 Characteristics of Crystal |
| Piezoelectric effect of Quartz Crystal |
| The quartz crystal has a character when the pressure is applied to the direction of the crystal axis, the electric change generates on the quartz crystal plate, and on the contrary, when the electricity is applied to the quartz crystal plate, the distortion occurs inside the crystal plate. So, that’s why we called it’s the piezoelectric effect of quartz crystal. |
| Equivalent Circuit of Crystal |
| The equivalent circuit of a quartz crystal is shown to explain the basic elements governing the crystal characteristics and performance. It consists of a motional capacitance Cl, inductance Ll, series resistance Rl, and a shunted capacitance C0. The first three parameters are known as the "motional parameters" of the quartz crystal element. See Fig. 1 |
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| Figure 1 |
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| Series Resonance |
| When a crystal is operating at series resonance (Fs), it looks resistive in the circuit. Thus, impedance at Fs is near zero. In a well design series resonant circuit, correlation is not a problem and load capacitance does not have to be specified. See Fig. 2 |
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| Figure 2 |
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| Parallel Resonance |
| When a crystal is operating at parallel resonance (Fs<Fl<Fa), it looks inductive in the circuit. Thus, function of a load capacitance is very important in selecting the stable point of oscillation. In parallel circuit design, load capacitance CL shall be specified. See Fig. 3 |
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| Figure 3 |
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| Difference between AT Cut AND BT Cut Crystals |
| AT cut crystals and BT cut crystals possess different angle cut (35 degrees on AT fundamental vs. 49 degrees on BT cut). Both types have the same vibration mode (thick-ness-shear). However, the BT cut crystal on the 50MHz fundamental is slightly thicker (2mils) compared to its AT cut (1.3mils). AT cut and BT cut have different temperature vs. frequency curves. (See Fig 4, 5 & 6) |
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| Figure 4. Frequency Temperature curves for AT Cut |
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| Figure 5. Typical Temperature characteristic for AT Cut Crystal |
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| Figure 6. Typical Temperature characteristic for BT Cut Crystal |
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| Thickness of AT Cut Crystal vs. BT Cut Crystal |
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