CW / FMCW / FSK
Contents
1 CW (Continuous Wave Radar)
CW radars which are the simplest form of radars operate continuously on a stable high frequency. It can be used to detect un stationary targets as distance measurement cannot be performed. Velocity information is extracted by the Doppler effect caused by motion between the 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 the known frequency difference between received and transmit signals and time offset in contrast to CW radars. The frequency shift is determined by mixing the received signal from the target with the transmitted signal.
Figure one demonstrates the 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, the target distance can be determined with high accuracy. Target distance is linear to the frequency shift (delta f in the graph above)and a greater difference in frequency refers to a high range. The frequency shift information is extracted by performing Fourier transform on digitalized data and the target range can be determined from the spectrum.
f= (B* 2R)/(T*c)
f=beat frequency B=Bandwidth
Range of the target affected 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 frequency is switched between at least two frequencies. Since the frequency changes are less than the bandwidth of FMCW radars, the Doppler shift effect of the reflected target is the same but the phase is changed correlated with changes in the target range. Range and velocity measuring is applicable for FSK radars. f1 and f2 in the below pictures correspond to carrier frequencies of the radar sensor. Velocity information of the target is obtained from Doppler frequency however range is determined by comparing the phase of before and after frequency switched signals voltages.
