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PULSE MODULATION

PULSE MODULATION. Three different sampling methods of PAM. Pulse Amplitude Modulation (PAM) Modulation in which the amplitude of pulses is varied in accordance with the modulating signal.

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PULSE MODULATION

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  1. PULSE MODULATION

  2. Three different sampling methods of PAM

  3. Pulse Amplitude Modulation (PAM) Modulation in which the amplitude of pulses is varied in accordance with the modulating signal

  4. Pulse Width Modulation (PWM)Modulation in which the duration of pulses is varied in accordance with the modulating signal

  5. Pulse Position Modulation (PPM)Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the modulating signal.

  6. SAMPLING • Process of converting a continuous time signal to an equivalent discrete time signal. • Continuous time signal x(t) is applied at input of multiplexer. • Other input of multiplexer is train of impulse. • At the output of multiplexer we get the sampled version of x(t)

  7. PAM Amplitude of pulse is proportional to amplitude of modulating signal. Band width of transmitting channel depends on width of pulse. Instantaneous power of transmitter varies. Noise interference is high. Complex system. Similar to A.M. DIFFERENCE PWM • Width of pulse is proportional to amplitude of modulating signal. • Band width of transmitting channel depends on rise time of the pulse. • Instantaneous power of transmitter varies. • Noise interference is minimum. • Simple to implement. • Similar to F.M. PPM • Relative position of pulse is proportional to amplitude of modulating signal. • Band width of transmitting channel depends on rise time of the pulse. • Instantaneous power remains constant. • Noise interference is minimum. • Simple to implement. • Similar to P.M.

  8. Flat Top PAM • The top of pulses of this PAM is flat. • Noise interference at the top of transmitted pulse can be easily removed. • Due to this it is widely used. • Better than Natural PAM • Because in case of Natural PAM, the varying top signal is when received at receiver, it becomes quite difficult to determine shape of top of pulse due to noise (which is always present). • Thus errors are introduced in the receiving signals due to wich we prefer a flat top PAM.

  9. Fig. shows sampled and hold circuit to Produce FLAT TOP sampled P.A.M. Sampled and hold circuit consist of 2 flat switches and a capacitor. The sampling switch is closed for a short duration by a short pulse applied to gate G1 of transistor. During this period the capacitor is charged up to a voltage equal to the instantaneous value of input signal x(t) When the sampling switch is opened the capacitor hold the charge. The discharge switch is then closed by a pulse applied to the gate G2 pf the other transistor. Due to this capacitor discharge to zero volts. Hence the output sampled and hold circuit consist of Flat Top Samples. Flat Top PAM cont.

  10. PULSE TIMEMODULATION(PTM) • There are two types of Pulse Time Modulation • Pulse Width Modulation (PWM) • Pulse Phase Modulation (PPM) • In both PWM and PPM some time parameters of the pulse is modulated. • In PWM the width of pulse is varied. • In PPM position of the pulse is varied. • Amplitude of pulses remain constant for both PWM and PPM

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