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Modulation

posted Jan 15, 2012, 7:54 PM by Charles Boling   [ updated Jan 15, 2012, 8:15 PM ]

Introduction

Modulation is essentially when two signals are combined into one, containing information from both.  Specifically, one signal is used to control a higher frequency signal, essentially encoding itself.  For example, your voice, an ultra-low frequency signal, is used to alter the properties of a higher frequency radio signal, known as the carrier wave.

Types of modulation

There are two main methods used to encode voice signals on a radio carrier:

  • AM (Amplitude Modulation)

The instantaneous amplitude of the voice wave varies the amplitude, or power ("Volume", if you prefer) of the radio wave.
  • FM (Frequency Modulation)

The instantaneous amplitude of the voice wave varies the frequency of the radio wave.

Pictures!

Here you see a time-domain (oscilloscope) representation of a simple tone modulating a radio wave using the two methods described above.
(picture from Wikipedia. Click to animate it.)

Advantages of each

AM was the first method developed, and in its basic form, is dirt simple to produce and detect.  You can hear multiple stations transmitting on the same frequency simultaneously, even if one is weaker than another.

FM offers improved resistance to noise, both natural and man-made,  and does not require great linearity in transmitter or receiver (In fact, non-linear receiver processing is often used to amplify and "flat-top" the received signal to reduce AM noise).  When two stations simultaneously transmit, unless they're pretty close to the same power, the stronger one generally pretty well hides the weaker one.  (This is known as the "capture" effect.)

Related modulation modes

There are variations of each of these two techniques.  For example, it can be argued that FM is a special type of Phase Modulation (PM), which is often used for encoding digital signals.  Likewise, SSB (Single Sideband) modes are simply AM signals with certain parts of the the output suppressed.

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