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One embodiment is directed towards a FMCW radar having a single antenna. The radar includes a transmit path having a voltage controlled oscillator controlled by a phase-locked loop, and the phase-locked loop includes a fractional-n synthesizer configured to implement a FMCW ramp waveform that ramps

One embodiment is directed towards a FMCW radar having a single antenna. The radar includes a transmit path having a voltage controlled oscillator controlled by a phase-locked loop, and the phase-locked loop includes a fractional-n synthesizer configured to implement a FMCW ramp waveform that ramps from a starting frequency to an ending frequency and upon reaching the ending frequency returns to the starting frequency to ramp again. The radar also includes a delay path coupled between a coupler on the transmit path and a mixer in a receive path. The delay path is configured to delay a local oscillator reference signal from the transmit path such that the propagation time of the local oscillator reference signal from the coupler to the mixer through the delay path is between the propagation time of signal reflected off the antenna and the propagation time of a leakage signal through a circulator.

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1. A frequency modulated continuous wave (FMCW) radar comprising: a single antenna;a transmit path coupled to the single antenna and configured to provide a FMCW signal thereto, the transmit path including a voltage controlled oscillator controlled by a phase-locked loop, the phase-locked loop inclu

1. A frequency modulated continuous wave (FMCW) radar comprising: a single antenna;a transmit path coupled to the single antenna and configured to provide a FMCW signal thereto, the transmit path including a voltage controlled oscillator controlled by a phase-locked loop, the phase-locked loop including a fractional-n synthesizer integrated circuit (IC) configured to implement a FMCW ramp waveform that ramps from a starting frequency to an ending frequency and upon reaching the ending frequency returns to the starting frequency to ramp again;a receive path coupled to the single antenna and configured to include a reflected version of the FMCW signal as a reflected signal and a portion of the FMCW signal leaking through a circulator as a leakage signal, the receive path including components configured to filter and sample a signal in the receive path; anda delay path coupled between a coupler on the transmit path and a mixer in the receive path, the delay path including a delay element configured to delay a local oscillator reference signal from the transmit path such that the propagation time of the local oscillator reference signal from the coupler to the mixer through the delay path is between the propagation time of the reflected signal and the propagation time of the leakage signal, wherein the mixer is configured to subtract the local oscillator reference signal from the signal in the receive path. 2. The FMCW radar of claim 1, wherein the antenna has a substantially flat group delay over the entire swept bandwidth of the FMCW signal. 3. The FMCW radar of claim 1, wherein the circulator provides at least 20 dB of isolation from signals in the transmit path to the receive path. 4. The FMCW radar of claim 1, wherein the antenna has a voltage standing wave ratio of less than 1.2 to 1 and a return loss of greater than 20 dB. 5. The FMCW radar of claim 1, wherein the transmit path also includes an amplifier coupled to the output of the voltage controlled oscillator to amplify a transmitted signal in the transmit path, wherein the amplifier is controlled such that the transmitted signal is less than 23 dBm. 6. The FMCW radar of claim 1, wherein the fractional-n-synthesizer IC has a phase detector frequency of at least 100 MHz. 7. The FMCW radar of claim 6, wherein a phase detector frequency of the fractional-n-synthesizer IC is set to 160 MHz. 8. The FMCW radar of claim 1, comprising a master clock coupled to the fractional-n-synthesizer to provide a clock signal for the fractional-n-synthesizer, wherein the master clock has a phase noise equal to or better than −150 dBc/Hz at 100 kHz offset. 9. The FMCW radar of claim 1, wherein a return from the ending frequency to the starting frequency is also a ramp. 10. The FMCW radar of claim 1, wherein the ramp from the starting frequency to the ending frequency is linear. 11. The FMCW radar of claim 10, wherein the fractional-n-synthesizer is configured to implement the FMCW linear ramp signal by scheduling frequency steps to be added to or subtracted from the starting frequency until the ending frequency is reached. 12. The FMCW radar of claim 1, wherein a processing device is coupled to the fractional-n-synthesizer and configured to control operation of the fractional-n-synthesizer, wherein the fractional-n-synthesizer is configured to synthesize a frequency sweep bandwidth or a frequency deviation of the FMCW linear ramp signal based on a commanded frequency deviation or frequency sweep bandwidth to maintain the FMCW linear ramp signal within the desired range. 13. A method of transmitting a frequency modulated continuous wave (FMCW) signal, the method comprising: providing a plurality of fixed frequency steps to a voltage controlled oscillator using a fractional-n-synthesizer, wherein the plurality of fixed frequency steps are configured to implement a linear ramp in frequency from a starting frequency to an ending frequency and upon reaching the ending frequency returning to the starting frequency;amplifying an output of the voltage controlled oscillator such to create a transmit signal, wherein amplifying the output includes amplifying the output by less than 23 dBm;propagating the transmit signal from an antenna;sensing reflections of the transmit signal;sensing signals at the antenna, wherein a receive path signal includes signals sensed by the antenna, reflections of the transmit signal off of the antenna, and leakage of the transmit signal through a circulator;subtracting a portion of the transmit signal, referred to as a local oscillator reference signal, with the receive path signal to cancel out phase noise in the receive path signal from the reflections and leakage of the transmit signal; andanalyzing the receive path signal to determine a distance to an object off of which the transmit signal has reflected. 14. The method of claim 13, comprising: delaying the portion of the transmit signal that is subtracted from the receive path signal with the receive path signal for a set delay, such that the propagation time of the local oscillator reference signal from is between the propagation time of the reflection of the transmit signal and the leakage of the transmit signal. 15. The method of claim 14, comprising: providing a clock signal to the fractional-n-synthesizer having phase noise equal to or better than −150 dBc/Hz at 100 kHz offset. 16. A frequency modulated continuous wave (FMCW) radar altimeter system comprising: a single antenna;a transmit path coupled to the single antenna and configured to provide a FMCW signal thereto, the transmit path including a voltage controlled oscillator controlled by a phase-locked loop, the phase-locked loop including a fractional-n synthesizer integrated circuit (IC) configured to implement a FMCW ramp waveform that ramps from a starting frequency to an ending frequency and upon reaching the ending frequency returns to the starting frequency to ramp again;a receive path coupled to the single antenna and configured to receive a reflected version of the FMCW signal, the receive path including components configured to filter and sample a signal in the receive path;a delay path coupled between a coupler on the transmit path and a mixer in the receive path, the delay path including a delay element configured to delay a local oscillator reference signal from the transmit path such that the propagation time of the local oscillator reference signal from the coupler to the mixer through the delay path is the same as the propagation time of a reflected signal from the coupler off of the single antenna and to the mixer, wherein the mixer is configured to provide cancellations of an undesired leakage and antenna reflected signal from a desired signal in the receive path;a first processor coupled to the receive path and configured to process samples of the signal in the receive path and produce altitude bin data therefrom, wherein the first processor is coupled to the transmit path and provides a control signal to the transmit path to implement the FMCW ramp waveform;an altitude computation processor coupled to the first processor and configured to process the altitude bin data to determine distance to ground values; andan output device to provide indications to a pilot based on the distance to ground values. 17. The FMCW radar altimeter system of claim 16, wherein the antenna has a substantially flat group delay across the entire modulation bandwidth and a voltage standing wave ratio of less than 1.2 to 1 and a return loss of greater than 20 dB. 18. The FMCW radar altimeter system of claim 16, wherein the transmit path also includes an amplifier coupled to the output of the voltage controlled oscillator to amplify a transmitted signal in the transmit path, wherein the amplifier is controlled such that the transmitted signal is less than 23 dBm. 19. The FMCW radar altimeter system of claim 16, comprising a master clock coupled to the fractional-n-synthesizer to provide a clock signal for the fractional-n-synthesizer, wherein the master clock has a phase noise equal to or better than −150 dBc/Hz at 100 kHz offset, and wherein the fractional-n-synthesizer IC has a phase detector frequency of at least 100 MHz. 20. The FMCW radar altimeter system of claim 16, wherein a return from the ending frequency to the starting frequency is also a ramp, and wherein the ramp from the starting frequency to the ending frequency is linear; wherein the fractional-n-synthesizer is configured to implement the FMCW linear ramp signal by scheduling fixed frequency steps to be added to or subtracted from the starting frequency until the ending frequency is reached.