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Series & Parallel Resonance
A crystal operating in a series resonant circuit will appear resistive and is said to be operating at its series resonant frequency, Fs. At the series resonant frequency impedance will be near zero, load capacitance will be infinite and the crystal will be operating at a frequency below parallel or load resonance.

At the parallel or load resonance, the crystal will appear inductive in the circuit and the impedance will reach its peak at the parallel resonant frequency. The changes in the circuit reactance values will "pull" the frequency of the crystal. Load, or parallel capacitance, the dynamic capacitance of the total circuit measured across the terminals of the crystal, must specified. In parallel circuit design, the load capacitance should be selected to operate the crystal at the stable point across the Fs to Fl reactance curve. When a load capacitance is connected in series with the crystal unit, a different frequency result called the load or parallel resonant frequency. The same frequency occurs if the same value of capacitance is connected in parallel with the crystal unit. In practice, the total load capacitance is the sum of the stray capacitance and the load capacitor. The frequency change caused by the load capacitance is called DELTA F.

Resonator Reactance vs. Frequency

Effects of Load Capacitance on f vs. T

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