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Explain aliasing error and aperature effect.

Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 05

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Aliasing Error:

Aliasing errors occur when components of a signal are above the Nyquist frequency (Nyquist theory states that the sampling frequency must be at least two times the highest frequency component of the signal) or one half the sample rate.

Aliasing errors are hard to detect and are almost impossible to remove using software. The solution is that to use a high enough sampling rate, or if it is not possible then to use an anti-aliasing filter in front of the analog-to-digital converter (ADC) to eliminate the high frequency components before they get into the data acquisition system.

The effect of aliasing can be reduced by :

  • By using Pre-alias filter to limit band of frequency of the required signal Hz.
  • Keeping sampling frequency greater than twice the maximum frequency i.e. fs > 2w.
  • Ant aliasing is a band limiting low pass filter (LPF) and pass x (t) through it before sampling. This filter has cut off frequency at fc = w so it will strictly bandlimit signal x (t) before sampling.

Aperature Effect: Aperture effect is the effect due to which the higher frequency components of a signal are suppressed. Aperture effect appears when the sampling is flat top. And the larger the time period of the sampling pulses, the more severe is the aperture effect.

The distortion caused by the use of pulse amplitude modulation to transmit an analog message signal is referred to as the aperture effect. This distortion may be corrected by connecting an equalizer in cascade with the low pass reconstruction filter.

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