Wednesday, February 4, 2009

Modulation - Amplitude (AM), Frequency (FM) and Phase

For Ham Radio purposes, modulation is a small variation of a sine wave (also called carrier wave) that carries information. In the case of Amplitude Modulation (AM) and Frequency Modulation (FM) the information that is carried is generally a wave itself. Phase modulation is commonly used for digital information and you can have large jumps in the phase to represent a digital signal but the carrier wave isn't changed drastically.

Let's start from a basic sine wave, or carrier wave. Incidently, carrier wave is where the abbreviation CW comes from, indicating Morse Code communication. A carrier wave imparts no information other than that it is present. CW communication imparts it's information by turning the carrier wave on and off. In a sense, this is modulating the carrier wave. An ideal carrier wave does not change at all in frequency or amplitude. For AM or FM signals this is the frequency shown on the radio. In order to send anything interesting with this wave we need to vary it or modulate it.

Amplitude is a measure of the strength of a wave. The stronger the signal is, the greater the amplitude will be. Now imagine that you have a radio where you can change the signal strength, the amplitude, by simply turning a knob, much like a volume knob for audio. If you are transmitting a simple carrier wave then as you turn the knob up and down you are varying the amplitude. Now let's say you have really fast hands. If you turn the knob up and down just right, 440 times a second you would produce an output that was the carrier wave, modulated at 440Hz which happens to be the tone A above middle C. This is an AM signal. To produce a pure sine wave of 440Hz you would have to turn the knob just right so that the amplitude would go up and down over time in the shape of a sine wave. This is, in essence what an AM transmitter does. An AM receiver gets the sent 440Hz wave by filtering out the carrier wave.

You can extend the idea so that whatever audio frequency signal you put in, a human voice for example, you can vary the amplitude of the carrier wave to transmit that signal and the receiver can recover it.

Let's take the same carrier wave as in Amplitude Modulation; but now when you turn the knob you change the frequency by a little bit instead of the amplitude. Once again, if you turn the knob just right you can modulate the signal with a 440Hz sine wave and a receiver can recover that pure tone. In this case the receiver has to use a different method to recover the audio signal but it is still a matter of detecting the modulation of the carrier and producing only the audio signal without the carrier. One term you have probably heard is deviation. If you don't have enough deviation, your audio signal is weak; if you have too much your audio signal can be distorted by overloading the detector circuits. Deviation is the measure of how much the frequency changes from the carrier frequency when the change is at it's most. That is, what is the largest amount that the frequency deviates from the carrier when the audio signal is applied.

Phase modulation changes the phase of the carrier and that is how it imparts information. The phase is an indication of how far into the sine wave you have progressed. We measure phase in degrees based on the mathematical sine function. The wave starts at zero degrees goes to a maximum at 90 degrees, back to zero a 180 degrees, etc. With digital signals you can suddenly change the phase to indicate a digital change from 0 to 1. This is a topic that really needs it's own session so I will hold off on saying more about digital modes. One thing to note is that phase and frequency are very closely related and varying the phase will vary the frequency and vice versa. In fact, there has been equipment used to create FM signals by using phase variation in conjunction with other signal processing.

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