[AMRadio] Physical Reality of Sidebands


lwill at voicenet.com lwill at voicenet.com
Mon Jan 17 15:36:41 EST 2005


Bob,

You are correct.  If I was home I'd send along the basic formula for 
modulation.  In FLA right now on vacation. It is multiplication just as a mixer is 
(the formula is the same really).  All AM modulation and demodulation is 
multiplication but in vector notation.  Check any college text on information 
theory.

regards,

Larry W3LW


On Mon, 17 Jan 2005 14:17:54 -0500, "Bob Bruhns" <bbruhns at erols.com> 
wrote :

> Hi Gary,
> 
> The carrier is not multiplied with the sideband frequencies to
> generate AM, it is multiplied with the baseband modulating
> frequency.  In my example, three sine waves corresponding to a
> carrier and two sideband frequencies are added together to produce
> the AM signal, but a relatively low frequency is multiplied with the
> carrier in the AM transmitter to produce the carrier and sideband
> frequencies in the reverse example.  Interestingly, if the AM
> signal, with or without carrier, is multiplied at the receiver with
> a regenerated carrier of proper frequency and phase, then the
> baseband modulating signal is recovered.
> 
> In the transmitter, the multiplication occurs in the modulated
> stage.  In plate-modulated AM, the carrier is multiplied by the
> plate voltage.  Normalizing plate voltage to unity, it is multiplied
> by +1 without modulation.  Modulation varies the B+, and therefore
> the multiplication factor.  The normalized multiplication factor
> varies from 0 to +2 with 100% sine wave modulation.
> 
>   Bacon, WA3WDR
> 
> 
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