MHz Quartz Frequency Pull Calculator
Effective load capacitance and frequency deviation from the layout circuitry and stray capacitance
Crystal (per datasheet)
Circuit / Layout
Pulling curve: frequency deviation (ppm) versus effective load capacitance
Always assess the calculated deviation against your crystal's frequency tolerance (e.g. ±10 or ±20 ppm). Frequency decreases as load capacitance rises; an excessive Cstray or oversized load capacitors pull the frequency down.
Formulas & notes used
Effective load capacitance: CL,eff = (CL1·CL2)/(CL1+CL2) + Cstray
Frequency pull: Δf/f0 = (C1/2)·( 1/(C0+CL,eff) − 1/(C0+CL,nom) )
Pulling sensitivity: S = (C1/2)/(C0+CL,eff)² (in ppm/pF)
Recommendation (symmetric): CL1 = CL2 = 2·(CL,nom − Cstray)
Idealised model. Cstray covers all parasitic capacitances at the crystal pins (traces, pad/pin capacitance, possibly the oscillator IC input capacitance). The datasheet is authoritative.
