Syllabus Sections:-

Frequency multipliers

4d.1 Understand that frequency multipliers use harmonics to generate frequencies above an oscillator's fundamental frequency (e.g. in a microwave transmitter)

Typically you could use an 8MHz crystal multiplied by 18 to achieve an output frequency of 144MHz and higher multiplication to achieve microwave frequency. There is no local oscillator.

The concept that is being introduced to you is that a single lower frequency crystal could be multiplied up to a harmonically related frequency and then filtered and amplified prior to feeding to an aerial etc. Which frequency is used as the initial frequency is all down to the circuit designer, with mode and cost being principal considerations.

Operate in Class "C" to make the harmonics

A suitable frequency multiplier circuit will have the transistor biased into class "C" to distort the waveform to such an extent that it produces harmonics of the fundamental frequency that can be used by the multiplier stage.

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