Continuous-wave Doppler radar

Continuous wave (CW) Doppler radars use continuous ous waves in place of pulses, in which the doppler is used to measure the frequency change caused by a dynamic target and is displayed as the relative velocity Goes because this broadcast is continuous. A simple Doppler radar is shown in the figure.

The circulator in the image is used to generate an isolation between the transmitter and receiver. The use of a duplexer in continuous transmission is futile. The isolation of the special circulator is of the order of 304B due to which some part of the transmitter signal leaks into the receiver. This facilitates that this signal can be mixed with the signal returning from the target to the detector. The difference between these two signals is called the doppler frequency. Due to its being in the Audi range in many Doppler applications, the output of the detector can be amplified by an audio amplifier and mounted on a frequency counter. The counter is just like a normal counter, with only its output displayed in kilometers or miles per hour in place of the actual frequency in herts. Doppler's radar system is very simple, but its sensitivity is very low. The detector used in this method is not helpful in detecting high frequencies because at low frequencies it produces a very high modulation noise. The modified receiver of the device with this view is shown in the figure.

According to the figure, the microscopic portion of the transmitter output power is mixed with the output of a local oscillator and this mixture is fed into the receiver mixer. The receiver mixer also receives the Doppler shift signal from the antenna and produces an output differential frequency (30 MHz + Doppler frequency). The output of this mixer is amplified and re-modulated, and the signal of the second detector is in the Doppler frequency because its sign is terminated so that it tells. It is not possible that the target is approaching or moving away. The receiver system is generally similar to a super heterodyne in that additional sensitivity is achieved by lowering the noise as the output of the diode mixer is now near 30 MHz at which the FM noise terminates. In this, separate antennas are shown for transmitter and receiving but this is not necessary. A circulator can be used to use the same antenna. Using separate antenna can ensure complete isolation of transmitter and receiver.

Thus the receiver can be shielded from the humming of the transmitter. This type of radar has a CW power of 100 W or less. Gun or IMPATT diode CW magnetrons are used for high power transmitters. These operate in the frequency range of pulse radar.

Advantages

Following are the benefits of CW Doppler radar

It can accurately calculate relative velocity
It reduces power loss
Their circuits are relatively simple and the size of the equipment used is small.
There is no effect in the presence of permanent targets.
It can also operate up to zero range because the receiver is on all the time
Through this, the speed of high range targets can be measured quickly and accurately.
By applying some additional circuits, it can measure the target's velocity along with its direction.

Application

Following are the applications of CW Doppler radar -

Used for speed measurement in aircraft navigation
Used in the rate off - climb scale
Used in radar speed motors used by police

Limitations

CW Doppler radar has the following limitations

Its range is also limited due to its transmit power being limited.
Being in the target space reduces the clarity in its display, filters are used along with the circuit to remove it.
Doppler radar is unable to index the range of the target, it can only tell the target speed
In this, the transmint signal is amodulated, due to which it is not able to detect the timing of the receiving signal.