Explaining a statement regarding aliasing in a document I am reading this document and in the section "Aliasing and Anti-Aliasing", the author make the following statement: 


  Consider a signal with two frequency components: $f_1 = 10\,\text{Hz}$ which is
  our desired signal and $f_2 = 20\,\text{Hz}$ which is noise. Let's say that we
  sample the signal at  $f_s = 30\,\text{Hz}$. The first frequency component,
  $f_1$, will generate the following frequency components at the output
  of the multiplier, $10\,\text{Hz}, 20\,\text{Hz}, 40\,\text{Hz}, 50\,\text{Hz}, 70\,\text{Hz}$ and so on. The
  second frequency component $f_2 = 20\,\text{Hz}$ will generate the following
  frequency components at the output of the multiplier, $20\,\text{Hz}, 10\,\text{Hz},
&gt; 50\,\text{Hz}, 40\,\text{Hz}, 80\,\text{Hz}$ and so on.


It's probably obvious to most but could someone explain where those generated frequency components come from. Thanks.

The background is that I've started reading Richard Lyons' DSP text and I really like it so far except that now I hit the bandpass sampling section and I didn't follow it. So, I've been looking for other material. If anyone knows of some good material ( books or papers ) on bandpass sampling ( I've printed out the dsp.stackexchange material and will check that out next ), it's appreciated.
The frequencies are $\left|10\,\mathrm{Hz}+k\cdot30\,\mathrm{Hz}\right|, k\in \mathbb{Z}$ and the same for $20\,\mathrm{Hz}$.

Note that real signals always have (complex conjugate) signal components for positive and negative frequencies, so a $10\,\mathrm{Hz}$ signal has components at $\pm10\,\mathrm{Hz}$.  Consequently aliasing in a $30\,\mathrm{Hz}$ grid will also reach $\mp 20\,\mathrm{Hz}$.