3.MICROWAVE SYSTEM

A microwave is signal measuring a wavelength of 30.05 cm or less which is converted to 984MHZ. This puts all the frequencies above 1GHZ under the same name (microwaves).The highest range of the microwave is 10 PetaHZ, which are light frequencies. But practical maximum range is 300 GHZ with a wavelength of 0.1 cm. It transmits very high power, very short duration pulse distance between two object that can be determined by measuring the time span for the signal of a known velocity after being reflected back.

The pulse generator sets the pulse width and the pulse repetition rate (tine between pulses) of the transmitted signal. Modulator turns on power oscillator for a given time at a rate determined by the pulse generator. The oscillator oscillates at the RF carrier frequency to send out short burst of high-frequency energy.

The HF and high power energy is carried by the waveguide to the horn antenna and parabolic reflector to concentrate energy into a narrow pencil beam to the target.

Transmit Receive (TR) tube short – circuits the waveguide going to the mixer so as to protect the diode mixer from high- energy pulse from power oscillator. The anti-transmit receive (ATR) box is gas – filled tube like TR but in series with waveguide that connects the power oscillator to the antenna. ATRs form a low – impedance path for power oscillator signal going to the antenna during high – energy pulse. It only becomes a high impedance at receive mode. Distance between ATR and TR must always be ¼ wavelength.

Echo box is used for testing the system when no targets are within range but turned far off carrier frequency during normal operation. A pulse is transmitted while the echo box is motor – tuned through resonance to indicate a return signal.

The mixer gets signal from the antennae and from the local oscillator too. The oscillator operates at a frequency higher than RF signal. The IF amp selects the difference frequency at about 30MHZ from the out put of the mixer. The IF has 5 Amplifying stages for a higher gain over 10MHZ bandwidth.

AFC is equivalent to phase-locked loop circuit. After the mixer acting as a comparator and amplified by IF circuit. The signal is then converted to DC in the AFC circuit relative to frequency change and is used to control the frequency of a VCO.

Video amplifier with wideband frequency response to accommodate multiple odd harmonics of the fundamental square wave modulating signal. The product of the video amplifier is used to intensely modulate the CRT monitor. The deflection oscillator generates a trapezoidal wave signal to be amplified by power amplifier.

The amplifier signal is then applied to deflection coil round the neck of the CRT
The synchronizing circuit notes each pulse from the generator and starts CRT sweep at the center of the display monitor as each pulse leaves transmitter.

For full time – distance indication, Range markers are added to display the full adder.
Motor servo system controls the spread and position of the rotating antenna assembly.


Reference:
Electronic communications Modulation and Transmissions by Robert J Schoenbeck.
RF Circuits Design Wireless System Volume 3 By Gilmore Besser,
The Design of CMOS Radio frequency Integrated Circuits By Thomas H Lee.
And http:/wikipedia.org/wiki/transmissionline/
Dr. David J., Jefferies, School of electronics engineering, IT and Mathematics, University of Survey England.