Question E4E04:
How can conducted noise from an automobile battery charging system be suppressed?  

Choices:  
A: By installing filter capacitors in series with the alternator leads  
B: By installing a noise suppression resistor and a blocking capacitor at the battery  
C: By installing a high-pass filter in series with the radio’s power lead and a low-pass filter in parallel with the antenna feed line  
D: By installing ferrite chokes on the charging system leads    

Correct Answer is  D: By installing ferrite chokes on the charging system leads  

To quiet conducted noise from an automobile charging system, you want to add impedance to the high-frequency noise riding on the DC leads while leaving DC current unaffected. Ferrite chokes do exactly that.

Why D is correct (install ferrite chokes on the charging system leads):
- Ferrite chokes add a frequency-dependent impedance in series with the leads, greatly attenuating RF/hash without reducing DC charging current.
- For RF noise, a ferrite’s impedance rises with frequency. Approximate series impedance:
  $$Z_{\text{ferrite}} \approx R(f) + j\,\omega L(f), \quad \text{with } \omega = 2\pi f$$
  and the inductive part
  $$X_L = 2\pi f L$$
  which gets large at higher frequencies, opposing the noise.
- Wrapping multiple turns through the core increases inductance roughly as:
  $$L \propto N^2$$
  so each extra turn boosts high-frequency attenuation.
- Use common-mode chokes (both + and − leads through the same core) and place them close to the noise source (alternator) or at the radio’s power input.

Why the other choices are incorrect:
- A: “Filter capacitors in series with the alternator leads”
  - A series capacitor would block DC charging current—impractical and potentially damaging. Capacitors for noise suppression are used shunt-to-ground, not in series with high-current DC leads.

- B: “Noise suppression resistor and a blocking capacitor at the battery”
  - A series resistor in the battery line would drop voltage and dissipate heat with changing load currents—unsafe and not used in automotive power distribution.
  - A “blocking capacitor” in series would again block DC. Proper bypassing uses capacitors in parallel (across the line to chassis), not series.

- C: “High-pass in series with the radio’s power lead and low-pass in parallel with the antenna feed line”
  - A high-pass in series with the power lead would pass high-frequency noise and impede or block DC—exactly the opposite of what you want.
  - A low-pass in parallel with the antenna feed would shunt RF from the antenna, detuning or shorting desired signals—wrong place and function for conducted power-line noise.

Useful rules of thumb:
- Add series impedance for noise, not for DC: use ferrite chokes or inductors in series with power leads; use capacitors shunt-to-ground for bypassing.
- Choose ferrite mix appropriate for the noise band (e.g., mix 31 for HF/low VHF noise, mix 43 for mid-VHF); clamp-on cores are convenient.
- Multiple turns through a ferrite core increase suppression at lower HF:
  $$Z \propto N^2 \text{ (roughly, at frequencies where the core is inductive)}$$
- Place chokes as close to the noise source as possible, and also at the radio input if needed.