What is the equivalent circuit of a quartz crystal?

To understand how a quartz crystal behaves in an RF circuit, engineers model it using standard electronic components. The motional arm of the crystal—which represents its actual mechanical mass, physical compliance, and friction—is modeled as a series RLC circuit (a Resistor, an Inductor, and a Capacitor in series). However, the crystal itself is physically sandwiched between two metal electrodes to connect it to the circuit. These metal plates act exactly like a standard capacitor, creating a parallel "shunt" capacitance across the entire series RLC circuit. Therefore, the complete equivalent circuit is a series RLC path in parallel with a shunt capacitor. This unique arrangement is important because it gives the crystal both a series resonant frequency and a slightly higher parallel resonant frequency.