Quartz Frequency Pull Calculator

A quartz frequency pull calculator determines how the frequency of a quartz crystal changes as a result of the actual load capacitance in the circuit. The calculation is based on the effective load capacitance, which consists of the load capacitors CL1 and CL2 as well as the parasitic stray capacitance Cstray. This allows you to estimate the frequency deviation relative to the nominal load capacitance specified in the datasheet. The page also displays the pull curve—that is, the relationship between the frequency deviation and the effective load capacitance. This allows developers to determine early on whether the circuit design is still within the crystal’s permissible frequency tolerance.

The frequency of a crystal decreases as the effective load capacitance increases. This is caused by larger load capacitors or excessive parasitic capacitance in the layout and at the crystal pins. The calculator uses an idealized model based on the frequency pull formula to determine the relative frequency deviation. It is particularly important to compare the calculated deviation with the frequency tolerance specified in the datasheet, for example, ±10 ppm or ±20 ppm. This allows you to assess whether the selected circuit configuration supports the circuit’s target accuracy.

The effective load capacitance is calculated using the formula CL,eff = (CL1·CL2)/(CL1+CL2) + Cstray. Here, CL1 and CL2 represent the two load capacitors at the crystal terminals, while Cstray encompasses all parasitic capacitances in the actual circuit. These include, for example, trace capacitance, pad and pin capacitance, and, if applicable, the input capacitance of the oscillator IC. This parameter is critical because not only the nominal capacitor values but also the layout influence the actual crystal frequency. To ensure a robust design, the result should always be compared with the specifications in the datasheet.

The stray capacitance Cstray has a direct effect on the effective load capacitance and thus on the resulting crystal frequency. Even small parasitic capacitances from trace patterns, pads, pins, or the oscillator input can measurably lower the frequency. Therefore, in practice, it is not sufficient to consider only the nominal values of CL1 and CL2. The calculator helps incorporate these parasitic effects into the estimation of the frequency deviation. Especially in precision applications, Cstray is a key factor in meeting the required ppm tolerances.

For a symmetrical circuit configuration, the relationship CL1 = CL2 = 2·(CL,nom − Cstray) is recommended. This allows the nominal load capacitance of the crystal to be approximated accurately, taking parasitic capacitances into account. This approach is particularly helpful when both load capacitors are to be designed with the same value. However, the model remains idealized, so real-world deviations due to component tolerances and circuit details are possible. Therefore, the specifications in the data sheet for the quartz used are always binding for the final design.

PETERMANN-TECHNIK combines technical expertise in frequency technology with practical tools for developers and buyers. The Quartz Frequency Pull Calculator helps users quickly and transparently evaluate the influence of load capacitance and stray capacitance on the actual frequency. At the same time, it clearly demonstrates that the calculated values must always be considered in the context of the quartz specification and the data sheet. This builds confidence and helps prevent typical design errors in oscillator circuits early on. In addition, frequency experts are available to answer individual questions when an application-specific evaluation is required.