Frequency multipliers are often used to multiply the frequency of the master oscillator or to increase the modulation index in the case of phase or frequency modulation [4, 5, 9, 16, 22]. The Class C frequency multiplier has the same schematic as the Class C power amplifier (see Figure 2-1) and operates in much the same way. The only difference is that the collector resonant circuit is tuned to the desired harmonic, suppressing all other harmonics. Assuming that the parallel LC output circuit is ideal, tuned to the n ^th harmonic, a sinusoidal output voltage is obtained.

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The output power is given by

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and taking into account that

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the collector efficiency is

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The collector efficiency is highest if V ₀ = V _dc.

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Finally, the power output capability (for V ₀ = V _dc) is given by

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The variation of the maximum collector efficiency h _max with the conduction angle q _c, for a Class C amplifier ( n = 1), a doubler ( n = 2), and a tripler ( n = 3), is shown in Figure 2-102. Note that the collector efficiency decreases as the multiplying order n increases. Also note that a Class B circuit cannot be used as a frequency tripler h _max ( q _c = 90 ) = 0 because a half-wave sinusoidal waveform does not contain the third harmonic (see Figure...