Radar System Engineering

518 THE RECEIVING SYSTEM—INDICATORS [SEC.1312
appreciable sector of the scan remote from the one of interest, in which
case the switching can be slower and in some cases mechanical. In the
first case the index appears continuously, and in the second it appears
during an appreciable fraction of the scan. In either event, the operator
is able to make this setting while the index is present. The method is not
applicable to a rotating-coil system because of its inertia. Fortunately,
however, this type of display is itself exceedingly accurate in angle, so
that the mechanical cursor can be used on the centered PPI display with
little error except that due to parallax.
13.12. Range and Height Indices; Synchronization. +ince the
providing of electronic range indices is frequently intimately associated
with synchronization, these topics can best be discussed together. The
discussion will be simplified if movable indices are described first.
Methods oj Obtaining Movable Marlcers.—In general, four methods are
used for obtaining a continuously movable marker. In increasing order
of the precision that can be obtained they are: (1) the cathode-coupled
multivibrator; (2) the phantastron; (3) a timing circuit based on a linear
sawtooth; (4) the phase-shifting of a precision sinusoid.
The use of the multivibrator or the phantastron for time-delay purposes
has been discussed in the sections describing those devices and no
further description need be given.
The use of a sawtooth voltage wave for timing depends upon the fact
that the time taken for such a wave to reach a given voltage is proportional
to the voltage chosen. Figure 1336 illustrates a circuit by means
of which this principle can be very precisely applied. Tubes Vl, Vl, Vs,
and Vi constitute a precision sawtooth generator. The drop across the
condenser C and the lack of unity gain in the cathode follower (Sec.
13. 10) are compensated by the network composed of Ca and Rz which
integrates the sawtooth appearing on the cathode of Vs and thus provides
across C2 a correction proportional to L2. The sawtooth waveform is
applied to the plate of diode Vs, whose cathode has a positive bias of an
amount determined by the setting of the delay potentiometer. Because
of this bias, no signal passes through the diode until the sawtooth has
reached a definite amplitude, determined by the bias value. When the
critical amplitude is reached (at time tl on the waveform diagram), the
remainder of the sawtooth appears on the grid of VG. This partial
sawtooth is amplified by Vc, differentiated in the plate-loading transformer,
further amplified in VT, and ultimately used to trigger, the singlestroke
blocking oscillator circuit of Va, which produces the delayed pulse.
The slope control determines the range scale and the zero-set resistors
balance out the combined effects of the starting time of the switching
square wave, the starting voltage of the sawtooth, and the conduction
point of the diode. The critical circuit elements in addition to those in