The common crystal oscillator mostly is two feet, 3 foot's crystal oscillators are one kind of collection crystal oscillator and the electric capacity are a body's compound part. Because in the integrated circuit vibrates in the post peripheral circuit always by a crystal oscillator (or other resonant element) and two electric capacity composition return route, to be advantageous for the simplified circuit and the craft, the people then developed have produced this kind of compound. In its 3 pins, the middle 1 foot usually is 2 electric capacity connection together public end, other 2 pins are the crystal oscillator both sides, is also two electric capacity each one and the crystal oscillator connection both sides. Thus it can be seen, carried forward this kind of compound available same frequency crystal oscillator and two 100~200pF porcelain piece electric capacity quinine substitutes directly as usual.