Radar systems with external coherence
discriminator
Figure 1: Block diagram of moving target detection by external coherence.
discriminator
Figure 1: Block diagram of moving target detection by external coherence.
Radar systems with external coherence
The principle of radar systems with external coherence is similar to that of systems with internal coherence (pseudo-coherent or fully coherent). The difference is that the coherent reference signal is not phase-controlled by an internal coherent oscillator but by a passive interference caused by fixed clutter. A circuit determines whether the phase difference between a target and the adjacent fixed clutter is constant or variable from pulse to pulse.
In principle, the circuit is a simple classical radar with a non-coherent transmitter, usually equipped with a magnetron. In signal processing, the IF signal is divided into two paths, one of which is delayed by a small amount with a delay chain. The delayed part corresponds to a reference frequency. A phase discriminator measures the phase difference between the two signals. If this phase difference is constant from pulse to pulse, then both echo signals are caused by fixed targets and are suppressed by a low-pass filter in the display. If even one of the targets of these two echo signals moves, a Doppler frequency is generated at the output of the phase discriminator. In the simplest case, the display is realized acoustically in a headphone.
In case there is no matching fixed target for the moving target, a generator forcedly synchronized by the phase of the last echo signal can provide a phase reference with a damped oscillation in the reference path between the delay and the phase discriminator.