After the constant wave modulation, the following division is Pulse modulation. Pulse modulation is additionally separated into analog and digital modulation. The analog modulation methods are mostly categorized into Pulse Amplitude Modulation, Pulse Duration Modulation/Pulse Width Modulation, and Pulse Position Modulation.
Pulse Amplitude Modulation (PAM) is an analog modulating scheme in which the amplitude of the pulse carrier different proportional to the immediate amplitude of the message signal.
The pulse amplitude modulated signal, will follow the amplitude of the original signal, as the signal traces out the path of the whole wave. In natural PAM, a signal sampled at the Nyquist rate is reconstructed, by passing it through an efficient Low Pass Frequency (LPF) with exact cutoff frequency
The resulting data give details the Pulse Amplitude Modulation.
Still the PAM signal is passed through an LPF, it cannot recover the signal without distortion. Therefore to evade this noise, flat-top sampling is done as shown in the resulting figure.
Flat-top sampling is the procedure in which experimented signal can be signified in pulses for which the amplitude of the signal cannot be altered with admiration to the analog signal, to be sampled. The tops of amplitude remain flat. This procedure simplifies the circuit design.
Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM) or Pulse Time Modulation (PTM) is an analog modulating scheme in which the duration or width or time of the pulse carrier varies relative to the instantaneous amplitude of the message signal.
The width of the pulse differs in this method, but the amplitude of the signal rests constant. Amplitude limiters are used to create the amplitude of the signal constant. These circuits clip off the amplitude, to a chosen level and therefore the noise is limited.
The resulting figures clarify the types of Pulse Width Modulations.
There are three differences of PWM. They are −
These three types are shown in the above given figure, with timing slots.
Pulse Position Modulation (PPM) is an analog modulating scheme in which the amplitude and width of the pulses are kept continuous, while the position of each pulse, with position to the position of a reference pulse differs according to the immediate sampled value of the message signal.
The transmitter has to send synchronizing pulses (or simply sync pulses) to keep the transmitter and receiver in synchronism. These sync pulses help maintain the position of the pulses. The resulting figures clarify the Pulse Position Modulation.
Pulse position modulation is completed in agreement with the pulse width modulated signal. Every trailing of the pulse width modulated signal develops the starting point for pulses in PPM signal. Therefore, the position of these pulses is proportional to the width of the PWM pulses.
As the amplitude and width are constant, the power handled is also constant.
The synchronization between transmitter and receiver is a must.
The comparison between the above modulation processes is presented in a single table.
|Amplitude is varied||Width is varied||Position is varied|
|Bandwidth depends on the width of the pulse||Bandwidth depends on the rise time of the pulse||Bandwidth depends on the rise time of the pulse|
|Instantaneous transmitter power varies with the amplitude of the pulses||Instantaneous transmitter power varies with the amplitude and width of the pulses||Instantaneous transmitter power remains constant with the width of the pulses|
|System complexity is high||System complexity is low||System complexity is low|
|Noise interference is high||Noise interference is low||Noise interference is low|
|It is similar to amplitude modulation||It is similar to frequency modulation||It is similar to phase modulation|
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