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.
