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Radar System

Monostatic Ranging Sensor (FMCW)

MONOSTATIC RANGING SENSOR — FMCW RADAR FREQUENCY-MODULATED CONTINUOUS WAVE ANTENNA CIRC 3-Port RX LNALow Noise MIXER LPFBeat AMPIF Gain ADCSample FFTRange VCOSwept TX LO TAP (DELAYED BY ROUND TRIP) RAMP GENTriangle FREQUENCY vs TIME TX RX Δf R = c × f_beat / (2 × BW / T_sweep)   |   f_beat = beat frequency, BW = chirp bandwidth, T = sweep time
Component Descriptions

Signal Chain Walkthrough

FMCW (Frequency-Modulated Continuous Wave) radar determines target range by measuring the frequency difference (beat frequency) between the transmitted chirp and the delayed return from the target. Unlike pulsed radar, FMCW transmits continuously, enabling very high range resolution with low peak power.

VCO + Ramp Generator

A voltage-controlled oscillator (VCO) is swept linearly in frequency by a ramp (triangle or sawtooth) waveform. The frequency sweeps over bandwidth BW in sweep time Tsweep. Wider chirp bandwidth gives finer range resolution: ΔR = c/(2×BW).

Mixer (Dechirp)

The mixer combines the received echo with a sample of the current transmit frequency. Because the echo is delayed by the round-trip time, it has a different instantaneous frequency than the current TX. The mixer output is a constant beat frequency proportional to range: fbeat = 2×R×BW/(c×Tsweep).

FFT Processing

The beat signal is digitized and processed with an FFT. Each FFT bin corresponds to a specific range. Multiple targets at different ranges appear as distinct spectral peaks. The FFT length determines the number of resolvable range cells.

Advantages Over Pulsed

FMCW radar operates with low peak power (milliwatts vs. kilowatts for pulsed), has no minimum range limitation, and achieves range resolution through bandwidth rather than pulse width. It is the dominant architecture for automotive radar (77 GHz), level sensing, and short-range industrial applications.

Typical Specifications

Component Specifications

ComponentParameterTypical Value
VCOCenter Frequency24 - 77 GHz
VCOChirp Bandwidth200 MHz - 4 GHz
VCOSweep Time1 - 100 ms
TX PowerAverage0 to +15 dBm
LNANoise Figure1.0 - 3.5 dB
Range ResolutionΔRc / (2 × BW)
Range Resolution@ 1 GHz BW0.15 m (15 cm)
ADCSample Rate1 - 100 MSPS
Design Note: FMCW range resolution depends only on chirp bandwidth: ΔR = c/(2×BW). A 1 GHz chirp gives 15 cm resolution; a 4 GHz chirp (77 GHz automotive) gives 3.75 cm. VCO linearity directly affects range accuracy and sidelobe levels in the FFT output. PLL-based chirp generators (e.g., fractional-N synthesizers) provide superior linearity compared to open-loop VCO sweeps.
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