Concept- Transmission is continuous here, the circulator is used to provide isolation between the transmitter and the receiver. Since transmission is continuous, it would be pointless to use a duplexer. The isolation of a typical circulator is of the order of 30dB, so hat some of the transmitted signal leaks into the receiver. The signal can be mixed in the detector with returns from the target, and the difference is the Dopler frequency. Being generally in the audio range in most Doppler application, the detector output can be amplified with an audio amplifier before being applied to a frequency counter. The counter is a normal one, except that its output is shown as kilometers or miles per hour, rather than the actual frequency in hertz. The used to accommodate the high incoming frequency is not a very good device at the audio output frequency, because of the modulation noise which it exhibits at low frequencies.
Applications: 1. It is used to give Doppler information contained in echo signal. 2. It is used to measurement of relative velocity to distinguish moving target from stationary objects