FMCW
Understanding FMCW Radar
FMCW radar works by transmitting a signal whose frequency increases linearly with time (a chirp). When this signal reflects from a target, it returns with a time delay proportional to the target range. Mixing the transmitted and received signals produces a beat frequency proportional to range.
FMCW Principles
- Range measurement: Beat frequency = chirp rate x 2R/c, where R is range.
- Velocity measurement: Doppler shift between chirps measures target velocity.
- Range resolution: Determined by chirp bandwidth: delta_R = c/(2 x BW).
- Multiple targets: FFT of beat signal resolves multiple targets at different ranges.
Advantages over Pulsed Radar
- Low peak power (continuous transmission vs. high-power pulses)
- Simple hardware (no high-voltage pulse generation)
- Fine range resolution with moderate bandwidth
- Compact, low-cost implementation suitable for mass production
Range: R = fb x c x T_chirp / (2 x BW)
Range resolution: delta_R = c / (2 x BW)
Max range: R_max = fs x c x T_chirp / (4 x BW)
77 GHz automotive example:
BW = 4 GHz, delta_R = 3.75 cm
T_chirp = 50 us, chirp rate = 80 MHz/us
Frequently Asked Questions
How does FMCW radar work?
FMCW radar transmits a frequency-sweeping signal (chirp) and mixes the reflected return with the transmitted signal. The resulting beat frequency is proportional to target range. FFT processing of the beat signal resolves multiple targets simultaneously.
What is the advantage of FMCW over pulsed radar?
FMCW uses continuous low-power transmission instead of high-peak-power pulses, enabling simpler, more compact, and less expensive radar hardware. This makes FMCW ideal for mass-production applications like automotive radar, where millions of sensors are deployed.
What determines FMCW range resolution?
Range resolution = c/(2 x BW), where BW is the chirp bandwidth. A 4 GHz chirp at 77 GHz provides 3.75 cm range resolution. Wider bandwidth improves resolution but requires wider-bandwidth components.