3-6
Bearing to the target is indicated by the target's angular position in relation to an imaginary line
extending vertically from the sweep origin to the top of the scope. The top of the scope is either true north
(when the indicator is operated in the true bearing mode) or ship's heading (when the indicator is operated
in the relative bearing mode).
PPI Block Diagram
The basic block diagram, figure 3-4, illustrates the major units of a plan position indicator.
Synchronization of events is particularly important in the presentation system. At the instant a radar
transmitter fires (or at some predetermined time thereafter), circuits which control the presentation on the
indicator must be activated. These events must be performed to a high degree of accuracy to ensure
accurate range determination. The synchronization of these events is provided by the gate circuit.
Figure 3-4.—Basic ppi block diagram.
GATE CIRCUIT.—The gate circuit develops pulses which synchronize the indicator with the
transmitter. The gate circuit itself is synchronized by trigger pulses from the synchronizer. It then
provides timing for the intensity gate generator, sweep generator circuit, and the sweep control circuit.
SWEEP CONTROL CIRCUIT.—The sweep control circuit converts mechanical bearing
information from the antenna into voltages which control sweep circuit azimuth.
SWEEP GENERATOR CIRCUIT.—The sweep generator circuit produces currents which deflect
an electron beam across the crt. Varying voltages from the sweep control circuit are applied to deflection
coils. Gate voltages determine sweep rate, and therefore, the effective distance (range) covered by each
sweep. Sweep potentials consist of separate north-south and east-west voltages; the amplitudes of these
voltages determine sweep azimuth. The sweep generator is synchronized by an input from the gate circuit.
INTENSITY GATE GENERATOR.—The intensity gate generator provides a gate which unblanks
the crt during sweep periods. The intensity of the trace appearing on the crt is determined by the dc level
of this gate. This circuit is also synchronized by the gate circuit.
