Question E4E03:
Which of the following types of noise are removed by a noise blanker?  

Choices:  
A: Broadband white noise  
B: Impulse noise  
C: Hum and buzz  
D: All these choices are correct    

Correct Answer is  B: Impulse noise  

A noise blanker is designed to detect very short, high-amplitude pulses and momentarily mute (blank) the receiver to remove those spikes from the audio/IF.

Why B (Impulse noise) is correct:
- Impulse noise consists of brief spikes (microseconds to milliseconds) from sources like ignition systems, power-line arcing, electric fences, or lightning.
- The blanker triggers on these spikes and blanks the receiver for a very short interval, effectively removing the click/pop without greatly affecting desired signals.

Why the other choices are incorrect:
- A: Broadband white noise is continuous Gaussian/hiss-type noise. A blanker cannot gate out a continuous signal. Use narrower bandwidth, DSP noise reduction, or better SNR instead.
- C: Hum and buzz are periodic low-frequency disturbances (e.g., 50/60 Hz and harmonics or switch-mode supply buzz). They are not brief impulses, so a blanker is ineffective. Use filtering, common-mode chokes, grounding fixes, or notch filters/DSP for tones/heterodynes.
- D: Not all types are removed; only short-duration impulse noise is addressed by a noise blanker.

Useful formulas and rules of thumb:
- Short pulses are broadband: $$\Delta f \approx \frac{1}{t_p}$$ where $t_p$ is pulse width. Very short $t_p$ spreads energy widely, which is why ignition noise is heard across the band.
- A blanker works best when pulse duration is much shorter than the receiver’s IF time constant: $$t_p \ll \frac{1}{B_{IF}}$$
- Tool selection:
  - Noise blanker: impulse noise (spikes).
  - DSP noise reduction: continuous hiss/white noise.
  - Notch/auto-notch: single tones/heterodynes (not buzz/hum as a whole).