nature of the collapsed building. They are 1. L or S band frequency say 1150 MHZ 2. UHF band frequency say 450 MHZ. Let us see the advantages and disadvantages of both the systems later. Schematic diagram of the microwave life-detection system The circuit description is as follows: PHASE LOCKED OSCILLATOR: The phase locked oscillator generates a very stable electromagnetic wave say 1150 MHZ with output power say 400 mW. DIRECTIONAL COUPLER 1(10 DB): This wave is then fed through a 10 dB directional coupler and a circulator before reaching a radio frequency switch , which energizes the dual antenna system. Also, the ten dB directional coupler branches out of the wave (40 mW ) which is then divided equally by a directional coupler2(3dB). DIRECTIONAL COUPLER2(3dB) : One output of the 3 dB directional coupler2 (20mW) drives the clutter cancellation unit. Other output serves as a local reference signal for the double balanced mixer. ANTENNA SYSTEM : The dual antenna system has two antenna, which are energized sequentially an electronic switch. Each antenna acts separately. CLUTTER CANCELLATION UNIT: The clutter cancellation unit consists of A digitally controlled phase shifter I A fixed attenuator A RF amplifier A digitally controlled attenuator WORKING: CLUTTER CANCELLATION OF THE RECEIVED SIGNAL: The wave radiated by the antenna I penetrates the earthquake rubble to reach the buried person . The reflected wave received by the antenna2 consists of a large reflected wave from the rubble and a small reflected wave from the
person’s body. The large clutter from the rubble can be cancelled by a clutter canceling signal . The small reflected wave from the person’s body cannot be cancelled by a pure sinusoidal canceling because it is modulated by their movements. The output of the clutter cancellation circuit is automatically adjusted to be of equal amplitude and opposite phase as that of the clutter from the rubble . Thus, when the output of the clutter cancellation circuit is combined with the directional coupler3 ( 3 dB ), the large clutter from the rubble is completely cancelled Now, the output of the directional coupler3 (3dB) is passed through a directional coupler4 (6dB) One-fourth of the output is directed is amplified by a RF pre-amplifier and then mixed with a local reference signal in a double balanced mixer. Three-fourth of the output is directed by a microwave detector to provide a DC output, which serves as the indicator for the degree of the clutter cancellation . When the settings of the digitally controlled phase shifter and the attenuator are swept the microprocessor control system, the output of the microwave detector varies accordingly. DEMODULATION OF THE CLUTTER CANCELLED SIGNAL: At the double balanced mixer, the amplified signal of the reflected wave from the person’s body is mixed with the local reference signal. The phase of the local reference signal is controlled by another digitally controlled phase shifter2 for an optimal output from the mixer.
The output of the mixer consists of the breathing and heart beat signals of the human plus some unavoidable noise. This output is fed through a low frequency amplifier and a band pass filter (0.4 HZ) before displayed on the monitor . The function of the digitally controlled phase of the local reference signal for the purpose of increasing the system sensitivity. The reflected signal from the person’s body after amplification by the pre-amplifier is mixed with the local reference signal in a double balanced mixer. MICROPROCESSOR CONTROL UNIT: The algorithm and flowcharts for the antenna system and the clutter cancellation system are as follows : ANTENNA SYSTEM: Initially the switch is kept in position1 i.e Signal is transmitted through the antenna1 Wait for some predetermined sending time ,Ts Then the switch is thrown to position2 i.e Signal is received through the antenna2 Wait for some predetermined receiving time Tr Go to step 1 Repeat the above procedure for some predetermined time, T. CLUTTER CANCELLATION SYSTEM: Send the signal to the rubble through antenna1. Receive the signal from the rubble through antenna2. Check the detector output .if it is within the predetermined limits go to step5. Otherwise send the correction signal to the digitally controlled phase shifter1 and the attenuator and go to step1.