Radar System Engineering

528 THE RECEIVING SYSTEJ{—INDICATORS [SEC. 1314
plate of VI triggers off the flopover. This method is extremely useful
when a variable sweep is involved, since it avoids the necessity of ‘‘ tracking”
two potentiometers.
13.14. B-scope Design.—The essentials for providing a type B display
have been shown in Fig. 121 and described in Sec. 12.2. Either an
electrostatic or a magnetic cathode-ray tube can be used, the former
being much cheaper but far less satisfactory. Except for the final
amplifiers the circuit requirements in a given situation are similar, no
matter which tube type is used.
An electrostatic B-scope can be built around the range sweep circuits
of the A-scope of Fig. 13”42, i the only changes necessary being to move
the video and marker signals to the intensity-modulating elements of the
CRT and to provide the azimuth sweeps. A potentiometer is almost
invariably used as the azimuth data transmitter for an electrostatic
tube. It may drive the deflecting plates directly, in which case it must
have two brushes delivering peak potentials of from one to two hundred
volts. Alternatively, a push-pull direct-coupled amplifier is inserted.
If the potentiometer has only one brush, phase inversion is accomplished
by common cathode-coupling like that used in amplifier V,, V, of Fig.
13.43b.
Mfignetic B-scopes. —Magnetic B-scopes of varying degrees of complexity
have been used. If the center of the display is to remain fixed
in angle, a potentiometer is usually employed and the only essential
depart ure from A-scope design is in the amplifiers. In many applications,
however, more flexibility is desired, requiring more complicated
methods. Figure 13.43 illustrates an example in which both the center
of the sector and the scale factor in each direction can be chosen at will.
“In each diagram the top row comprises the azimuth sweep circuits.
An audio oscillator VI excites the first of a pair of differentially connected
synchros by using its rotor in an oscillating circuit. The excited synchro
is rotated in synchronism with the scanner, the second being manually
oriented to select the desired sector (Sec. 13.4). As many as five or six
B-scopes with independent sector selection can be operated in this way
from a single oscillator and antenna synchro. The modulated signal
from the second synchro is passed through an impedance-changing
cathode follower Vz, demodulated by a phase-sensitive rectifier Vt (Sec.
13.9), then smoothed by an RC-filter. The azimuth deflecting circuit is
completed by the push-pull amplifier VA, Vh, in which the latter tube
receives its exciting signal by virtue of the common cathode and screen
resist ors.
If continuous rotation is used, rather than sector scanning, the sweep
] The circuit of Fig. 13.41 is not suited for a cathode-ray tube using accelerating
potentials as high as those required for a B-scope with a persistent screen.