Crystal Oscillator

Load capacitors, start-up gain margin, drive level and ppm budget for a Pierce crystal oscillator

Crystal (Datasheet)

Nominal Frequency(F)

MHz

Load Capacitance(Cₗ)

Shunt Capacitance(C₀)

ESR (max)(ESR)

Motional Capacitance (Cₘ)

Oscillator & Board

MCU Datasheet Gives

Oscillator Transconductance(gₘ)

mA/V

Stray Capacitance (pins + traces)(Cₛ)

Load Capacitors

C1 / C2 Selection

Standard Value Series

Capacitor Tolerance(±t)

Stray Variation(±ΔCₛ)

Drive Level

Max Drive Level (crystal)(DLₘₐₓ)

µW

Swing at OSC_IN(Vₚₚ)

Probe Capacitance (if measured)(Cₚ)

External Resistor (RExt)

Frequency Budget

Frequency Tolerance @ 25 °C(±)

ppm

Stability over Temperature(±)

ppm

Aging (service life)(±)

ppm

Clock Accuracy Requirement

Design looks good

Within Spec

107

Gain Margin (×)?

45.6

Drive Level (µW)?

±73.0

Freq. Error (ppm)?

Results

Ideal C1 = C2(2(Cₗ−Cₛ))?

10.00pF

C1 (OSC_IN)(C₁)?

10.0pF

C2 (OSC_OUT)(C₂)?

10.0pF

Effective Load(Cₗ)?

10.00pF

CL Error(ΔCₗ)?

0.00pF

Motional Cap. (used)(Cₘ)?

28.00fF

Frequency Offset(Δf/f)?

0.00ppm

Trim Sensitivity(TS)?

-48.44ppm/pF

Oscillator gm (used)(gₘ)?

25.0mA/V

Critical gm(gₘ(crit))?

233.7µA/V

Gain Margin(gₘ/gₘ(crit))?

107.0×

Gain Margin (Vittoz exact)(gₘ/gₘ(exact))?

104.0×

ESR Limit for 5×(ESRₘₐₓ)?

244Ω

Max Negative Resistance(Rₙ(max))?

244Ω

External Resistor(Rₑₓₜ)?

0Ω

Load Resistance(Rₗ)?

231Ω

Drive Level(DL)?

45.6µW

Max Swing at DLₘₐₓ(Vₚₚ)?

2.96V

Max Crystal Current(pp)?

1.86mA

Crystal Contribution(±)?

35.0ppm

Load Contribution(±)?

38.0ppm

Total Freq. Error(±)?

73.0ppm

Allowed (Requirement)(±)?

100ppm

Gain Margin vs. Crystal ESR

AN2867 (linear gm_crit)
Exact (Vittoz)

Frequency Pulling vs. Load Capacitance

Created by Catlynn Nguyen & Jie Chai · 2026