Folding Frequency

• Let $f_{\mbox{in}}$ be the input signal and $f_{\mbox{out}}$ be the signal at the output (after the lowpass filter).
  • If $f_{\mbox{in}}$ is less than the Nyquist limit,
    $$f_{\mbox{out}} = f_{\mbox{in}}.$$

  • If $f_{\mbox{in}}$ is greater than Nyquist but less than the sampling rate,
    $$f_{\mbox{out}} = f_s - f_{\mbox{in}}.$$

  Figure 5: Folding of a sinusoid sampled at $f_s = 2000$ samples per second.

  

• The folding occurs because of the negative frequency components.