CW / FMCW / FSK
1 CW (Continuous Wave Radar)
CW radars which are the simpliest form of radars operate continuously on stable high frequency. It can be used to sdtetct un stationary targets as distance measurement cannot be performed. Velocity information is extracted by Doppler effect caused by motion between target and the radar.
f = doppler frequency
λ = wavelength
V = target velocity
1.1 CW Radar Application
Please visit Speed Measurement section.
2 FMCW ((Frequency-Modulated Continuous Wave Radar)
FMCW radar generates linearly modulated pulses over time and commonly used frequency modulations are sawtooth and triangular, this can be done conveniently by interfacing a Radar front end (via frequency divider output) with an off-chip phase-locked loop (PLL) that can generate the modulation signal. Determining the range information of a target can be done due to known freuqnecy difference between received and transmit signals and time offset in contrast to CW radars. Frequency shift is determined by mixing the recevied signal from target with transmitted signal.
Figure one demonstrates sawtooth modulated FMCW radar principle, from there time offset and frequency shift between received(blue) and transmitted(orange) signal can be obtained.Using FMCW radar, target distance can determined with high accuracy. Target distance is linear to the frequency shift (delta f in graph above)and greater difference in frequency referes to a high range. The frequency shift information is extracted by performing Fourier tranform on digitiliazed data and target range can be determined from the spectrum.
f= (B* 2R)/(T*c)
f=beat frequency B=Bandwidth
Range of the target affacted by great numbers of factors such as antenna gain, Radar cross section (RCS), Wavelength, atmospheric effects etc.
2.1 FMCW Radar Application
Please visit Applications section.
3 FSK (Frequency Shift Keying)
FSK is another radar technology and unlike CW and FMCW radars, transmitting freuqency is switched between at least two frequencies. Since the frequency changes is less than bandwidth of FMCW radars, Doppler shift effect of reflected target is the same but the phase is changed correlated with changes in target range .Range and velocity measuring is applicable for FSK radars. f1 and f2 in the below pictures corresponds to carrier frequencies of the radar sensor. Velocity information of target is pbtained from Doppler frequency however range is determined by comparing the phase of before and after frequency switched signals voltages.