*Question E6D02:
What is the equivalent circuit of a quartz crystal?*  

*Correct Answer is  A: Series RLC in parallel with a shunt C representing electrode and stray capacitance*  

### AI Response: (may contain errors)  

The equivalent circuit of a quartz crystal is important for understanding its behavior in electronic circuits. 

### Correct Answer: 
**A: Series RLC in parallel with a shunt C representing electrode and stray capacitance**

#### Explanation:
- **Series RLC**: The crystal itself behaves like a series resonant circuit consisting of a resistor (R), inductor (L), and capacitor (C). This series combination represents the crystal's resonant frequency, where it can oscillate.
- **Shunt C**: The shunt capacitor represents the stray capacitance and the capacitance of the electrodes. This is important because it affects the overall impedance and frequency response of the crystal in a circuit.

### Why Other Choices Are Incorrect:
- **B: Parallel RLC, where C is the parallel combination of resonance capacitance of the crystal and electrode and stray capacitance**  
  - Incorrect because the crystal operates as a series resonant circuit, not a parallel one. The series configuration is essential for defining its resonant behavior.

- **C: Series RLC, where C is the parallel combination of resonance capacitance of the crystal and electrode and stray capacitance**  
  - Incorrect because it misrepresents the configuration of the capacitance. The stray capacitance is not in parallel with the resonant circuit; it is shunted.

- **D: Parallel RLC, where C is the series combination of resonance capacitance of the crystal and electrode and stray capacitance**  
  - Incorrect because it suggests a parallel configuration, which does not accurately reflect the behavior of the crystal. Additionally, combining capacitances in series does not apply here.

### Useful Formulas and Rules of Thumb:
- The resonant frequency ($f_0$) of a series RLC circuit can be calculated using:
  
  $$f_0 = \frac{1}{2\pi\sqrt{LC}}$$

  where:
  - $L$ = inductance in henries
  - $C$ = capacitance in farads

- For quartz crystals, the series resonance is typically characterized by a sharp peak in impedance at the resonant frequency, which is crucial for applications in oscillators and filters.

Understanding these concepts will help you grasp how quartz crystals function in circuits and prepare you for related questions on the exam.