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

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] book/video (MIT courseware, whatever) recommendations?
Date: Tue, 22 Feb 2011 19:23:06 -0500
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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.


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

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