Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
The correct answer is B: Provide the crystal with a specified parallel capacitance. To ensure a crystal oscillator operates on the frequency specified by the crystal manufacturer, you must provide the crystal with a specified parallel capacitance (load capacitance). The crystal's frequency is specified for a particular load capacitance.
Crystal manufacturers specify the crystal frequency for a specific parallel load capacitance (typically 12 pF, 18 pF, or 30 pF). If the actual circuit capacitance doesn't match the specified load capacitance, the oscillator will operate at a slightly different frequency. To get the exact specified frequency, you must match the load capacitance. This is typically done with a trimmer capacitor or by designing the circuit to have the correct capacitance. The load capacitance affects the crystal's effective frequency through the crystal's motional parameters.
Exam Tip
Crystal frequency accuracy = Match load capacitance. Remember: To get the crystal's specified frequency, provide the specified parallel load capacitance. Mismatched capacitance causes frequency error.
Memory Aid
"**C**rystal **F**requency = **C**orrect **L**oad **C**apacitance (think 'CF = CLC')"
Real-World Application
You're building a crystal oscillator using a 14.000 MHz crystal specified for 18 pF load capacitance. You add an 18 pF trimmer capacitor in parallel with the crystal. By adjusting the trimmer to exactly 18 pF, the oscillator operates at exactly 14.000 MHz as specified. If the capacitance is different, the frequency will be slightly off.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Parallel inductance doesn't set the crystal frequency. The crystal frequency is determined by load capacitance, not inductance.
Option C: Incorrect. Bias voltage doesn't determine the crystal frequency. Crystals are passive devices - their frequency is determined by mechanical properties and load capacitance.
Option D: Incorrect. Bias current doesn't determine crystal frequency. Crystals don't require bias current - they're driven by AC signals.
题目解析
The correct answer is B: Provide the crystal with a specified parallel capacitance. To ensure a crystal oscillator operates on the frequency specified by the crystal manufacturer, you must provide the crystal with a specified parallel capacitance (load capacitance). The crystal's frequency is specified for a particular load capacitance. Crystal manufacturers specify the crystal frequency for a specific parallel load capacitance (typically 12 pF, 18 pF, or 30 pF). If the actual circuit capacitance doesn't match the specified load capacitance, the oscillator will operate at a slightly different frequency. To get the exact specified frequency, you must match the load capacitance. This is typically done with a trimmer capacitor or by designing the circuit to have the correct capacitance. The load capacitance affects the crystal's effective frequency through the crystal's motional parameters.
考试技巧
Crystal frequency accuracy = Match load capacitance. Remember: To get the crystal's specified frequency, provide the specified parallel load capacitance. Mismatched capacitance causes frequency error.
记忆口诀
**C**rystal **F**requency = **C**orrect **L**oad **C**apacitance (think 'CF = CLC')
实际应用示例
You're building a crystal oscillator using a 14.000 MHz crystal specified for 18 pF load capacitance. You add an 18 pF trimmer capacitor in parallel with the crystal. By adjusting the trimmer to exactly 18 pF, the oscillator operates at exactly 14.000 MHz as specified. If the capacitance is different, the frequency will be slightly off.
错误选项分析
Option A: Incorrect. Parallel inductance doesn't set the crystal frequency. The crystal frequency is determined by load capacitance, not inductance. Option C: Incorrect. Bias voltage doesn't determine the crystal frequency. Crystals are passive devices - their frequency is determined by mechanical properties and load capacitance. Option D: Incorrect. Bias current doesn't determine crystal frequency. Crystals don't require bias current - they're driven by AC signals.
知识点
Crystal oscillator, Load capacitance, Parallel capacitance, Frequency accuracy
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.