Radar System Engineering

434 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 121
of auxiliary equipment, the chief functions of which are to prepare the
geometrical and other data, apart from thesignals, forusebytheindicator,
to control the indicator functions, and to assist in the making of
measurements. No satisfactory descriptive term exists for this auxiliary
equipment, but in combination with the indicator it is known as the
“indicating (or indicator) equipment. ” The echo signals must be
amplified greatly and demodulated before being presented to the indicator,
usually in the form of video signals. The path which they follow in
this process will be spoken of as the “signal channel,” and the equipment
involved as the “receiver.” On some occasions the system also contains
computers or other devices which aid in the ultimate disposal of the data.
The receiver must deliver a maximum of desirable, and a minimum
of undesirable, information to ihe indicator. It should have as little
inherent noise as possible, Since such noise determines the ultimate limit
in signal detectabilityy. It should afford sufficient amplification to
realize this ultimate limit and sufficient dynamic range to allow wide
latitude in useful signal intensities. The bandwidth must be chosen to
provide sufficiently rapid transient response so that the details of the
signals will be preserved; but it must not be so great as to decrease unduly
the signal-to-noise discrimination. In many cases, special design characteristics
enhance the discrimination of certain kinds of echo in comparison
to other undesired echoes or to radiations from other transmitters.
The indicating equipment, which is almost entirely responsible for
the purely geometrical aspects of the display problem, must share with
the receiver the responsibility for the discernibility of the signals with
respect to noise, interference, and signals from other targets. The indicator
proper is almost always a cathode-ray tube whose screen presents the
radar display. Synthesis of such displays involves combining the signal
intelhgence with antenna scanning angles and uther geometrical factors
to provide an intelligible picture of the dispositions and other characteristics
of the radar targets. Some of the displays are, in principle, facsimile
representations of the actual geometrical situation; others are deliberately
deformed to improve some particular type of observation or measurement.
A wide variety of displays has been discussed in Chap. 6. The basic
types are listed here for reference:
1. De$ection-modulated displays, in which the echo signals are used
to deflect the beam laterally on the tube face. In practice, the
other rectangular coordinate is invariably range (A-scope, R-scope,
etc.).
2. Intensity-modulated displays, in which the signals serve to brighten
the screen, and hence appear as bright spots or patches against a
background that is partially illuminated by the receiver noise
These fall into several categories: