1.0 Introduction
The AM transmitter generates such high power that it's prime requirement is efficiency. Amplitude modulation can be generated at any point after the radio frequency source as a matter of fact, even a crystal oscillator could be amplitude modulated except that this would be an unnecessary interference with its frequency stability. If the output stage in a transmitter is plate modulated the system is called high-level modulation.
An AM transmitter which may be either low level or high level modulated have a stable RF source and buffer amplifier followed by RF power amplifier. The audio voltage is processed or filtered so as to occupy the correct bandwidth and compressed somewhat of reduce the ration of maximum to minimum amplitude.
In IM transmitter, the prime requirement of an FM system is a variable output frequency, with the variation proportional to the instantaneous amplitude or the modulation voltage.
The power and auxiliary stages of FM transmitter are similar to those in AM transmitter except that FM has an advantage, since it is a constant amplitude modulation system all the power amplifier can be operated in class c and that is very efficient.
1.1 Modulation (Analog and Digital)
Modulation is the systematic transformation of a carrier wave in accordance with the message signal. To a large extent the success of a communication system in any given mission depends on the modulation so much so that the type of modulation is a pivotal decision to system design. There are two basic types of modulating techniques the analog and digital modulation. Analogue modulation uses sinusoidal waveform as the carrier signal while digital modulation uses a discrete or pulse train as the carrier signal.
Analogue modulation being a continuous process is obviously suited to signal that are continually varying with time. The carriers are at a frequency much higher modulating signal. The modulation process is thus characterized by frequency translation. Pulse modulation is discontinuous or discrete process in the sense that the pulses are present only at a certain interval of time. in the past, analogue modulation methods have been very largely exploited and still are because of the capital investment in existing systems and theirs basic simplify. The two most important methods of analogue modulation are amplitude modulation.
1.2 Amplitude Modulation (AM)
In amplitude modulation the amplitude of a carrier the modulating voltage whose frequency is invariably lower than that of the carrier varies signal. AM is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage. Let the carrier voltage and the modulating voltage Vc and Vm respectively be represented by
- Vc = VC Sin Wct
- Vm = Vm Sin Wmt
Note that phase angle has been ignored in both expressions since it is unchanged by the amplitude modulation process from the definition of AIG amplitude vc. of the unmodulated carrier will have to be made proportion to the instantaneous modulating voltage vm sin wmt where the carrier is amplitude modulated.
1.3 Frequency Modulation
Frequency modulation is a system in which the amplitude of the carrier is made constant whereas its frequency is varied about its unmodulated frequency in a way and manner determined by the amplitude of the modulating signal. When the information signal is positive the carrier frequency is increased above its unmodulated value.
The increase in carrier frequency varies linearly with the instantaneous value of the information reaching a maximum when the modulating signal reaches its peak value. The converse applies when the modulating signals is negative ie the instantaneous carrier.
Frequency being decreased in proportion to the instantaneous value the modulating or information signal.
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