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Modulation

Once radio waves are produced, something must be done so that they can convey a message. In early radio, the transmitter was simply turned on and off with a telegraph key and messages were sent in Morse code. When receivers were developed that were capable of receiving something other than the "white noise" of spark gap transmitters, new types of transmitters were developed that could send voice.

Continuous Wave (CW)

Continuous wave is the term used for sending Morse code by turning the transmitter on and off.[1] This may be a bit confusing as the wave coming from the transmitter is not continuous; it is interrupted by the operator as the transmitter is turned on and off. Originally, continuous wave meant a steady sine wave output as produced by an electronic oscillator, as opposed to a damped wave produced by a spark gap transmitter. In the modern context, CW means that the wave is not modulated.

Amplitude Modulation (AM)

The first voice transmitters used amplitude modulation.[2] Amplitude modulation (AM) varies the output power of the transmitter proportionally to the signal that is modulating the transmitter. This is like Bell's "voice shaped currents" that convey information in a telephone system. To transmit a 1,000 Hz tone, the power output of an AM transmitter increases and decreases 1,000 times per second.

Using the analogy of flashing lights for Morse code, amplitude modulation works like this: When speaking into a microphone sound waves cause the diaphragm to vibrate. The diaphragm is connected to a coil of wire that vibrates in the field of a permanent magnet. This causes "voice-shaped currents" to flow in and out of the coil. These currents are then amplified and used to drive the signal light. The final result is that as the voice coil in the microphone moves one way, the lights get brighter and when the voice coil moves the other way the lights get dimmer. We could then say that we have "voice-shaped light." Speaking into the microphone would cause the lights to brighten and dim at a combination of rates between about 100 times per second and about 4,000 times per second.

Amplitude modulation works precisely the same way except it is radio waves instead of light waves that get stronger and weaker. The first experimental AM transmitter simply had a carbon microphone between the transmitter and the antenna. This microphone acted as a variable resistor when it reacted to sound and varied the current to the antenna.

AM is not commonly used for radio communication because it is not immune to noise like Frequency Modulation. However, it is still used for lower frequencies (such as shortwave and AM broadcast) and for the video portion of an analog television broadcast (where that is still done). AM is also used for Aircraft Communication.

Frequency Modulation (FM)

Let's return to the light analogy for radio. Frequency modulation is like changing the color of the light as the microphone vibrates. That is, as the diaphragm of the microphone moves one way the light becomes bluer and when the microphone diaphragm moves the other way the light becomes redder.

Frequency Modulation (FM) varies the frequency of the transmitter's oscillator proportionally to the modulating signal. For example, consider a commercial broadcast FM transmitter operating at 100,000,000Hz (100MHz) that is transmitting a 1,000Hz tone. The transmitter's output frequency falls to as low as 99,925,000 Hz and rises to as high as 100,075,000Hz (75kHz above and below the assigned frequency), swinging back and forth between these two frequencies 1,000 times per second.

FM is popular because it has high noise immunity. Noise form lightning and other sources is effectively filtered out by an FM receiver.

FM has a phenomenon called the capture effect. If two signals reach the same receiver, only the strongest one is heard. The weaker one is completely rejected. This is desirable for most applications. However, for aircraft communication, this means that if two pilots try to talk at the same time, the controller will only hear one and will not know that a second is trying to communicate. With AM, the two competing transmissions will cause a squealing noise, telling the controller (or other pilots) that more than one pilot is trying to communicate. This should prevent the controller or a pilot from completely missing out on a critical communication.[3]

Phase Modulation (PM)

With phase modulation, neither the amplitude nor the frequency of the signal changes. Instead, the timing of the arrival of the waves is varied. Phase modulation has become important as part of quadrature amplitude modulation, which is the standard for digital signals.

Quadrature Amplitude Modulation (QAM)

Quadrature amplitude modulation is a combination of amplitude modulation and phase modulation. It has become the standard for digital signals. It typically uses two amplitudes and eight different phases. This allows four bits of digital information to be sent in the same timeframe that only one bit can be sent with AM or FM. There are eight phase possibilities, which depicts three bits of information. The amplitude has two possibilities, depicting another bit. This gives four bits in total.

The "quadrature" in QAM comes from the method of generating the signal. Two AM sine waves that are 90 degrees out-of-phase (in-quadrature) are mixed. By manipulating the amplitudes of the two signals, any phase angle or amplitude can be generated.

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1When radio is used to send Morse code it is called radiotelegraphy.
2When radio is used to carry two-way voice communication it is called radio telephony.
3Unfortunately, using AM did not prevent the lost communication that lead to the deadliest plane crash of all time, the 1977 Tenerife Airport disaster.
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