Radar System Engineering

SEC. 122] A TYPICAL RECEIVING SYSTEM 435
a. True plots of a plane surface in which range and an angle are
combined as polar coordinates (PPI).
b. Rectangular plots of two cartesian components of range, in
general with unequal normalizations’ (stretched PPI, RHI).
c. Rectangular plots of the polar coordinates, range and angle
(B-scope, E-scope.)
d. Rectangular plots of azimuth angle and elevation angle
(type C).
e. Modifications of the above types to indicate a third dimension
in certain simple situations.
3. Presentations, not properly displays, in which the cathode-ray
tube is used as a two-dimensional meter (spot error indicator).
Additional types of deliberate deformation of the basic displays and
certain approximations used for technical reasons will appear later in
detailed descriptions of the production of the various displays.
Observations and measurements are aided by various indices. Practically
all the displays that include range are provided with a set of precisely
timed ‘‘ electronic markers” which occur at convenient regular
intervals on the display itself. These are sometimes supplemented by a
manually controlled continuously movable marker, which removes the
necessity for interpolation. Measurements of angle can be made by
means of similar electronic indices, but fixed lines etched on a transparent
overlay plus, perhaps, a movable mechanical cursor, are often used.
12.2. A Typical Receiving System.—Figure 12.1 illustrates the
principal parts and some of the subdivisions of a typical receiving system
containing a cathode-ray-tube indicator.
The receiver is always of the superheterodyne type and consists of the
signal channel and the local oscillator, together with frequency-control
circuits for the latter. Strictly speaking, this should include all equipment
concerned with the received signal, beginning with the antenna and
ending with the input terminals of the indicator proper. However, in
practically all radar applications the antenna, the TFi tube, and the r-f
line connecting them are shared with the transmission channel, and the
techniques in these sections are dictated somewhat more by the transmitter
requirements than by those of the receiver. We shall, therefore,
consider the receiver as beginning at the point where the signal channel
branches from that which is shared with the transmitter.
R-f signals from the antenna by way of the TR switch enter a crystal
mixer where they are combined with the c-w output of a tuned local
oscillator to form a heterodyned signal at the desired intermediate frequency,
usually 30 to 60 IUc/sec. This signal is led from the mixer into
an intermediate-frequency amplifier of very special characteristics, where
1If the normalizations are equal the display is, of course, equivalent to Category a