TABLE OF CONTENTS The detector stage converts the intermediate frequency signals into video signals as in a classical superheterodyne receiver. The intermediate frequency signal has the following types of modulation:
Amplitude modulation;
Phase (or frequency) modulation.
In early radars the output of the detector was amplified and displayed, as shown in Figure 9.1. In radars that observe or track individual objects (fire control radars, battlefield observation radars, for example) as an aid to identifying each type of scatterer the range gated echoes are passed to headphones. With a staring radar the sound in the headphones allows the operator to identify, for example, propeller modulation of aircraft in chaff and the movement of animals, people, or vehicles.
Between the echoes there is principally receiver noise without any perceptible carrier frequency. The noise pulses occur at random and have a bidirectional Gaussian distribution shown in Figure 2.19, called polyphase or bottlebrush noise. The echo signals are modulated on a notional carrier, the intermediate frequency representation of the transmitter pulse, and have an amplitude proportional to the scatterer size and inversely proportional to the fourth power of their range. The phase of the echo signal with respect to the coherent oscillator (COHO) depends on its fine range. The phase angles repeat themselves each half of a wavelength in range. The frequency of the carrier is the intermediate frequency plus or minus the Doppler frequency of the scatterer.
There is a problem illustrating signals mixed with noise. The form of a single echo...
