Crystals above 30 MHz (up to >200 MHz) are generally operated at series resonance where the impedance appears at its minimum and equal to the series resistance. For these crystals the series resistance is specified (<100 Ω) instead of the parallel capacitance. To reach higher frequencies, a crystal can be made to vibrate at one of its overtone modes, which occur near multiples of the fundamental resonant frequency. Only odd numbered overtones are used. Such a crystal is referred to as a 3rd, 5th, or even 7th overtone crystal. To accomplish this, the oscillator circuit usually includes additional LC circuits to select the desired overtone.
crystals above 30 mhz (up to >200 mhz) are generally operated at series resonance where the impedance appears at its minimum and equal to the series resistance for these crystals the series resistance is specified (<100 ω) instead of the parallel capacitance to reach higher frequencies a crystal can be made to vibrate at one of its overtone modes which occur near multiples of the fundamental resonant frequency only odd numbered overtones are used such a crystal is referred to as a 3rd 5th or even 7th overtone crystal to accomplish this the oscillator circuit usually includes additional lc circuits to select the desired overtone