Question E7H07:
How can an oscillator’s microphonic responses be reduced?  

Choices:  
A: Use NP0 capacitors  
B: Reduce noise on the oscillator’s power supply  
C: Increase the gain  
D: Mechanically isolate the oscillator circuitry from its enclosure    

Correct Answer is  D: Mechanically isolate the oscillator circuitry from its enclosure  

Microphonics are frequency or phase changes in an oscillator caused by mechanical vibration. Vibrations flex components and wiring, changing $L$ or $C$ and thus the oscillator frequency.

Why D is correct
- Mechanically isolating the oscillator circuitry (rubber grommets, foam mounts, sub-chassis, mass loading, rigid PCB) reduces the vibration reaching sensitive parts, directly reducing microphonic FM/PM.

Why the others are not
- A: Use NP0 capacitors — NP0/C0G parts have very low temperature coefficient and low piezoelectric effect, so they help, but they do not address all vibration paths (inductors, crystals, wiring, enclosure). Mechanical isolation is the primary fix.
- B: Reduce noise on the oscillator’s power supply — This lowers hum and phase noise from the supply, not vibration-induced frequency modulation.
- C: Increase the gain — Higher loop gain generally makes the oscillator more sensitive to small parameter changes and can worsen microphonics.

Useful relationships and rules of thumb
- LC oscillator sensitivity:
  $$f \approx \frac{1}{2\pi\sqrt{LC}}, \quad \frac{\Delta f}{f} \approx -\tfrac{1}{2}\frac{\Delta C}{C} - \tfrac{1}{2}\frac{\Delta L}{L}$$
  Vibration changes $L$ and $C$ via movement and piezoelectric effects.
- g-sensitivity (rule of thumb for oscillators/crystals):
  $$\frac{\Delta f}{f} = S_g \cdot a_g$$
  where $S_g$ is fractional change per g, and $a_g$ is acceleration in g.
- Practical tips:
  - Physically isolate/damp the oscillator module (correct answer).
  - Use rigid, short leads; brace coils; avoid microphonic dielectrics (high-K ceramics).
  - Use NP0/C0G or mica in resonant networks (helps but not the primary cure).
  - Buffer the oscillator to reduce load-induced coupling.