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| Modulation |
Modulation is used to move the intelligence signal to the frequency band of the transmission channel. This is done by using a high-frequency waveform (carrier) to carry the messages. In the modulator, the modulating signal (message) systematically alter the carrier wave with the variation in the modulating signal.
Keywords
- Modulation
- Amplitude Modulation
- Frequency Modulation
- Phase Modulation
- Sideband Frequency
- Modulation Index
- Amplitude Shift Keying
- Suppressed Carrier Double-Sideband (SCDSB) Modulation
- Modulation methods
- Single Sideband Modulation
- Modulation
Modulation is used to move the intelligence signal to the frequency band of the transmission channel. This is done by using a high-frequency waveform (carrier) to carry the messages.
In the modulator, the modulating signal (message) systematically alter the carrier wave with the variation in the modulating signal. The message may alter carrier amplitude, frequency, or phase.
In the demodulator, the variations of the modulated carrier are detected and the original message waveform is reconstructed.
Modulation is a process. In which a signal feature is changed and then mixed with another signal. So that it can be transmitted. Such a process is of great importance in telecommunication network system and broadcasting.
It has the ability to send a variety of data such as sound, video and digital data. It also helps to receive data.
Includes the basic types of modulation
- Amplitude modulation (AM)
It changes the amplitude of the signal.
- Frequency Modulation
It changes the Frequency of the Signal.
- Phase Modulation (PM)
It changes the phase of the signal. These different types are used according to different conditions and needs.
- Amplitude Modulation
The original carrier wave has a constant peak value and a much higher frequency than the modulating signal, the message. When the modulating signal is applied, the peak value of the carrier varies in accordance with the instantaneous value of the modulating signal and the outline wave shape or envelope, of the modulated wave follows the shapes of the original modulating signal, as shown in figure.
Figure
Thus the unique property of amplitude modulation is that the envelope of the modulated carrier has the same shape as the message.
It can be shown with the help of the simple mathematical analysis that when a sinusoidal wave at carrier frequency fc Hz is amplitude modulation by a sinusoidal modulating signal at message frequency fm Hz.
The modulated wave contains three frequencies.
The original carrier frequency,
fc Hz;
The sum of the carrier and modulating signal frequency,
(fc + fm) Hz;
The difference between the carrier and modulating signal frequency
(fc – fm) Hz;
Figure
- Sideband Frequency
The sum of and different frequencies are new, produced by the amplitude modulation (AM) process and are called sideband frequency.
In this cause, the bandwidth of the modulation signal is
(fc +fm)—(fc – fm) = 2 fm.
If the modulating signal contains multiple frequency components, a band of frequencies such as in speech or music, the AM process transfers the message spectrum with the carrier. The message spectrum appear after the modulation on both side of the carrier and the required bandwidth is doubled.
figure 2.1 (b)
Show in an example where the original message with baseband bandwidth W modulates a carrier at the frequency fc. Each individual frequency that the message contains produce upper and lower side frequencies around the carrier frequency and complete upper and lower side bands that contain all the frequencies of the message are obtained.
- Modulation Index
The ratio between the amplitude of the modulating frequencies and the carrier frequencies is called modulation index and is expressed as percentage. At 100% modulation the carrier amplitude range between double the unmodulated level and zero. Normally 100% modulation is not adopted
- Amplitude Shift Keying
If the message is in digital format, the amplitude of the carrier is changed rapidly from one value to another. This is called “keying” because in early wireless telegraph systems, the carrier was switched on and off with each keystroke by an operator. This type of digital AM is called (amplitude shift keying) ASK.
AM is the oldest modulation method but is still commonly used in modern communication systems together with phase modulation. The original AM has further developed into the suppressed carrier double-sideband modulation.
- Suppressed Carrier Double-Sideband (SCDSB) Modulation
In the case of AM modulation, the carrier is in the air even when there is no information to be transmitted. It can be shown that even with the maximum information, amplitude, at least 50% of the total power is spent on the carrier waves in AM. In the SCDSB, (also called DSB, for short) scheme, the carrier wave is suppressed and all the power is used in for sideband that carry the information.
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| Suppressed Carrier-Sideband Modulation |
- Modulation methods
- (a) SCDSB, where the carrier is suppressed to save power and the DSB is used as a subprocess in FM-stereo broadcasting.
- (b)SSB, where only one side band is transmitted to save bandwidth and the SSB is used in analog carrier system.
- (c) VSB, where one sideband and a small fraction of the other are transmitted for improved low-frequency response and the VSB is used in TV-video transmission.
The cost incurred for power saving with the help of SCDSB, is having to use more complicated transmitters and receivers, but this is no longer important with the present technology. The detector is in the receiver cannot find the message by following the envelope only. The receiver carrier wave has a phase reveal every time the message crosses zero and in addition to the amplitude, the phase has to be detected. SCDSB is used for example for stereo information processing in present analog FM-radio broadcasting system and together with phase modulation in digital radio transmission system.
- Single Sideband Modulation
The conversional AM doubles the bandwidth of the message-wasting bandwidth in addition to power. Suppressing one of the sidebands reduces the transmission bandwidth and leads to single-sideband modulation, SSB.
- Bandwidth Of Transmission Channel
The bandwidth of a transmission channel is an especially important restriction of the carrier system in the telecommunication network. SSB is used in the analog carrier systems that are designed to transmit as many telephone channels as possible through a bandwidth-limited channel such as a cable. The SSB doubles the capacity (the number of speech channels) compared with AM and SCDSB.
Single sideband is specialized from amplitude modulation which offers some important advantage over convenrstional amplitude modulation.
Probably most important is the ability of single sideband (SSB) to convey twice as much information as an DSB-AM signal of equal bandwidth or for a given amount of information, SSB requires only half the bandwidth of an AM signal.
This is important because the need for communication is continuously growing, and as it does, the available spectrum space has become scarce, especially in the highly populated area of the frequency spectrum such as the high-frequency band (3 MHz to 30 MHz). SSB provides more channels of communication than DSB-AM and therefore utilizes the availability spectrum more efficiently.
- Advantage of SSB Convenrstional AM Lines
A second advantage of SSB over convenrstional AM lines lies in transmitted power efficiencies. In convenrstional AM, Two-third of the total power is in the carrier, yet none of the intelligence is found there. All the intelligence is conveyed by the sideband, which continues to only one-third the total power of a fully modulated AM signal.
- SSB Communication system
As will be seen, SSB communication systems eliminate the carrier, thus conserving two-thirds of the power. In addition, since the two sidebands of a convenrstional AM signal contain the same information, one of the sidebands can be eliminated without any loss of intelligence.
- Fully Modulated AM Signal Stripped
A fully modulated AM signal stripped of its carrier, and one of its sidebands becomes a single sideband signal. It contains the same intelligence as the original AM signal, but with only one-sixth of the power and one-half the bandwidth.
These two factors combine to give the SSB system an 8 dB gain advantage over an AM system.
A third advantage of SSB over convenrstional AM is the elimination of heterodyne interference which results when two or more AM carrier fall within the bandwidth of a convenrstional AM receiver.
- Nonlinear Mixing Results
Nonlinear mixing results when a convenrstional AM receiver is tuned to receive two closely spaced signal simultaneously.
