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Radar System

Monostatic Speed Sensor (CW Doppler)

MONOSTATIC SPEED SENSOR — CW DOPPLER RADAR ANTENNA TO TARGET CIRC 3-Port RX LNALow Noise MIXER LPFDoppler AMPIF Gain DSPFreq Counter VELOCITY OUTPUT CW SOURCEOscillator TX LO TAP f_doppler = 2 × v × f_tx / c   |   v = velocity, f_tx = transmit frequency, c = speed of light
Component Descriptions

Signal Chain Walkthrough

A monostatic CW Doppler radar uses a single antenna for both transmit and receive. The CW source provides a continuous unmodulated carrier. The reflected signal from a moving target is shifted in frequency by the Doppler effect. Mixing the received signal with a sample of the transmitted signal produces a beat note at the Doppler frequency, which is proportional to target velocity.

CW Source

A stable oscillator (Gunn diode, DRO, or synthesizer) generates the continuous-wave transmit signal. A portion of this signal is tapped and used as the LO for the receive mixer, ensuring coherent detection.

Circulator

Routes the CW source output to the antenna for transmission, and routes the received echo from the antenna to the LNA. Circulator isolation (20-30 dB) limits the amount of transmit leakage reaching the receiver.

Mixer + LO Tap

The mixer combines the received echo with a sample of the CW source. The output is the Doppler frequency shift: fd = 2vftx/c. For a 24 GHz sensor, a 1 m/s target produces a 160 Hz Doppler shift.

DSP / Frequency Counter

Measures the Doppler frequency and converts it to velocity. FFT-based processing can resolve multiple targets at different speeds simultaneously.

Typical Specifications

Component Specifications

ComponentParameterTypical Value
CW SourceFrequency10 - 77 GHz
CW SourceOutput Power+5 to +20 dBm
CirculatorIsolation20 - 30 dB
LNANoise Figure1.0 - 3.0 dB
LNAGain15 - 25 dB
MixerConversion Loss6 - 10 dB
Doppler LPFBandwidth10 Hz - 50 kHz
Design Note: The main limitation of monostatic CW Doppler is TX-to-RX leakage through the circulator. This leakage power can saturate the LNA or mixer. Techniques to mitigate include: high-isolation circulators, TX leakage cancellation circuits, and careful impedance matching of the antenna to minimize reflections back into the receiver.
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