The block diagram on the figure illustrates the principle of a fully coherent radar. The fundamental feature is that all signals are derived at low level and the output device serves only as an amplifier. All the signals are generated by one master timing source, usually a synthesiser, which provides the optimum phase coherence for the whole system. The output device would typically be a klystron, TWT or solid state. Fully coherent radars exhibit none of the drawbacks of the pseudo-coherent radars, which we studied in the previous section.
The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.
The function of the mixer stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal.
After conversion to the intermediate frequency, the signal is amplified in several IF-amplifier stages. Most of the gain of the receiver is developed in the IF-amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF-stages.
In this system the transmitting pulse is caused with a small performance in a waveform generator. It is taken to the necessary power with a Power Amplifier following. The Power Amplifier would typically be a klystron, Travelling Wave Tube (TWT) or solid state.
The StaLO is also very stable CW RF oscillator, which generates the local RF frequency simultaneously for up-conversion in the transmitter and down-conversion in the receiver. Minimum FM noise (or phase noise) of the StaLO is an important characteristic. This is because such noise would limit the overall MTI improvement factor, as fixed clutter would inherit a Doppler component from the transmission. Similar arguments apply to FM noise added by the output device.
The COHO is a very stable CW (Continuous Wave) oscillator locked to the IF frequency (The COHO frequency is generally derived from a master crystal oscillator) and constitutes the internal phase reference. The COHO provides the coherent reference signal to the Phase Sensitive Detector and also through a frequency divider generates the system PRF in the Synchronizer.
The function of this mixer stage is to convert the StaLO- Frequency and the COHO- Frequency upwards into the phase-stabile continuous wave transmitter-frequency.
The IF-signal is passed to a phase sensitive detector which converts the signal to base band, while faithfully retaining the full phase and quadrature information (I & Q- processing) of the Doppler signal.
The indicator presents to the observer a continuous, easily understandable, graphic picture of the position of radar targets. In recently radars the indicator would be a computer display.
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