1. First, the original signal passes through a frequency selection circuit, which only allows signals with specific frequencies to pass through. The frequency selection circuit can be an LC oscillator, a quartz crystal oscillator or other types of oscillators. This step is to ensure that we get the fundamental frequency sine signal.

2. Then, the fundamental frequency sine signal passes through a rectification circuit, which is half-wave rectification or full-wave rectification. Half-wave rectifier circuit only transmits half cycle of sine wave, while full-wave rectifier circuit transmits all cycle of sine wave. In this process, the waveform of the signal is shaped into DC signal and cosine wave signal.

3. Next, the DC signal and cosine wave signal pass through a squaring circuit. The circuit will square the amplitude of the input signal and then integrate the result to generate a new signal. This new signal is a periodic signal with second harmonic.

4. Finally, this periodic signal with the second harmonic passes through another frequency selection circuit, and only the signal with twice the fundamental frequency is allowed to pass. This step is to obtain a dual-frequency sine signal.