» SystemVue Index

  » Introduction

  » Natural Sampling

  » Flat Topped Sampling

  » Aperture Error

  » Aliasing

  » Quantization Distortion

  » MSWord Version



A great deal of signal processing is required to convert an analog signal to the digital domain. This collection of SystemView models will examine this process, and its consequences. Some of these include:

  • Natural & flat topped sampling
  • Aperture error
  • Aliasing
  • Quantization distortion

Nyquist Sampling

In any sampling process, it is necessary to obtain enough samples to be representative of the whole. The Nyquist theorem states that the minimum number is 2 samples per Hertz. In actual practice it is often necessary to take more than this minimum amount.

In a telephony codec, the ratio is .

Aliasing occurs if less than 2 samples/Hz are obtained. Notice what happens in the time domain, if a sine wave is sampled at less than the Nyquist rate:


Aliasing also occurs in computer simulations if the System Clock is not set to a high enough rate for the model. In most cases, choose a value of about 5 10 times the highest frequency in the model. Don't forget that pulses consist of many high frequency components, therefor the System Clock might have to be set 10 times higher yet again!

There are two types of sampling: natural and flat-topped. Of these two, natural sampling is the easier to analyze. However, in practice, flat-topped sampling is used in an ADC.