Abstract:
Systems and methods for allowing dual-mode radar operation. An exemplary transmission system includes a hybrid coupler that receives a signal produced by a synthesizer and couples the received signal to two output ports. A pulse transmitter receives a pulse transmit-activate signal from a controller, receives an input signal from the hybrid coupler and, if the activate signal has been received, amplifies the received signal based on a predefined desired pulse output transmission setting. A frequency-modulation continuous-wave (FMCW) transmitter receives an FMCW transmit-activate signal from the controller, receives an input signal from the hybrid coupler and, if the activate signal has been received, amplifies the received input signal based on a predefined desired FMCW output transmission setting. An isolator protects the pulse transmitter during FMCW operation and also the FMCW transmitter from receiving power reflected off of pulse transmitter components.

Description:
COPENDING APPLICATION 
       [0001]    This application relates to copending U.S. patent application Ser. No. ______ (attorney docket No. H0034625 (HOOO-1-1892)), the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    There does not currently exist a radar system that allows both marine and aviation applications to provide a combination of long-range and moderate-range resolution, in addition to very short minimum range (&lt;10 feet) to modest range (5-7 nautical miles (NM)) with very high-range resolution on the order of three to ten feet. Current commercial marine radar systems employ either pulse or pulse-compression methods for moderate (˜1 NM) to long-range capability with modest- to high-range resolution. Examples include Honeywell&#39;s RDR 4000 nonlinear frequency modulation (NLFM) pulse-compression radar, Kelvin Hughes LFM Pulse Compression Marine Radars, JRS Solid State Marine Radar, NGC/Sperry Marine Solid State Pulse Compression Radar system. Marine radars currently are pulsed (all suppliers) or frequency-modulation continuous-wave (FMCW) (Navico) types of systems. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention provides systems and methods for allowing dual-mode radar operation. An exemplary transmission system includes a hybrid coupler that receives a signal, produced by a synthesizer, and couples the received signal to two output ports. A pulse transmitter receives a pulse transmit-activate signal from a controller, receives an input signal from a first one of the two output ports of the hybrid coupler and, if the activate signal has been received, amplifies the received input signal based on a predefined desired pulse output transmission setting. A frequency modulation continuous wave (FMCW) transmitter receives an FMCW transmit-activate signal from the controller, receives an input signal from the other output port of the hybrid coupler and, if the activate signal has been received, amplifies the received input signal based on a predefined desired FMCW output transmission setting. An isolator protects the pulse transmitter during FMCW operation and also the FMCW transmitter from receiving power reflected off of pulse transmitter components. 
         [0004]    The present invention avoids large mechanical switches (coax or waveguide) that would be required to achieve very low insertion loss. The present invention avoids switch-time limitations when interleaving FMCW and pulse compression mode. 
         [0005]    The present invention allows the highest possible efficiency and lowest possible losses when accommodating both transmitters. The present invention allows very short minimum range of just a few meters in FMCW mode, with high range resolution for multiple applications, and permits high-power pulse-compression mode for long-range detection with variable-range resolution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0007]      FIGS. 1 and 2  are block diagrams of exemplary transmitter systems formed in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]      FIG. 1  shows a portion of an exemplary radar system  20  for a vehicle (e.g., boat, aircraft) that is configured to provide a dual-mode, low-power radar system. The system  20  induces a dual-mode transmitter  24 , a synthesizer  26 , a controller  28  (e.g., field-programmable gate array (FPGA)), an antenna  30 , and, optionally, a circulator  32 . A complete radar system would include other components, such as a digital signal processor and output device. 
         [0009]    The transmitter  24  receives a base signal from the synthesizer  26 , then generates a transmission signal from the received base signal according to an operational mode (i.e., transmission frequency) determined by the controller  28 . 
         [0010]    In one embodiment, a user manually sets the operational mode using a user interface associated with the controller  28 . In one embodiment, the operational mode is automatically set by the controller  28 , based on received information. Exemplary received information includes information about the operational state of the vehicle associated with the system  20 . 
         [0011]    If the antenna  30  is a transmit/receive antenna, then the circulator  32  is included for the coordination of signals sent from the transmitter  24  to the antenna  30  and signals received at the antenna  30 , then sent to a receiver system (not shown). 
         [0012]    The first and most obvious method to solve this problem is to suggest the use of simple single-pole, double-throw (SPDT) switches at the input and output of the high-power transmitter so that, in low-power mode, exciter power levels are bypassed around the high-power transmitter and, in high-power pulse mode, the bypass is eliminated and the high-power transmitter is directly connected to the antenna. 
         [0013]    However, this method is exceptionally wasteful because it dissipates a substantial amount of the available pulse transmitter power (20 to 30% at a minimum) in the insertion loss of the output switch and requires a set of mechanical coaxial or waveguide switches that add significant size and weight and have a large cost impact. This method also does not allow for rapid interleaving of the transmit waveforms. Rapid continuous switching of mechanical switches (ultra-low loss, high isolation) will also result in high failure rates and low overall mean time between failures (MTBF). Use of PIN diode switches is possible but far higher insertion losses occur with switching limits placed on the maximum transmit power for the diodes. Using switches with any loss requires the transmitter power to be increased, so that the final power delivered to the antenna is maintained at the required level. This dramatically increases cost, size, weight, and power (CSWAP). 
         [0014]    As shown in  FIG. 2 , the transmitter  24  includes a hybrid coupler  66 , a pulsed transmitter  60 , an isolator  68 , a frequency-modulated continuous-wave (FMCW) transmitter  62 , a directional coupler  70  and a lowpass filter (LPF)  72 . The hybrid coupler  66  receives a base signal from a synthesizer  26 . An exemplary synthesizer includes a direct digital synthesizer (DDS) with a fractional N synthesizer, such as that and others described in copending U.S. patent application Ser. No. ______ (Attorney Docket No. H0034625 (HOOO-1-1892)), which is hereby incorporated by reference. 
         [0015]    The synthesizer  26  provides two different signals that determine the operating mode of the transmitter  24 . When the synthesizer  26  generates long continuous linear frequency modulation waveforms that resemble a triangle (linear frequency ramp up followed by linear frequency ramp down) with the transmitter  24  operating continuously, then the low power FMCW transmitter  62  is used. In continuous wave mode (FMCW) the waveforms can have durations as long as milliseconds. When high power pulses are required then the synthesizer  26  generates short bursts of linear or non-linear frequency modulation with durations on the order of a few to several microseconds. In the high power pulse mode the high power transmitter path (the pulsed transmitter  60 ) is operational. Each of these waveforms is typically stored in an FPGA to be used as a suite of possible transmitted waveforms as conditions require. 
         [0016]    The low-power transmitter (the FMCW transmitter  62 ) is protected by the reverse isolation of the directional coupler  70  and an optional variable attenuator  80 . The isolator  68  prevents energy from coupling in an undesired direction from the directional coupler  70 . When operating in low-power mode (i.e., FMCW transmission mode), the isolator  68  prevents energy from reflecting from the pulsed transmitter  60  that has been switched off. 
         [0017]    The FMCW transmitter  62  is protected from high power levels of the high power pulsed transmitter  60  by the reverse isolation of the directional coupler  70  and the variable attenuator  80 . The isolator  68  is provided for two reasons: 1) protect the high power pulsed transmitter  60  from a failure in the antenna  30  that would cause large amounts of the transmitter power to be reflected back to the amplifier and potentially cause serious permanent damage; 2) when in the FMCW mode power is coupled primarily in the direction of the output circulator  32  but some of the power is sent towards the high power transmitter  60  that is turned off. Without the isolator  68  that power from the FMCW transmitter  62  will then reflect power back towards the output circulator  32  and will therefore combine with the intended transmit power with undefined phase and amplitude. This could result in large levels of amplitude and phase modulation in the resulting transmitted signal that will cause serious distortion when finally received. This distortion will reduce receiver sensitivity and range resolution. The isolator  68  causes the power that travels in the undesired direction towards the high power transmitter  60  to be absorbed in the load of the isolator  68  and is not reflected back towards the output circulator  32 . 
         [0018]    The high-power pulsed transmitter  60  is directly connected to the antenna  30  at all times so that the signals produced do not incur any significant insertion losses en route to the antenna  30 . Power for the FMCW transmitter  62  is increased to the 1 to 2 watt (W) level with only 0.1-0.2 W being coupled to the antenna  30 . The directional coupler  70  transfers power to the output transmission line at a reduced coupling level. That coupling level provides isolation from the very high power transmitter  60  reaching the low power transmitter  62 . Typically directional couplers have a FORWARD transfer loss of 3 to 20 dB. So for a 10 dB coupler a 2 Watt transmitter corresponds to 33 dBm, reduced by the 10 dB coupler the directional coupler 33-10 dB=23 dBm or 0.20 Watts sent in the forward direction to antenna  30 . The reverse or undesired isolation will be about 15 to 20 dB. That means that a 100 Watt high power transmitter (50 dBm) is reduced 50-20 dB reverse isolation=30 dBm or 1 Watt. The attenuator is then set for 30 dB and the FMCW transmitter  62  will only experience 30 dBm-30 dB=0 dBm or 1 milliwatt. While this is a bit wasteful, this power level is very easily achieved (vs. 40-300 W) at low CSWAP. 
         [0019]    By using the hybrid coupler  66  and the directional coupler  70  (or alternately a waveguide directional coupler for transmitters &gt;40 W) “switching” transmit modes can be done as quickly as the modulator/synthesizer can generate appropriate waveforms with very low losses compared to the typical design using Single Pole, Double Throw Switches. The only significant losses in the proposed system are in the input driver stages due to the hybrid isolated power divider (hybrid coupler). These losses, however, are low cost and occur at the &lt;1 W level where overcoming those losses is low CSWAP. 
         [0020]    In the present invention, the directional coupler experiences insertion loss and directivity losses typically &lt;0.4 decibel (dB). The hybrid coupler  66  provides continuous load to the synthesizer  26  at all times. The synthesizer  26  is always on so that it is ready to generate any desired waveform. Turing the PLL off or causing the load to change dramatically introduces large changes to the PLL that will cause large changes in output frequency. This instability requires time after the load change to smooth out and produce a stable output. Waiting for the synthesizer to stabilize slows the pulse transmission rate or the switching rate between FMCW and Pulse modes. 
         [0021]    In the FMCW mode objects at a very short minimum range of just a few meters can be detected. When in the high-power pulse-compression mode, objects at long range can be detected with variable-range resolution. 
         [0022]    In one embodiment, the directional coupler  70  during pulse transmission output provides 10 dB directional coupling towards the antenna  30  (e.g., 30 dBm input and 20 dBm output to the antenna). The load of the directional coupler  70  is sized for pulse transmitter power minus directivity (e.g., 50 dBm−20 dBm=30 dBm peak). The directional coupler  70  has &gt;20 dB coupler directivity to protect the FMCW transmitter  62  (e.g., 50 dBm−20 dB=30 dBm). 
         [0023]    In one embodiment FMCW variable attenuator provides ˜20 dB of added attenuation. The variable attenuator  80  switches in added attenuation for protection during high-power pulse transmission mode. Also, the variable attenuator  80  may be used to adjust output power in FMCW mode. The variable attenuator  80  is used to precisely control transmit power to just below the self jam level. The FMCW transmitter  62  is biased OFF during high-power pulse mode under control of a control signal sent from the FPGA. The FMCW transmitter  62  is a 2 W CW device that is on only during FMCW mode. 
         [0024]    The hybrid coupler  66  includes an isolated load that absorbs power reflected from the “Off FMCW Transmitter” to protect the PLL operation. 
         [0025]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.