FREQUENCY MODULATED CW RADAR

FREQUENCY MODULATED CW RADAR:

The transmitter signal in a CW radar is not modulated. therefore it can neither provide range of the target nor sense which particular cycle of oscillation is being received at any instant. this major drawback can be eliminated by frequency modulation of the transmitted signal even though it increases the bandwidth.

a saw tooth generator frequency modulates the CW carrier. thus transmitted signal frequency increases linearly with the increasing amplitude of the modulating signal.

Earth acts as a target which is stationary with respect to the aircraft.

for a given altitude of aircraft, a known time will be required for the signal to travel from earth to aircraft and hence during this time a definite change in single frequency would take place.If this frequency change can be measured,an indication of the height of the aircraft is obtained.the frequency counter is switched on first when the modulating signal has zero frequency(i.e. the  transmitter frequency equals fc ) and this signal frequency is picked up after its reflection from the earth and used to switch off the counter. the final reading of the counter will give an indication of the change in frequency and hence the altitude of the aircraft,which is displayed on the indicator.

If the relative velocity of the aircraft and earth is not zero, there will be a Doppler frequency shift in addition to the frequency change.

This frequency shift provides a measure of the relative velocity of the aircraft.however the time difference between the transmission and reception of a particular cycle of the signal will be constant and hence the average frequency difference will also be constant. therefore accurate altitude measurement can still be made on the basis of average frequency difference.

 If 2Δf is the range of frequency and T is the modulation period, rate of change of frequency is given by

df/dt=2Δf/(T/2)

APPLICATION:

FMCW radar is mostly used as altimeter in aircraft due to shorter ranges and lower power requirements as compared to pulsed radars,smaller size for aircraft installation and smaller transmitter powers.
typical equipment developed for FMCW radar has a power less than 1 watt and operates at carrier frequency of 440MHz with modulating frequency of 120 hz and frequency deviation of either 2 or 20 MHz.a