Small triodes such as the 6C4 operate satisfactorily as frequency multipliers, and can deliver output well into the v-h-f range. Resistor R normally will have a value in the vicinity of 100,000 ohms.
given output, because a moderate amount of power is delivered to the amplifier load by the driver stage of a grounded-grid amplifier.
Quartz crystals and variable-frequency oscillators are not ordinarily used for direct control of the output of high-frequency transmitters. Frequency multipliers are usually employed to multiply the frequency to the desired value. These multipliers operate on exact multiples of the excitation frequency; a 3.6-Mc. crystal oscillator can be made to control the output of a transmitter on 7.2 or 14.4 Mc., or on 28.8 Mc., by means of one or more frequency multipliers. IXhen used at twice frequency, they are often termed frequency doubters. A simple doubler circuit is shown in figure 16. It consists of a vacuum tube with its plate circuit tuned to twice the frequency of the grid driving circuit. This doubler can be excited from a crystal oscillator or another multiplier or amplifier stage.
Doubling is best accomplished by operating the tube with high grid bias. The grid circuit is driven approximately to the normal value of d-c grid current through the r-f choke and grid-leak resistor, shown in figure 16. The resistance value generally is from two to five times as high as that used with the same tube for straight amplification. Consequently, the grid bias is several times as high for the same value of grid current.
Neutralization is seldom necessary in a doubler circuit, since the plate is tuned to twice the frequency of the grid circuit. The impedance of the grid driving circuit is very low at the doubling frequency, and thus there is little tendency for self-excited oscillation.
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