Question E7H02:
What is a microphonic?  

Choices:  
A: An IC used for amplifying microphone signals  
B: Distortion caused by RF pickup on the microphone cable  
C: Changes in oscillator frequency caused by mechanical vibration  
D: Excess loading of the microphone by an oscillator    

Correct Answer is  C: Changes in oscillator frequency caused by mechanical vibration  

Correct answer: C — Changes in oscillator frequency caused by mechanical vibration

Why C is correct:
- Microphonics are mechanical vibrations in components (crystals, inductors, capacitors, tubes) that convert motion into unintended frequency modulation of an oscillator. Vibrations change $L$ or $C$, so the oscillator frequency shifts:
  $$f_0 \approx \frac{1}{2\pi\sqrt{LC}}$$
  Small changes give:
  $$\frac{\Delta f}{f_0} \approx -\tfrac{1}{2}\left(\frac{\Delta L}{L} + \frac{\Delta C}{C}\right)$$
- The result is audible “pinging,” warble, or FM/PM on the signal when the chassis is tapped or vibrated.

Why the other choices are incorrect:
- A: An IC used for amplifying microphone signals — That’s a microphone preamp, not “microphonic.”
- B: Distortion caused by RF pickup on the microphone cable — That’s RF feedback or RF ingress, not microphonics.
- D: Excess loading of the microphone by an oscillator — Mixing concepts; loading can affect audio response, but it’s not microphonic behavior.

Useful rules of thumb to prevent microphonics:
- Use mechanically stable components: NP0/C0G capacitors, rigid inductors, low-microphonic crystal holders.
- Physically damp or isolate oscillators (grommets, foam, rigid mounting).
- Keep leads short; secure components to minimize vibration.
- If vibration modulates frequency like FM, the modulation index is:
  $$\beta = \frac{\Delta f}{f_m}$$
  where $f_m$ is the vibration frequency. Lowering $\Delta f$ (by mechanical stability) reduces the effect.