Modeling the frequency dependence of a crystal oscillator on load capacity requires the impedance of the crystal in the oscillator feedback loop. The impedance is simplified for most purposes by the network of Figure 1. The motional electrical elements are commonly named Li, Ci and Ri where the i script denotes the vibrational mode of the crystal; 1 for fundamental, 3 for third overtone etc. There is one element, C0, which is the static crystal holder capacity and is common to all modes. The nomenclature used by crystal vendors for fundamental mode crystals, L1, C1, RR (rather than R1) and C0, is for direct correspondence with measured crystal data. Additionally crystal vendors can provide FR for the low impedance motional series crystal resonant frequency and FL as the high impedance parallel resonant frequency for a series of external load capacitors, CL. Both frequencies are provided as either mode may be used depending on the application.Refer to application note AN-831 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.