PLAN POSITION INDICATOR (PPI).

The ppi scope shown in figure 3-3, is by far the most used radar display. It is a polar coordinate display of the area surrounding the radar platform. Own ship is represented as the origin of the sweep, which is normally located in the center of the scope, but may be offset from the center on some sets. The ppi uses a radial sweep pivoting about the center of the presentation. This results in a map-like picture of the area covered by the radar beam. A long-persistence screen is used so that the display remains visible until the sweep passes again.

Figure 3-3. - PPI scope.

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.

VIDEO AMPLIFIER. - The video amplifier circuit amplifies the video signal from the receiver and applies it to the crt intensity-modulating element (control grid).

POWER SUPPLY. - The power supply produces all voltages needed to operate the indicator. It also includes protective devices and metering circuits.

Although not shown in the basic block diagram, many indicators contain circuits which aid in range and bearing determination. These circuits are also synchronized by the gate circuit.

Sweep Deflection

In modern indicator systems, electromagnetic deflection of the crt electron beam is preferred to electrostatic deflection. Reasons for this choice are (1) increased control of the beam, (2) improved deflection sensitivity, (3) better beam position accuracy, and (4) simpler construction of the crt.

The primary difference between electromagnetic and electrostatic cathode-ray tubes lies in the method of controlling deflection and focusing of the electron beam. Both types employ electron guns and use electrostatic fields to accelerate and control the flow of electrons. The physical construction of a crt employing electromagnetic deflection is similar to an electrostatic type. The construction of a crt employing electromagnetic deflection is shown in figure 3-5.

Figure 3-5. - Electromagnetic crt construction.

The electron gun in figure 3-5 is made up of a heater, cathode, control grid, second or screen grid, focus coil, and anode (composed of a special coating). Focusing the electron beam on the face of the screen is accomplished by the focus coil. A direct current through the windings sets up a strong magnetic field at the center of the coil. Electrons move precisely along the axis of the tube and pass through the focusing field with no deflection. This is because they move parallel to the magnetic field at all times.

Any electron which enters the focusing field at an angle to the axis of the tube has a force exerted on it that is perpendicular to its direction of motion. A second force on this electron is perpendicular to the magnetic lines and is, therefore, constantly changing in direction. These forces cause the electron to move in a helical or corkscrew path shown in figure 3-6. With the proper velocity of the electron and strength of the magnetic field, the electron will be caused to move at an angle which allows it to converge with other electrons at some point on the crt screen. Focusing is accomplished by adjusting the current flow through the focusing coils.

Figure 3-6. - Helical motion of electron through a uniform magnetic field.

The focused electron beam is deflected by a magnetic field that is generated by current flow through a set of deflection coils, as shown in figure 3-5. These coils are mounted around the outside surface of the neck of the crt. Normally, four deflection coils (N, S, E, and W) are used, as shown in figure 3-4. Two coils in series are positioned in a manner that causes the magnetic field produced to be in a vertical plane. The other two coils, also connected in series, are positioned so that their magnetic field is in a horizontal plane. The coils (N-S) which produce a horizontal field are called the VERTICAL DEFLECTION COILS and the coils (E-W) which produce a vertical field are called the HORIZONTAL DEFLECTION COILS. This may be more clearly understood if you recall that an electron beam will be deflected at right angles to a deflecting field. The deflection coils are illustrated in view A of figure 3-7. View B shows the N-S windings in schematic form.

Figure 3-7. - Deflection yoke.

Electron deflection in the electromagnetic crt is proportional to the strength of the magnetic fields. Magnetic field strength depends on current in the coils. The sweep circuits associated with electromagnetically deflected cathode-ray tubes must provide currents, rather than voltage, to produce the desired beam deflection.

A sawtooth current is required to produce a linear trace. A deflection coil may be considered equivalent to the circuit shown in view A of figure 3-8. Because of the inductance of the coil, a trapezoidal voltage must be applied across the coil to produce a sawtooth of current through it. This is illustrated in view B. (Refer to NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, for a review of wave shaping.)

Figure 3-8. - Deflection coil equivalent circuit and waveform.

Sweep Rotation

Azimuth indication of the ppi requires that the range trace rotate about the center of the screen. A very simple means of achieving sweep rotation is to cause the deflection coil to rotate about the neck of the crt in synchronization with the antenna motion. This method, however, has the disadvantages of inaccuracy and maintenance complications inherent to any mechanical gear-train assembly.

Most modem ppi systems employ fixed deflection coils and use special circuits to electronically rotate the magnetic field. Figure 3-9 illustrates a method of electronically producing a rotating sweep. In view A, a range sweep current, i, is applied to the vertical deflection coils only. The resulting magnetic field, represented by F, lies along the axis of these coils. The resulting range trace, shown by the short straight line, is vertical because the electron beam is deflected perpendicular to the magnetic field. In view B, range sweep currents are applied to both sets of coils, and the resultant magnetic field takes a position between the axes of the two sets of coils. Because of this shift of the magnetic field, the range trace is rotated 45 degrees clockwise from its previous position. In view C, the sweep current is applied to the horizontal deflection coils only, and the range trace lies 90 degrees clockwise from its original position. Further rotation is obtained if the polarities of the deflection coil currents are varied in proper sequence, as illustrated in views D and E.

Figure 3-9. - Trace rotation.

To synchronize sweep rotation with antenna rotation, you must convert antenna azimuth (bearing) information into electrical signals. These signals, usually provided by synchros, control the amplitudes and polarities of the sawtooth sweep currents applied to the deflection coils.

Figure 3-10 illustrates the waveforms of current required to produce a rotating range sweep. The amplitudes of the sawtooth sweep currents are varied sinusoidally (like a sine wave), corresponding to the rotation of the antenna. Notice that there is a 90 degree phase difference between the amplitude variations of the horizontal and vertical waveforms.

Figure 3-10. - Deflection coil currents.

CRT Screen Persistence

A ppi requires a crt in which the screen is coated with a long-persistence phosphor. This is necessary because each target reflects energy for only a short period of time during each rotation of the antenna. Therefore, the target indication on the face of the crt must be able to continue to glow during the portion of antenna rotation when the target is not reflecting energy.

Q.4 What coordinates are presented on a ppi scope?
Q.5 What type of deflection is preferred for a crt electron beam?
Q.6 Which of the two types of deflection coils (fixed or rotating) is used most often?