On Thu, 05 Jul 2007 00:00:45 -0700, Ron Baker, Pluralitas! wrote:

Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?

This is close, but not to scale:
http://en.wikipedia.org/wiki/Amplitude_modulation
The animation shows the "envelope".

What would it look like on a spectrum analyzer?

One vertical "spike" at 1 MHz with smaller spikes at .9 and 1.1 MHz. The
height of the two side spikes, depends on the depth of modulation.
In this case, the carrier is in the middle, and the sidebands are on the
sides.

Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?

whatever 0.9 MHz superimposed on 1.1 MHz looks like. ;-)

What would that look like on a spectrum analyzer?

One spike at each input frequency, 0.9 and 1.1 MHz. If they're mixed
nonlinearly, then you get modulation, as above.

Hope This Helps!
Rich