Thickness-fix principle
The Dicke-Fix principle is a method for suppressing short interference signals in radar devices .
This form of radar signal processing is implemented in the IF amplifier . The principle was perfected around 1960 by Robert Dicke (Defense Research Telecommunications Establishment in Ottawa). A broadband amplifier is followed by a limiter and then the signal is processed further in a narrowband signal-adapted IF amplifier. This procedure attenuates signals that are too long ( clutter ) or that are too short (noise peaks and impulse interference).
Working principle
The basic equipment consists only of a broadband, limiting IF amplifier, followed by an IF amplifier with optimal bandwidth . The clipping level is set very low so that the peak levels of noise interference are limited and a constant false alarm rate is achieved over the entire range. Depending on the interference, the bandwidth can vary between 10 and 20 MHz and is therefore much larger than the reciprocal of the pulse duration
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At test point 1, a mixture of useful signals, interference pulses and noise can still be seen. The interference pulses even have a greater amplitude than the useful signals . After the limitation (test point 2), all signals have approximately the same maximum amplitude.
The following filter shows resonance behavior for the expected pulse duration. Signals with exactly this pulse duration have a slightly larger amplitude at test point 3 than the interference signals. The bandwidth of the filter is therefore equal to the reciprocal of the pulse duration:
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After demodulation , the video amplitudes of the useful signals are greater than the interference signals (test point 4). The interference signals can therefore be suppressed with a threshold value circuit. At the filter output, the echo signals with the expected bandwidth (i.e. with the expected pulse duration) have a greater amplitude than the interference pulses and the noise and can therefore be more easily recognized as echo signals in the threshold value circuit (test point 5).
application
This circuit can very well reduce the effect of swept jammers up to an order of magnitude of 20 to 40 dB without significantly impairing the sensitivity. The thickness-fix principle, on the other hand, has no effect in the case of strong unmodulated interference, as this drives the receivers to their limits. The Dicke-Fix principle was specially developed to protect the receiver from noise interference, from interference from fast swept or narrow-band pulse-modulated jammers. The thickness-fix principle is z. B. used in the air defense radar device MPR .