmission. According to one example, the transmitted signal loses about ten decibels at the directional coupler, and is then transmitted from the radar. At step406, the received signal is received at the antenna. At step408, the received signal is sent through a low loss path of the directional coupler to an amplifier. According to one feature, the received signal loses less than one decibel of amplitude during transmission from the antenna to the amplifier. The amplifier amplifies the received signal at step410. At step412, the transmitted and received signals are mixed.

In one embodiment, the received I signals are mixed with the transmitted I signals and the received Q signals are mixed with the transmitted Q signals. Thus, according to one embodiment, the transmitted signal is sent through the directional coupler to a power divider, and the power divider divides the transmitted signal into I and Q signals. Similarly, according to another embodiment, the received signal is sent from an amplifier to a power divider which divides the received signal into I and Q signals.

Accordingly, various aspects and embodiments are directed to a system and method of isolating the received signal from the transmitted signal in a homodyne radar, as discussed above. According to one embodiment, a directional coupler is used to send the received signal to a low noise amplifier with minimal losses, while allowing high losses to the transmitted signal. According to one feature, using the low loss path of the directional coupler for the received signal allows for a high signal to noise ratio of the received signal, and for the received signal to be amplified before being mixed in the mixing stage of the radar system. According to one feature, a low amplitude transmitted signal is sufficient for short-range radars, and therefore, connecting the directional coupler to minimize loss in the receive path (but allowing loss in the transmit path) is acceptable. As discussed above, this result may be particularly beneficial for short-range radars where only low transmit power, in particular transmit power that is significantly lower that the power used to drive the mixing stage, is needed.