Question E4E09:
What undesirable effect can occur when using a noise blanker?  

Choices:  
A: Received audio in the speech range might have an echo effect  
B: The audio frequency bandwidth of the received signal might be compressed  
C: Strong signals may be distorted and appear to cause spurious emissions  
D: FM signals can no longer be demodulated    

Correct Answer is  C: Strong signals may be distorted and appear to cause spurious emissions  

To understand a noise blanker (NB): it detects short, high-amplitude impulse noise (e.g., ignition clicks) and momentarily “blanks” (mutes) the IF or RF signal. This time-domain gating can inadvertently affect desired signals.

1) Why the correct answer is correct (C)
- The NB briefly turns the signal path off during detected impulses. If a strong desired signal is present, this “on-off” gating effectively amplitude-modulates that signal with short pulses.
- Time-domain multiplication by a blanking waveform $b(t)$ spreads energy in frequency: if the received signal is $s(t)$, the output is $s(t)\,b(t)$ with spectrum
  $$S_{\text{out}}(f) = S(f) * B(f)$$
  where $*$ denotes convolution. Because $B(f)$ from short pulses is wide, the result includes distortion products that resemble spurious emissions on the received audio or spectrum display.
- Practically: strong nearby signals may trigger or interact with the blanker, causing audible distortion and visible “splatter-like” artifacts.

2) Why the other choices are incorrect
- A: Echo effect in speech range — That’s characteristic of some DSP noise reduction algorithms (due to filtering delays), not of an NB’s brief muting action.
- B: Audio bandwidth compressed — Bandwidth compression is associated with audio processing (e.g., speech compression), not noise blanking. The NB doesn’t “compress” AF bandwidth; it blanks short impulses in the IF/RF path.
- D: FM signals can no longer be demodulated — FM receivers already use limiters to remove amplitude noise. An NB does not prevent FM demodulation; it’s simply less beneficial for FM because FM is amplitude-insensitive.

3) Useful formulas and rules of thumb
- Model: NB output is the product $s(t)\,b(t)$; short-duration blanking pulses have broad spectra, so convolution $S * B$ can spread energy and create apparent spurs.
- Rule of thumb:
  - Use NB only for impulse noise; turn it off when strong signals are present or when it audibly distorts signals.
  - Set the NB threshold just above the impulse noise level to minimize unnecessary blanking.
  - IF-based NBs after a roofing filter usually cause fewer overload effects than wideband RF blankers.