Re: [Discuss-gnuradio] book/video (MIT courseware, whatever) recommendations?

[Marcus D. Leech]

On 02/22/2011 07:04 PM, Brett L. Trotter wrote:
> Long story short, what's a good way to get a more solid grasp of how
> driving a DAC can create electromagnetic waves, and what can one do with
> those waves. I'd really really like to walk away understanding how
> complex numbers turn into constellations are really formed as an
> electromagnetic wave, etc, and the real guts of some basic things like
> FM and DSSS.
>
> -Brett
The first few chapters of the ARRL handbook actually make a fairly good 
introduction to
  radio in general.  Once you realize that an electronic circuit 
oscillating at some frequency
  can "launch" an EM wave into a suitably matched "antenna like thing", 
it's not hard to see
  how you can make that oscillation happen digitally, etc.

DSP/SDR uses digital representations of the underlying mathematics of 
creating signals.
  In the analog world, we use analog components to *approximate* (to 
varying degrees of
  fidelity) the mathematical transformations of radio communications.

Consider, very briefly, amplitude modulation:

You have a "carrier" frequency, at some Mhz, that is modulated in 
*amplitude* by a much-lower-frequency audio wave.
  In the analog world, you use a mixer to do this (with suitable 
futzing-about that is nearly-always necessary in the
  analog world).  But once you realize that amplitude modulation is 
nothing more than *multiplication* of two waveforms
  (or, the digital representation of those waveforms), then AM becomes 
quite trivial to generate.

Here's a quick experiment you can try at home.  Assuming you have GnuPlot:

gnuplot> plot (sin(x*1000000)*sin(x*100))

You just plotted what amounts to a simple amplitude modulated signal, 
where the "audio" frequency is a fraction of the carrier
  frequency. It's not that realistic, but it's illustrative.


--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org