At the parallel resonance point the ESR reaches its maximum resistive value. This differs from crystal to crystal and is in the megaohm range. Hz from its 1.0 MHz specified frequency. Crystal Types Crystal oscillators are manufactured in two basic types for different modes of operation. For the remainder of this section, the crystal types are referred to by their mode of operation, one being fundamental and the other being third overtone.
Crystals can also implement fifth or greater overtone modes, with similar principles.
The fundamental mode crystals are designed and manufactured to oscillate at the crystal's fundamental frequency. The corresponding circuit configuration shown in Figure 1a works well for these types of crystals. Fundamental mode crystals can be found in various frequencies up to about 30 MHz. For frequencies beyond this, the second type of crystal and a different circuit configuration is required. Third overtone crystals are designed and manufactured to operate at the third overtone frequency from the crystal's fundamental.