Pulse Amplitude Modulation (PAM) (EC8491-UNIT-5)
Analog Pulse Modulation Techniques
Pulse Amplitude Modulation (PAM)
The Necessity of Digitization
▪ The conventional methods of communication used analog signals for long distance communications, which suffer from many losses such as distortion, interference, and other losses including security.
Why Digital?
• Unwanted electrical noise or other interference further distorts the pulse waveform.
• Before it is degraded to an ambiguous state the pulse is amplified by a digital amplifier that recovers its original ideal shape. The pulse is thus reborn or regenerated. It is done by regenerative repeaters
ADVANTAGES OF DIGITAL TRANSMISSION
1. Noise immunity – More resistive to additive noise as it uses signal regeneration rather than amplification
2. Error detection and correction: Transmission errors can be detected and corrected easily. Probability of error is less
3. Multiplexing: Many baseband signals can be combined and transmitted using TDM
4. Use of signal regenerators - They sample the received noisy signal , reproduce an entirely new digital signal with same signal to noise ratio (S/N) as that of original transmitted signal so that the signal can be transmitted to a longer distance
5. Data integrity -due to signal regenerator or digital repeater
6. Data security -due to encryption
7. Implementation of digital technology – miniaturization, less power consumption, less cost, less weight
DISADVANTAGES OF DIGITAL TRANSMISSION
1. Requires more bandwidth
2. Requires precise time synchronization between the clocks used in transmitter and receiver
3. Need for additional complex circuitry for encoding and decoding
4. Quality of service (QoS) can degrade all of a sudden from very good to very poor if the SNR drops to a specified threshold level
Pulse Modulation
▪ In pulse modulation, some parameter of a pulse train is varied in accordance with the message signal. On this basis, we may classified to two families of pulse modulation:
1. Analog pulse modulation (PAM, PWM &PPM)
2. Digital pulse modulation (PCM)
1. Analog pulse modulation, in which a periodic pulse train is used as the carrier wave and some characteristic feature of each pulse (e.g., amplitude, duration, or position) is varied in a continuous manner in accordance with the corresponding sample value of the message signal.
Thus, in analog pulse modulation, information is transmitted basically in analog form but the transmission takes place at discrete.
Digital pulse modulation, in which the message signal is represented in a form that is discrete in both time and amplitude, thereby permitting transmission of the message in digital form as a sequence of coded pulses
1. Pulse Amplitude Modulation (PAM)
2. Pulse Width Modulation (PWM)
3. Pulse Position Modulation (PPM)
PAM
▪ Amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message signal.
▪ The width and positions of the pulses are constant in this modulation.
Types of PAM
Depending upon the shape of the PAM pulse, it is classified into two types.
1. Natural PAM.
2. Flat top PAM
Generation of PAM
Operation:
A sample of hold circuit is used to produce flat top sampled PAM. This consists of two FET switches and a capacitor.
Flat top PAM signals are generated by applying the input modulating signal changing switch. Sampling switch is closed for a short duration by a short pulse applied to gate G1 of the transistor.
During this period, the capacitor C quickly charged up to a voltage equal to the instantaneous sample value of the incoming signal.
Now, sampling switch is opened and capacitor C holds the charge. The discharge switch is then closed by a pulse applied to gate G2, Due to this the capacitor C is discharged to zero volts.
PAM signal sampled at Nyquist rate can be reconstructed at the receiver end , by passing it through an efficient Low Pass Filter (LPF) with exact cut off frequency of fs/2. This is known as Reconstruction Filter.
The low pass filter eliminates the high-frequency ripples and generates the demodulated signal. This signal is then applied to the inverting amplifier to amplify its signal level to have the demodulated output with almost equal amplitude with the modulating signal.
For a flat topped PAM, a holding circuit followed by a LPF gives demodulated signal
Advantages of PAM
• It is the simple process for modulation and demodulation
• Transmitter and receiver circuits are simple and easy to construct
Disadvantages of PAM
• In PAM noise cannot be removed easily.
• Transmission bandwidth required is too large.
• Transmission power is not constant.
Pulse Amplitude Modulation (PAM) (EC8702-UNIT-5)
UNIT V SAMPLING & QUANTIZATION • Low pass sampling,Aliasing, Signal Reconstruction,Quantization, Uniform & non-uniform quantization, quantization noise, Logarithmic Companding ,PAM, PPM, PWM, PCM,TDM, FDM, Anna University
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