Small continious wave radars are using the Doppler phenomena. Even the most simple door opener is able to detect the changes in the reflected signal caused by the Doppler shift. There are many other practical uses of the doppler shift from the medical imaging up to the astronomy.
The Doppler shift can be observed e.g. with a car's horn. When the horn is approaching, it will sound with higher pitch which means higher frequency and shorter wavelength. By a receding horn you will hear a lower pitch which refers to a lower frequency and longer wavelength.
Let's calcualte the doppler shift!
Assume that the car is moving with a constant speed:
The horn has also a constant frequency:
When the car is approaching the sound waves are "pushed together", the observer will hear a higher pitch:
When the car is receding the sound waves are "expanded", the observer hears a lower pitch:
Why is that important at the CW radars? -they process the doppler frequency by mixing the transmitter frequency with the recieved frequency. It is also called phase coherent mixer, because the same oscillator's signal is used for transmitting and mixing the recieved frequency. Another solution to use phase locked oscillators, but the microwave sensor's we are going to use are simple systems.
The mixer architecture of the CW radars are very similar to the heterodyn radio recievers.
Fig. Continious Wave IQ antenna with fixed frequency oscillator.
By using two mixers with 90° phase difference, the phase of the doppler frequency can also be measured which will tell us the direction of the movement. An approaching target will result a +fd while a receding a -fd.
How much is the doppler frequency in a 24GHz radar sensor?