Deep Dive: E7H02
The correct answer is C: Changes in oscillator frequency caused by mechanical vibration. A microphonic (or microphonics) refers to changes in oscillator frequency caused by mechanical vibration. When an oscillator is subjected to vibration, shock, or sound waves, the frequency can change due to mechanical effects on components. Microphonics can be caused by physical vibration of components (like crystals, capacitors, or inductors) in the oscillator circuit. When these components vibrate, their electrical properties change slightly, causing the oscillator frequency to shift. This is a common problem in mobile equipment, where engine vibration or road bumps can cause frequency instability. Microphonics can also be caused by sound waves (like loud audio) affecting sensitive components. The term comes from the fact that the oscillator 'hears' mechanical vibrations like a microphone.
Why Other Answers Are Wrong
Option A: Incorrect. An IC for amplifying microphone signals is a microphone amplifier, not a microphonic. Microphonics is an unwanted effect, not a component. Option B: Incorrect. Distortion from RF pickup on microphone cable is RF interference, not microphonics. Microphonics is about mechanical vibration affecting oscillators. Option D: Incorrect. Excess loading of a microphone by an oscillator doesn't describe microphonics. Microphonics is about vibration affecting oscillator frequency.
Exam Tip
Microphonic = Vibration causes frequency change. Remember: Microphonics are changes in oscillator frequency caused by mechanical vibration. The oscillator 'hears' vibrations like a microphone.
Memory Aid
**M**icrophonic = **M**echanical **V**ibration **F**requency (think 'M = MVF' = Mechanical Vibration Frequency change)
Real-World Example
You're operating mobile, and when you hit a bump in the road, your VFO frequency shifts slightly. This is microphonics - the mechanical shock from the bump causes components in your oscillator to vibrate, changing their electrical properties and shifting the frequency. To reduce this, you might mount the oscillator on shock-absorbing material or use components less sensitive to vibration.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E7H
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E7H topic.