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Receiver Channels

Amplitude Channel

Figure 1: simplest demodulator: envelope curve detector

Schematic symbols for demodulation: a serial diode followed by parallel capacitor for rectification and filtering. You will see the envelope only on the oscilloscope screen.

Figure 1: simplest demodulator: envelope curve detector

The signal at the receiver output can have different properties. In the simplest case, the intermediate frequency is rectified by a diode, and at the receiver output, you can measure a so-called video signal. Since this video signal of the receiver includes frequencies up to 5 MHz (depending on the bandwidth), one cannot speak as with a radio receiver of low frequency but the principle of the demodulation is the same as a radio in the medium wave (MW) broadcast band.

This type of receiver output signal provides the advantage of maximum sensitivity of the radar receiver. As major disadvantages are mentioned, a Doppler frequency analysis is not possible, and all interferences overlay the radar signal. Therefore, such an output signal in analog radars was shown only in the far area on the screen, an area in which there is no interference by fixed targets (clutter).

This kind of receiver output is used for the following applications in modern surveillance radars only:

Coherence Channel

To detect a Doppler frequency (and thus a speed), the received signal must pass through a demodulator which is phase-sensitively. (phase detector). At the output of the receiver, a signal of intermediate frequency is provided. This signal was amplified but only changed through a dynamic range compression up to this point.

Modern surveillance radars use this pathway only because a Doppler frequency analysis provides valuable information for automatic target detection.