Frequency Multipliers
Author: J.B. Hoag
In a frequency multiplier, the plate's tank circuit is tuned to a harmonic multiple of the frequency applied to the grid. The appearance of the circuit diagram of such a multiplier is the same as that of an ordinary amplifier and hence need not be repeated here (Fig. 13 J). Since the grid and plate circuits are tuned to different frequencies, it is not necessary to neutralize the amplifier, even when a triode is used. Frequently, multipliers operate on the second harmonic, and as such are called frequency doublers. Pushpull amplifiers can only be used for multiplication of the odd harmonics because the evennumbered harmonics are almost completely cancelled out in the tank circuit. With doublers, the Cbias, the r.f. input voltage, and the grid driving power must be considerably greater than the usual values for normal Class C amplifiers. The efficiency of these circuits ranges from 40 to 50 per cent. The tank circuit should have a high L/C ratio.

Fig. 13 J. Tuned transformer coupling for radio frequencies 
