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FMCW radar sensor: How it works. FMCW basics.

Learn the bascics of FMCW radar. And how FMCW radar sensors measure distances.

We use the latest frequency modulated continuous wave (FMCW) radar technology for our radar sensors for distance measurement or collision avoidance. Learn more about the FMCW principle and how radar distance sensors work!

FMCW radar: Frequency Modulated Continous Wave

FMCW radar: Frequency Modulated Continous Wave

The radar with the “chirp” – a continuously modulated radar signal

OndoSense uses frequency modulated continuous wave (FMCW) radars for distance measurement. An FMCW radar emits a radar frequency that continuously increases or decreases over a defined frequency band width (bw). This process is known as frequency modulation and is also called sweep or chirp. The frequency is usually modulated in a linear way. The duration of the recurring frequency modulation is called chirp time (t).

FMCW type: Sawtooth modulation with linear chirp

FMCW type: Sawtooth modulation with linear chirp

The radar signal increases linearly from minimum to maximum frequency (up chirp)

In addition to triangular modulation, the sawtooth modulation is one of the most common FMCW processes. In the sawtooth method, the radar signal continuously increases from the minimum to the maximum frequency (up chirp) – e. g. in the ISM band from 122 GHz to 123 GHz.

If the emitted radar signal hits an object, it will totally or partly reflect the radar waves back to the radar sensor. The received radar signal has a different frequency than the emitted signal. From the frequency shift (Δ f) of the two signals, the radar sensor can then determine the exact distance to the object.

How FMCW radars calculate distances

The distance (d) to the target object is calculated from the chirp time (t), the frequency difference (Δ f), the total frequency band width of the chirps, and the (approximate) speed of light (c) at which the radar waves move through the space.

The sawtooth process is also suitable for the so-called fast chirping: Very short chirps of less than 100 μs are generated in order to achieve, among other things, high measuring frequencies.

FMCW radar vs. CW radar (Doppler radar): the differences

In contrast to FMCW radar sensors, CW radars emit unmodulated radar waves.

FMCW radar: frequency modulation

FMCW radar: frequency modulation

CW radar: no frequency modulation

CW radar: no frequency modulation

What is the main difference between CW and FMCW radar sensors? In contrast to FMCW radars, CW radars (continuous wave radas) emit radar waves with a constant frequency and amplitude. Therefore, there is no frequency modulation with CW radar.

Compared to FMCW radar sensors, CW radar sensors are mainly used for speed measurement. This is done by determining the so-called Doppler frequency, which refers to the frequency difference between the transmitted signal and the received signal.

Using the Doppler frequency, the CW radar can then calculate the speed of moving objects. For this reason, CW radar sensors that measure speeds are also called Doppler radars. The police often use Doppler or CW radars as “speed cameras” to detect illegal speeding. Learn more about CW radar sensors in our radar glossary.

Differences between FMCW radars and pulse radars

Unlike FMCW radars, pulse radars do not emit a continuous radar signal.

Pulse radar: impulses instead of continuous radar waves

Pulse radar: impulses instead of continuous radar waves

In contrast to FMCW radar sensors with their continuous radar signal, pulse radar sensors send out short signal pulses at recurring intervals. The target object reflects these impulses back to the radar. The pulse radar can then calculate the distance to the target from the time difference between the transmitted and received signals.

As a result, you can use pulse radars to measure the distance to objects – just like FMCW radars. However, FMCW radar sensors achieve a significantly higher measurement accuracy than pulse radars when it comes to distance measurement. As a matter of fact, OndoSense radar sensors even offer a measurement accuracy in the micrometer range. However, pulse radar sensors with strong transmission power can detect the position of objects at distances of more than 100 km.

Impulse radars are ideal for positioning targets at very long distances. This is why tthey are used by the military sector, for air traffic control, in weather observation and in earth exploration. In the past, pulse radar sensors were also used as level radar sensors. However, the FMCW radar has now replaced the pulse radar in level measurement. Learn more about impulse radar sensors in our radar glossary.


Smart radar sensor for distance measurements.

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High-precision radar distance sensor.

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