Keying Methods

1-29 Keying Methods Keying a transmitter causes an rf signal to be radiated only when the key contacts are closed. When the key contacts are open, the transmitter does not radiate energy. Keying is accomplished in either the oscillator or amplifier stage of a transmitter. A number of different keying systems are used in Navy transmitters. In most Navy transmitters, the hand telegraph key is at a low-voltage potential with respect to ground. A keying bar is usually grounded to protect the operator. Generally, a keying relay, with its contacts in the center-tap lead of the filament transformer, is used to key the equipment. Because one or more stages use the same filament transformer, these stages are also keyed. A class C final amplifier, when operated with fixed bias, is usually not keyed. This is because no output occurs when no excitation is applied in class C operation. Keying the final amplifier along with the other stages is not necessary in this case. OSCILLATOR KEYING.—Two methods of OSCILLATOR KEYING are shown in figure 1-23. In view (A) the grid circuit is closed at all times. The key (K) opens and closes the negative side of the plate circuit. This system is called PLATE KEYING. When the key is open, no plate current can flow and the circuit does not oscillate. In view (B), the cathode circuit is open when the key is open and neither grid current nor plate current can flow. Both circuits are closed when the key is closed. This system is called CATHODE KEYING. Although the circuits of figure 1-23 may be used to key amplifiers, other keying methods are generally employed because of the high values of plate current and voltage encountered. Figure 1-23A.—Oscillator keying. Figure 1-23B.—Oscillator keying.