Friday, April 11, 2014

Amplitude Modulation in layman terms..

Here's a copy of a great post by Bob Felmly explaining what modulation is. The comment originated in a forum thread here:

There are two ways to explain amplitude modulation or AM.  The correct way and the one that is easy to understand and makes sense to the layman.

For starters, the carrier or continuous AC wave produced by an AM transmitter ideally is a pure sine wave.  Its strength depends on the power applied and the efficiency of the device.

Radio frequency power can travel large distances without wires.  Audio frequencies can not.  The radio frequency power is used to transport or carry the audio without wires connecting the origination point to the end point.

When you add your program audio to the radio frequency carrier, the loudness of the audio causes the amplitude of the carrier to change in step with the audio.  The audio is impressed on the carrier or modulates the carrier.

The audio is an AC signal.  The voltage of the audio alternately is positive and negative.  That causes the carrier to alternately increase and decrease at the audio rate.  The extra power is the audio.  If the carrier is 100 mW the audio needed for 100% modulation would be 50 mW.

The maximum the carrier can decrease is 100%.  If the audio is too loud the carrier can't go beyond 100% in the negative direction as the carrier would shut off at that point.  Trying to go beyond 100% in the negative direction creates RF interference over a wide range of frequencies and must be avoided.

Depending upon how the audio is impressed on the carrier, the carrier can increase more than 100% in the positive direction.  This does not have the same effect as more than 100% negative modulation because the carrier does not shut off it simply keeps getting larger.

When the carrier is modulated more than 100% in the positive direction this is called asymetrical modulation.  Technically, this is distortion but the end result is the received signal sounds louder.  Special modulators allow the positive going signal beyond 100% but limit the negative going signal to no more than 100%.

The other way to explain amplitude modulation is when the audio is mixed with the carrier, new frequencies are generated.  Among others you end up with the carrier frequency, carrier frequency plus audio frequency and carrier frequency minus audio frequency.  The new plus and minus frequencies are called sidebands.  When these three frequencies are mixed together the algebraic sum of the instantaneous voltages produces the modulated output signal.  A lot less words but a little more difficult for some to grasp.

The frequency difference between the plus and minus sidebands is the bandwidth of your signal.  Another words, the bandwidth of your signal is two times the highest audio frequency applied.  The AM broadcast band channel is 10 kHz so the maximum audio frequency should be 5 kHz but that seems to have been relaxed over the years.

OK, that's my stab at it.  Rip it up.
by MRAM 1500 kHz Charter Member - Association of Low Power Broadcasters Chairman - ALPB

A Technical Article About Amplitude Modulation...
Here is a LINK TO A GREAT ARTICLE which explains the process.

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