Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

PAM

Generation of PAM

• Pulse amplitude modulation is the basic form of pulse modulation in which the signal is sampled at regular and each sample is made proportional to the amplitude of the modulating signal at the sampling instant.
• The Fig1 shows the generation of PAM signal from the sampler which has two inputs i.e. modulating signal and sampling signal or carrier pulse.

• Thus the amplitude of the signal is proportional to the modulating signal through which information is carried. This is Pulse amplitude modulation signal.
• Fig2 shows the spectrum of pulse amplitude modulated signal along with the message signal and the sampling signal which is the carrier train of pulses with the help of the waveform plotted in time domain.
• Pulse Modulation may be used to transmitting analog information, such as continuous speech signal or data.

Demodulation of PAM

• For Demodulation of the Pulse Amplitude Modulated signal, PAM is fed to the low pass filter as shown in Fig3 below.

  

• The low pass filter eliminates high frequency ripples and generates the demodulated signal which has its amplitude proportional to PAM signal at all time instant.

• This signal is then applied to an inverting amplifier to amplify its signal level to have the demodulated output with almost equal amplitude with the modulating signal.

Generation of PWM

• PWM signal can be generated by using a comparator, where modulating signal and sawtooth signal form the input of the comparator. It is the simplest method for PWM generation.

• The PWM generation is explained with the help of the Fig5 given below.

• As shown in the figure, one input of the comparator is fed by the input message or modulating signal and the other input by a sawtooth signal which operates at carrier frequency.

• Considering both ±ve sides, the maximum of the input signal should be less than that of sawtooth signal.
• The comparator will compare the two signals together to generate the PWM signal at its output as shown in the third waveform of Fig6.
• The rising edges of the PWM signal coincides with the falling edge of the sawtooth signal.
• When the sawtooth signal is at the minimum value which is less than the minimum of the input signal, then the positive input of the comparator is at higher potential which gives the comparator output as positive.
• When the sawtooth signal rises and is at the maximum value, the negative input of the comparator is at higher potential, which will produce the comparator output to be negative.
• Thus the input signal magnitude determines the comparator output and its potential, which then decides the width of the pulse generated at the output.
• In other words we can say that the width of the pulse generated signal is directly proportional to the amplitude of the modulating signal.

Pulse Position Modulation

• In pulse duration modulation (PDM), the samples of the message signal are used to vary the duration of the individual pulses. This form of modulation is also known as pulse width modulation bor pulse length modulation.
• In PPM, the position of a pulse relative to its unmodulated time of occurrence is varied in accordance with the message signal as shown in fig for the case of sinusoidal modulation.
• Let $ 'T_s'$ denote the sample duration. Using the sample $m(nT_s)$ of a message signal m(t) to modulate the position of the nth pulse, we obtain the PPM signal.

$ s(t) = \sum_{n=-\infty}^{\infty} . g(t - nT_s - K_p m (nT_s) ) $

where $K_p$ is the sensitivity of the pulse position modulation.

Generation of PPM waves:

• The PPM signal which is generated is shown in fig. The message signal m(t) is first converted into a PAM signal by means of a sample and hold circuit, generating a staircase waveform u(t), which is shown in figure for the message signal $m(t)$

• Next, the signal $u(t)$ is added to a saw-tooth wave yielding the combined signal $v(t)$. The combined signal $v(t)$ is applied to a threshold detector that produces a very narrow pulse each time v(t) crosses zero in the negative going direction.The resulting sequence of "impulses" $i(t)$ is as shown. Finally, the PPM signal $s(t)$ is generated by using this sequence of impulses to excite a filter whose impulse response is defined by the standard pulse $g(t).$

Detection of PPM waves: Consider a PPM wave $s(t)$ with uniform sampling, and assume that the message signal $m(t)$ is strictly band limited. The operation of one type of PPM receiver may proceed as follows: Convert the received PPM wave into a PDM wave with the same modulation. Integrate this PDM wave using a device with a finite integration time, thereby computing the area under each pulse of the PDM wave. Sample the output of the integrator at a uniform rate to produce a PAM wave, whose pulse amplitudes are proportional to the signal samples $m(nT_s)$ of the original PPM wave $s(t)$. Finally, demodulate the PAM wave to recover the message signal $m(t)$.

Advantages of Pulse Position Modulation (PPM):

• Pulse position modulation has low noise interference when compared to PAM because amplitude and width of the pulses are made constant during modulation.
• Noise removal and separation is very easy in pulse position modulation.
• Power usage is also very low when compared to other modulations due to constant pulse amplitude and width.

Disadvantages of Pulse Position Modulation (PPM):

• The synchronization between transmitter and receiver is required, which is not possible for every time and we need dedicated channel for it.
• Large bandwidth is required for transmission same as pulse amplitude modulation.
• Special equipments are required in this type of modulations.