Basics of FMCW Radar – Radar Operation – Part 2

A continuous wave (CW) Doppler radar can be augmented via the use of frequency modulation. This type of radar is known as a frequency modulated continuous wave (FMCW) radar. The use of frequency modulation allows for the detection of both range and target velocity, as well as the ability to discriminate between multiple targets. A simple CW radar can only tell a target’s velocity and lacks range timing detection. The velocity measurement function works similarly to a simple CW radar, where the transmitted signal is continuously compared with a mixer to the receive signal. The resulting frequency difference is directly related to the velocity of a target as a result of the Doppler effect.

The time-of-flight of the signal can be determined by the difference in phase or frequency difference of the transmitted and received signals. Hence, a simple frequency comparison can be used to determine a targets range.

FMCW intrinsically enables a very low minimum range to the target, which is comparable to the wavelength of the carrier frequency. FMCW radars are also able to simultaneously detect the target range and velocity with a relatively simple circuit. These measurements can also be made with precision components and yield a very accurate range measurement. The low frequency output of an FMCW radar allows for simple realization of the detection circuitry, and can operate at relatively low power compared to pulsed radar.

FMCW radar, like pulsed radar, are still range limited by the transmitted power, antenna efficiency, and sensitivity of the receiver. The FMCW signal bandwidth also limits the maximum range of the radar, leading to a trade-off with range resolution. The range resolution, on the other hand, is limited by the bandwidth of the transmitted signal. A greater bandwidth FMCW signal allows for enhanced range resolution.

Various FM moduluations can be used with FMCW radar, with the common modulation patterns being sawtooth, triangular, square-wave, stepped, and sinusoidal. Each modulation pattern has its own benefits, where sawtooth modulation provides a relatively high maximum range and minimized influence of the Doppler frequency. A triangular modulation allows for relatively simple circuitry and processing to determine the difference between the frequency shift form time-of-flight and the Doppler frequency. Square-wave modulation, as a rudimentary form of frequency-shift keying (FSK), can provide very precise close range distance measurements, but cannot be used to discriminate between multiple targets. Stepped modulation enhances the maximum range for interferometric measurements.

 FMCW Radar block diagram