FMCW
Encyclopedia
Frequency Modulated Continuous-wave radar is a short range measuring radar set. This kind of radar is often used as “radar altimeter” to measure the exact height during the landing procedure of aircraft.
system where a known stable frequency continuous wave
radio
energy is modulated by a triangular modulation signal so that it varies gradually and then mixes with the signal reflected from a target object with this transmit signal to produce a beat signal.
Variations of modulation are possible (sine, sawtooth, etc), but the triangle modulation is used in FM-CW radars where both range and velocity are desired. As shown in the Figure the received waveform (green) is simply a delayed replica of the transmitted waveform (red). The time delay is a measure of the range.
With the advent of modern electronics, the use of Digital Signal Processing is used for most detection processing. The beat signals are passed through an Analog to Digital converter, and digital processing is performed on the result.
FM-CW radars can be built with one antenna using either a circulator, or circular polarization. Most modern systems use one transmitter antenna and multiple receiver antennas. Because the transmitter is on continuously at effectively the same frequency as the receiver, special care must be exercised to avoid overloading the receiver stages.
As explained in the literature, FMCW ranging for a linear ramp waveform is given in the following set of equations:
, where 
is the radar frequency sweep amount and 
is the time to complete the frequency sweep.
Then,
, rearrange to a more useful: 
, where 
is the round trip time of the radar energy.
It is then a trivial matter to calculate the physical one-way distance for an idealized typical case as:
, where 
is the speed of light.
Technical introduction
(FMCW) is a radarRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
system where a known stable frequency continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
energy is modulated by a triangular modulation signal so that it varies gradually and then mixes with the signal reflected from a target object with this transmit signal to produce a beat signal.
Variations of modulation are possible (sine, sawtooth, etc), but the triangle modulation is used in FM-CW radars where both range and velocity are desired. As shown in the Figure the received waveform (green) is simply a delayed replica of the transmitted waveform (red). The time delay is a measure of the range.
With the advent of modern electronics, the use of Digital Signal Processing is used for most detection processing. The beat signals are passed through an Analog to Digital converter, and digital processing is performed on the result.
FM-CW radars can be built with one antenna using either a circulator, or circular polarization. Most modern systems use one transmitter antenna and multiple receiver antennas. Because the transmitter is on continuously at effectively the same frequency as the receiver, special care must be exercised to avoid overloading the receiver stages.
As explained in the literature, FMCW ranging for a linear ramp waveform is given in the following set of equations:
, where is the radar frequency sweep amount and is the time to complete the frequency sweep.Then,
, rearrange to a more useful: , where is the round trip time of the radar energy.It is then a trivial matter to calculate the physical one-way distance for an idealized typical case as:
, where is the speed of light.