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Analog & Digital Communication Lab Experiments

Study of PAM, PPM and PDM



Aim

Study of PAM, PPM and PDM.

(a)Study of Pulse Amplitude Modulation using Natural & Flat top Sampling.

Apparatus Required:

  1. ST2110 with power supply cord.
  2. Oscilloscope with connecting probe
  3. Connecting cords.

Theory

Most digital modulation systems are based on pulse modulation. It involves variation of a pulse parameter in accordance with the instantaneous value of the information signal. This parameter can be amplitude, width, repetitive frequency etc. Depending upon the nature of parameter varied, various modulation systems are used. Pulse amplitude modulation, pulse width modulation, pulse code modulation are few modulation systems cropping up from the pulse modulation technique. In pulse amplitude modulation (PAM) the amplitude of the pulses are varied in accordance with the modulating signal. In true sense, pulse amplitude modulation is analog in nature but it forms the basis of most digital communication and modulation systems. The pulse modulation systems require analog information to be sampled at predetermined intervals of time. Sampling is a process of taking the instantaneous value of the analog information at a predetermined time interval. A sampled signal consists of a train of pulses, where each pulse corresponds to the amplitude of the signal at the corresponding sampling time. The signal sent to line is modulated in amplitude and hence the name Pulse Amplitude Modulation (PAM).

Natural sampling:In the analogue-to-digital conversion process an analogue waveform is sampled to form a series of pulses whose amplitude is the amplitude of the sampled waveform atb the time the sample was taken. In natural sampling the pulse amplitude takes the shape of the analogue waveform for the period of the sampling pulse as shown in figure.

sampling-pulse

Flat Top sampling:-After an analogue waveform is sampled in the analogue-to-digital conversion process, the continuous analogue waveform is converted into a series of pulses whose amplitude is equal to the amplitude of the analogue signal at the start of the sampling process. Since the sampled pulses have uniform amplitude, the process is called flat top sampling as shown in figure

flat-top-sampling

Circuit Diagram:-

circuit-diagram

Signal Reconstruction:-

Procedure:-

  1. Connect the circuit as shown in Figure.
  2. Output of sine wave to modulation signal input in PAM block keeping the switch in 1 KHz position.
  3. 8 KHz pulse output to pulse input.
  4. Switch ‘On’ the power supply & oscilloscope.
  5. Observe the outputs at TP(3 & 5) these are natural & flat top outputs respectively.
  6. Observe the difference between the two outputs.
  7. Vary the amplitude potentiometer and frequency change over switch & observe the effect on the two outputs.
  8. Vary the frequency of pulse, by connecting the pulse input to the 4 frequencies available i.e. 8, 16, 32, 64 kHz in Pulse output block.
  9. Switch ‘On’ fault No. 1, 2, 3, 4 one by one & observe their effect on Pulse Amplitude Modulation output and try to locate them.
  10. Switch ‘Off’ the power supply.

Related O/P Waveforms:-

related-o-p-waveforms

(b) Study of PPM using DC Input, Sine wave Input.

Apparatus Required:

  1. ST2110 with power supply cord.
  2. Oscilloscope with connecting probe
  3. Connecting cords.

Theory

The Amplitude and width of the pulses is kept constant in this system, while the position of each pulse, in relation to the position of a recurrent reference pulse is varied by each instantaneous sampled value of the modulating wave. As mentioned in connection with pulse width modulation, pulse-position modulations has the advantage of requiring constant transmitter power output, but the disadvantages of depending on transmitter receiver is synchronization.

(a)analog-signal(b)pulse-amplitude-modulation(c)pulse-width-modulation(d)pulse-position-modulation

There may be a sequence of signal sample amplitudes of (say) 0.9, 0.5, 0 and -0.4V. These can be represented by pulse widths of 1.9, 1.5, 1.0 and 0.6μs respectively. The width corresponding to zero amplitude was chosen in this system to be 1.0μs, and it has been assumed that signal amplitude at this point will vary between the limits of + 1 V (width = 2μs) and -1 V (width = 0μs). Zero amplitude is thus the average signal level, and the average pulse width of 1μs has been made to correspond to it. In this context, a negative pulse width is not possible. It would make the pulse end before it began, as it were, and thus throw out the timing in the receiver. If theb pulses in a practical system have a recurrence rate of 8000 pulses per second, the time between the commencements of adjoining pulses is 106 /8000 =125μs. This is adequate not only to accommodate the varying widths but also to permit time-division multiplexing. Pulse width modulation has the disadvantage, when compared with pulse position modulation, which will be treated next, that its pulses are of varying width and therefore, of varying power content. This means that the transmitter must be powerful enough to handle the maximum-width pulses, although the average power transmitted is perhaps only half of the peak power. On the other hand, puls width modulation still works if synchronization between transmitter and receiver fails, whereas pulse-position modulation does not, as will be seen.

For DC Input:

circuit-diagram:

Procedure:

  1. Connect the circuit as shown in Figure and also described below for clarity.
  2. a. Connect the DC output to input of PPM block.
  3. Switch ‘On’ the power supply & oscilloscope.
  4. Observe the output of PPM block at TP7.
  5. Vary the DC output while observing the output of PPM block.
  6. Switch ‘On’ the switched faults No. 1, 2, & 6 one by one & observe their effects PPM input and try to locate them.
  7. Switch ‘Off’ the power supply.

Connection Diagram:-

connection-diagram
related-wave-form

For Sine wave Inputs:

for-sine-wave-input

Circuit Diagram:

circuit-diagram-1

Procedure:-

  1. Connect the circuit as shown in Figure and also described below for clarity. a. Input of pulse position modulation blocks to sine wave output of FG block.
  2. Switch ‘On’ the power supply & oscilloscope.
  3. Keep the oscilloscope at 0.5mS / div, time base speed and in X-5 mode, and observe the pulse position modulated waveform at the pulse position modulation block output.
  4. Vary the amplitude of sine wave and observe the pulse position modulation, keep the amplitude preset in center. Here you can best observe the pulse modulation.
  5. Switch ‘On’ fault No. 1, 2, & 6 one by one & observe their effects in pulse position modulation output and try to locate them.
  6. Switch ‘Off’ the power supply.
ppm-output

(c) Study of PWM using different Sampling Frequency.

Apparatus Required:

  1. ST2110 with power supply cord.
  2. Oscilloscope with connecting probe
  3. Connecting cords.

Theory

In pulse width modulation of pulse amplitude modulation is also often called PDM (pulse duration modulation) and less often, PLM (pulse length modulation). In this system, as shown in Figure, we have fixed amplitude and starting time of each pulse, but the width of each pulse is made proportional to the amplitude of the signal at that instant.

(a)analog-signal(b)pulse-amplitude-modulation(c)pulse-width-modulation(d)pulse-position-modulation

there may be a sequence of signal sample amplitudes of (say) 0.9, 0.5, 0 and -0.4V. These can be represented by pulse widths of 1.9, 1.5, 1.0 and 0.6μs respectively. The width corresponding to zero amplitude was chosen in this system to be 1.0μs, and it has been assumed that signal amplitude at this point will vary between the limits of + 1 V (width = 2μs) and -1 V (width = 0μs). Zero amplitude is thus the average signal level, and the average pulse width of 1μs has been made to cores to it. In this context, a negative pulse width is not possible. It would make the pulse end before it began, as it were, and thus throw out the timing in the receiver. If the pulses in a practical system have a recurrence rate of 8000 pulses per second, the time between the commencements of adjoining pulses is 106 /8000 = 125μs. This is adequate not only to accommodate the varying widths but also to permit time-division multiplexing. Pulse width modulation has the disadvantage, when compared with pulse position modulation, which will be treated next, that its pulses are of varying width and therefore, of varying power content. This means that the transmitter must be powerful enough to handle the maximum-width pulses, although the average power transmitted is perhaps only half of the peak power. On the other hand, pulse width modulation still works if synchronization between transmitter and receiver fails, whereas pulse-position modulation does not, as will be seen.

Connection Diagram:-

circuit-diagram-2

Procedure:-

  1. Connect the circuit as shown in Figure and also described below for clarity.
  2. a. 1 KHz sine wave output of function generator block to modulation input of PWM block
  3. b. 64 KHz square wave output to pulse input of PWM block.
  4. Switch ‘On’ the power supply & oscilloscope.
  5. Observe the output of PWM block.
  6. Vary the amplitude of sine wave and see its effect on pulse output.
  7. Vary the sine wave frequency by switching the frequency selector switch to 2 KHz.
  8. Also, change the frequency of the pulse by connecting the pulse input to different pulse frequencies viz. 8 KHz, 16 KHz, 32 KHz and see the variations in the PWM output.
  9. Switch ‘On’ fault No. 1, 2, & 5 one by one & observes their effect on PWM output and tries to locate them.
  10. Switch ‘Off’ the power supply.