| Keith Dysart 2007-07-05, 10:33 am |
| On Jul 5, 10:01 am, John Fields <jfie...@austininstruments.com> wrote:
> On Thu, 5 Jul 2007 00:00:45 -0700, "Ron Baker, Pluralitas!"
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> <t...@aint.me> wrote:
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> The first example is amplitude modulation precisely _because_ of the
> multiplication, while the second is merely the algebraic summation
> of the instantaneous amplitudes of two waveforms.
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> The circuit lists I posted earlier will, when run using LTSPICE,
> show exactly what the signals will look like using an oscilloscope
> and, using the "FFT" option on the "VIEW" menu, give you a pretty
> good approximation of what they'll look like using a spectrum
> analyzer.
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> If you don't have LTSPICE it's available free at:
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> http://www.linear.com/designtools/software/
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> --
> JF
Since your modulator version has a DC offset applied to
the 1e5 signal, some of the 1e6 signal is present in the
output, so your spectrum has components at .9e6, 1e6 and
1.1e6.
To generate the same signal with the summing version you
need to add in some 1e6 along with the .9e6 and 1.1e6.
The results will be identical and the results of summing
will be quite detectable using an envelope detector just
as they would be from the modulator version.
Alternatively, remove the bias from the .1e6 signal on
the modulator version. The spectrum will have only
components at .9e6 and 1.1e6. Of course, an envelope
detector will not be able to recover this signal,
whether generated by the modulator or summing.
....Keith
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