The electric equivalent circuits of a crystal unit is generally represented as shown in Fig.7, and the four equivalent constants are labeled as follows: L₁: Equivalent series inductance C₁ : Equivalent series capacitance R₁: Equivalent series resistance C₀: Shunt capacitance	Fig. 7 Electric Equivalent Circuit of a Crystal Unit
The main characteristics of a crystal unit is derived by using these equivalent constants. As an example, Fig. 8 shows the frequency characteristic of impedance. Typical frequencies and performances of crystal units are shown below.	Fig. 8 Impedance of Frequency characteristics
The general crystal oscillation circuit is capacitive, and can be treated as series circuits with a negative resistance (-R) and load capacitance (C_L) as shown in Fig. 9	Fig. 9 Relationship between a Crystal Unit and the Oscillation Circuit
When the load capacitance values are C_L and series , f_L and f_S, respectively, the following relationship is established. Fig. 10 shows an example. An overtone crystal with a smaller C₁ value than a fundamental wave has a smaller frequency change rate with respect to CL than a fundamental wave. The C₁ value of small-size crystal units is smaller than that of large-size crystal units with the same frequency. For this reason, frequency change rate to the load capacitance is smaller for a small-sized crystal units. When the load capacitance (C_L) is connected to the crystal unit in series as shown in Fig. 9, the equivalent resistance increases according to the following formula.	Fig. 10 Load Capacitance vs. Frequency Change