Radar & Defense

CW Radar

Q: How does CW radar measure velocity without measuring range?

A continuous-wave radar transmits a constant single-frequency carrier. When the energy reflects off a moving target, the round-trip path length changes continuously, so the returned signal is shifted in frequency by the Doppler effect: f_d = 2*v_r*f0/c, where v_r is the radial velocity component and f0 is the carrier frequency. The receiver mixes the echo against a sample of the transmitted carrier and the resulting beat frequency is the Doppler shift itself, which maps one-to-one to radial speed. Because an unmodulated carrier carries no timing reference, there is no way to recover the propagation delay, so a pure CW radar reports velocity only and cannot determine target range. At 10.525 GHz an X-band traffic radar produces about 31.3 Hz of Doppler per mile per hour, so a 60 mph car returns roughly a 1.88 kHz tone that audio-band electronics resolve easily.

Q: Why is transmit-to-receive isolation the hardest part of a CW radar design?

Because the transmitter is on continuously, any leakage path into the receiver appears as a strong zero-Doppler signal that competes directly with weak target echoes. A monostatic CW radar that shares one antenna relies on a circulator or a reflectometer bridge for isolation, but these typically provide only 20 to 30 dB, and antenna VSWR reflects more transmit power straight back into the receiver. The leakage also drags in the transmitter phase noise, so the receiver noise floor near the carrier is set by the oscillator rather than by thermal noise. Practical CW sensors use separate transmit and receive antennas for 40 to 70 dB of physical isolation, low-phase-noise sources, and sometimes a feedthrough nulling loop that injects a cancelling sample of the leakage. Spillover from nearby fixed clutter is rejected by the receiver high-pass response that removes the DC and very-low-frequency components.

Q: What is the difference between CW, FMCW, and pulse-Doppler radar?

A pure CW radar transmits an unmodulated tone and recovers velocity from the Doppler shift but has no range capability and, in monostatic form, suffers from continuous transmit leakage. FMCW radar sweeps the carrier linearly in frequency so the beat between transmit and echo encodes round-trip delay, recovering both range and velocity while still transmitting continuously; it is the basis of automotive 77 GHz radar and radar altimeters. Pulse-Doppler radar transmits short pulses and listens between them, which gives clean range gating and removes the leakage problem, at the cost of greater peak power and a more complex transmitter. CW radar wins where simplicity, low cost, and high sensitivity to slow Doppler matter, such as police speed guns, motion detectors, and proximity fuzes.

/see-dubl-yoo ray-dar/

A radar architecture that transmits an uninterrupted, unmodulated carrier and detects moving targets from the Doppler shift of the reflected signal rather than from echo timing. By mixing the return against a sample of the transmitted wave, the receiver produces a beat tone equal to the Doppler frequency (f_d = 2v_rf₀/c), giving direct, unambiguous radial-velocity measurement with no blind speeds and excellent sensitivity to slow motion. The penalty is that an unmodulated tone carries no timing reference, so a pure CW radar reports velocity only and provides no range; adding a linear frequency sweep turns it into an FMCW radar that recovers range as well. Continuous transmission also makes transmit-to-receive isolation the dominant design challenge, which is why CW sensors favor separate antennas and low-phase-noise sources. The topology dominates police speed guns, motion detectors, proximity fuzes, and Doppler velocimeters from 10 to 94 GHz.

Category: Radar & Defense

Measures: Radial velocity (Doppler)

Typical Bands: X (10.525 GHz), K, W

How Doppler Sensing Replaces Range in CW Radar

Continuous-wave radar trades the range capability of a pulsed system for an extremely clean velocity measurement. The transmitter emits a single fixed frequency, and the only thing that changes between the transmitted and received waveforms is the Doppler shift imposed by a target's radial motion. A receiver that mixes the echo against the transmit reference, an arrangement known as homodyne or zero-IF detection, produces a beat frequency exactly equal to the Doppler offset. Because the shift is proportional to both velocity and carrier frequency, a higher band such as K-band (24 GHz) or W-band (77 to 94 GHz) yields more hertz per unit speed and finer velocity resolution than X-band. There are no blind speeds in a pure CW system, and stationary clutter sits at zero Doppler where it is easily filtered out.

The defining limitation is the absence of a timing reference. With no modulation imprinted on the carrier, there is no observable propagation delay, so the radar cannot tell whether a target is near or far, only how fast it is closing or receding. Designers who need range as well as velocity introduce a frequency or phase modulation, which produces the FMCW family and its relatives. Even then the carrier is never switched off, so the design must contend with the second hallmark of CW operation: the transmitter and receiver run simultaneously and must be isolated from each other.

That isolation requirement shapes nearly every CW radar layout. Any transmit energy that couples directly into the receiver appears as a large zero-Doppler signal and, worse, carries the transmitter's phase noise into the baseband. Near the carrier the receiver noise floor is therefore set by oscillator phase noise rather than thermal noise, which is why low-noise dielectric-resonator or PLL sources are preferred. Bistatic layouts with physically separated transmit and receive antennas achieve 40 to 70 dB of isolation, while compact monostatic sensors rely on circulators or reflectometer bridges that typically deliver only 20 to 30 dB, sometimes supplemented by an active feedthrough-nulling loop.

Governing Equations

Doppler Frequency Shift:
f_d = 2v_rf₀ / c = 2v_r / λ

Radial Velocity from Beat Tone:
v_r = f_d × c / (2f₀) = f_d × λ / 2

Received Echo Power (radar equation):
P_r = P_tG²λ²σ / [(4π)³R⁴]

Where v_r = radial velocity, f₀ = carrier frequency, c ≈ 3×10⁸ m/s, λ = wavelength, P_t = transmit power, G = antenna gain, σ = radar cross-section, R = range. Example: at f₀ = 10.525 GHz (λ ≈ 28.5 mm), a target closing at 60 mph (26.8 m/s) gives f_d ≈ 1.88 kHz, an audio-band tone.

CW versus FMCW versus Pulse-Doppler

Attribute	Pure CW	FMCW	Pulse-Doppler
Range capability	None (velocity only)	Yes (beat encodes delay)	Yes (range gating)
Velocity capability	Excellent, unambiguous	Good (from sweep slope)	Good, with PRF ambiguity
Tx/Rx isolation	Critical (continuous Tx)	Critical (continuous Tx)	Easy (time duplexing)
Peak vs avg power	Equal (100% duty)	Equal (100% duty)	High peak, low average
Limited by	Source phase noise	Sweep linearity	Transmitter peak power
Typical use	Speed guns, fuzes, motion	Automotive 77 GHz, altimeters	Air-surveillance, weather

Common Questions

Frequently Asked Questions

How does CW radar measure velocity without measuring range?

The radar transmits a constant single-frequency carrier; a moving target shifts the echo by the Doppler relation f_d = 2v_rf₀/c. Mixing the echo against the transmit sample yields a beat tone equal to that shift, mapping one-to-one to radial speed. An unmodulated carrier holds no timing reference, so propagation delay (and therefore range) cannot be recovered. At 10.525 GHz a 60 mph target returns roughly a 1.88 kHz audio tone.

Why is transmit-to-receive isolation the hardest part of a CW radar design?

Because the transmitter is on continuously, leakage into the receiver appears as a strong zero-Doppler signal and drags in transmitter phase noise, setting the near-carrier noise floor. Monostatic single-antenna designs using a circulator or bridge get only 20 to 30 dB, and antenna VSWR reflects more power back. Bistatic layouts with separate antennas reach 40 to 70 dB, often with low-phase-noise sources and a feedthrough nulling loop.

What is the difference between CW, FMCW, and pulse-Doppler radar?

Pure CW sends an unmodulated tone and recovers velocity only, with continuous leakage. FMCW sweeps the carrier so the transmit-to-echo beat encodes round-trip delay, giving range and velocity (automotive 77 GHz, altimeters). Pulse-Doppler transmits short pulses and listens between them for clean range gating and no leakage, at the cost of high peak power. CW wins where simplicity, low cost, and slow-Doppler sensitivity matter.

Related Terms

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